The Annotated Origin: A Facsimile of the First Edition of On the Origin of Species 0674032810, 9780674032811

Table of contents :
Introduction ix
On the Origin of Species
Coda: The Origin Evolving 491
References 497
Biographical Notes 509
Acknowledgments 527
Subject Index 529

Citation preview

Charles darvvir A Facsimile of the First Edition of On the Origin of Species ANNOTATED

BY

JAMES T. COSTA

$35.00

Charles Darwin's On the Origin of Species is the most important and yet least read scientific work in the history of science. Now JamesT Costa—ex¬ perienced field biologist, theorist on the evolution of insect sociality, and passionate advocate for teaching Darwin with DaPvvin in a society where a significant proportion of adults believe that life on earth has been created in its present form within the last 10,000 years—has given a new voice to this epochal work. By leading readers line by line through the Origin, Costa brings evolution's foun¬ dational text to life for a new generation.

The Annotated Origin is the edition of Darwin's masteoA/ork used in Costa's course at Western Car¬ olina University and in Harvard's Darwin Summer Course at Oxford. A facsimile of the first edition of 1859 is accompanied by Costa's extensive marginal annotations, drawing on his extensive experience with Darwin's ideas in the field, lab, and classroom. This edition makes available an accessible, useful, and practical resource for anyone reading the Ori¬ gin for the first time or for those who want to reread it with the ins.^^ working biologist can provide.

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The Annotated Origin

Digitized by the Internet Archive in 2018 with funding from Kahle/Austin Foundation

https://archive.org/details/annotatedoriginfOOdarw

The Annotated Origin A Facsimile of the First Edition of On the Origin of Species

Charles Darwin ANNOTATED BY JAMES T. COSTA

The Belknap Press of Harvard University Press Cambridge, Massachusetts, and London, England

•

2009

Copyright © 2009 by the President and Fellows of Harvard College All rights reserved Printed in the United States of America Library of Congress Cataloging-in-Publication Data Darwin, Charles, 1809-1882. [On the origin of species] The annotated Origin: a facsimile of the first edition of On the origin of species / Charles Darwin; annotated by James T. Costa, p. cm. Includes bibliographical references and index. ISBN 978-0-674-03281-1 (alk. paper) 1. Evolution (Biology). 2. Natural selection. 3. Darwin, Charles, 1809-1882. On the origin of species. I. Costa, James T., 1963- II. Title. QH365.02 2009 576.8'2-dc22 2008043895

For Leslie, who loves ginkgo leaves best

Contents

Introduction

ix

On the Origin of Species

Coda: The Origin Evolving

491

References

497

Biographical Notes

509

Acknowledgments

527

Subject Index

529

Introduction

Darwin’s Origin of Species is a living document that has, in many respects, even greater currency today than when it was first pub¬ lished in 1859. It is at once a founding treatise of a major scien¬ tific discipline, a philosophical argument for a novel worldview, and a masterly piece of science writing. A close reading throws open a window on a time and place, giving us insight into the cultural context in which its ideas were fermented and debated. That window also reveals much about scientific pursuit in the nineteenth century, a time when professional science was in its infancy and scholars began refining ideas about science as a way of knowing the natural world. As for the book as science litera¬ ture, we have the renowned naturalist Alfred Russel Wallace, co-discoverer of the principle of natural selection, to thank for derailing Darwin’s planned mega-tome on the subject and giv¬ ing us instead this remarkable book, On the Origin of Species by Means of Natural Selection. Darwin apologetically called the Origin an “abstract,” which never fails to elicit a chuckle from students braving its 490-odd pages; but indeed it is an abbrevi¬ ated version of the treatise he intended, lacking proper footnotes and the many argument-buttressing examples Darwin mar¬ shaled in that larger work. As a result, the Origin is written in a style far more accessible than the “big species book” might have been. The Origin has its dense passages, but in places the book is nothing short of lyrical. Its arguments are backed by case stud¬ ies, data, and observations, yet they are presented in a narrative style that was unusual for serious scientific books at the time, and that had the welcome effect of broadening the book’s audience. To a greater extent than perhaps any other watershed scien¬ tific document, the Origin rippled through society: its implica¬ tions were felt in literature, philosophy, politics, and art; the book was midwife to the Modern period to come in the next

century. Yet despite its initial impact, today readers see the Ori¬ gin as little more than a presentation of Darwin’s ideas of evolu¬ tion and natural selection; in fact, it is probably more misunder¬ stood than understood by the general public. Even within the biological sciences few venture far beneath its cover, and fewer still read the book in its entirety. There are several reasons for this inattention, chief among them the nature of the scientific pursuit. Today the extent and pace of scientific research are un¬ precedented, and the sheer quantity of new knowledge is so great that, in a discipline that emphasizes the latest cutting-edge findings, not much room is left in biology curricula for reading the old literature. Another problem is the distance between Dar¬ win’s time and our own. People, places, and many of the ideas discussed in the Origin mean little to a modern reader, and for many people Darwin’s Victorian prose poses its own stumbling block. The annotations presented here aim to remedy this situation. It is my hope that by highlighting guideposts along the path of Darwin’s “one long argument,” as he described the book—trans¬ porting the reader to Darwin’s time and place by fleshing out such details as biographical references, natural history observa¬ tions, and the intent or meaning of arguments that are obscure from a modern perspective—a new generation of students will be inspired to read Darwin. This book can also be seen as a guide to the making of the Origin, the behind-the-scenes ferment re¬ vealed in Darwin’s correspondence, notebooks, and diaries, and the writings of his contemporaries. My aim, finally, is to show readers the breathtaking sweep of Darwin’s method, in hopes that more educators and others will take a page from his playbook in making the case for biological evolution in school cur¬ ricula and beyond. Indeed, it is my hope that the very notion of

INTRODUCTION IX

having to defend biological evolution and its teaching in a court

ing would be the same upon publication of On the Origin of

of law will become a thing of the past, a phenomenon seen

Species. He declared that “Mr. Darwin has given the world a new

as a historical peculiarity of twentieth- and early twenty-first-

science, and his name should, in my opinion, stand above that

century American society, symptomatic of a particularly perni¬

of every philosopher of ancient or modern times. The force of

cious brand of scientific illiteracy.

admiration can no further go!!!” (Berry 2002). Wallace received

As the late astronomer Carl Sagan pointed out in his book of

his copy of the book in early 1860, while still far off in southeast

essays Demon Haunted World, ours is an age of contradictions.

Asia. He was not to return from his eight-year adventure in the

American society in particular has been a powerhouse of scien-

Malay Archipelago for another two years, and so his adulation

tihc innovation and advance made possible by the unrivalled

marks what was almost certainly the farthest-flung, most re¬

collective brainpower of its homegrown and immigrant scien¬

mote reverberation made by the Origin when it was published.

tists and prosperous economic conditions. Yet this is true even

Now, here we are reading the book 150 years later, and the rever¬

as a significant proportion of this same society embraces mysti¬

berations continue still.

cism and the supernatural, and evinces profound scientific illit¬

Charles Robert Darwin was an unlikely revolutionary. An

eracy (see, e.g.. Gross 2006). This national cognitive dissonance

indifferent student in youth, a medical school dropout, and a

can only spell disaster if science and the understanding and in¬

sometime

novation it brings become eclipsed by ignorance, fear, supersti¬

tempered young man really just wanted to ride, hunt, and natu¬

tion, and religious zealotry. Scientific knowledge of and by itself

ralize. By his own account he had the common child’s penchant

is no panacea, to be sure. But the importance of thoughtfully

for collecting—shells, coins, rocks, and minerals—the kind of

considering and debating the implications of scientific pursuits,

passionate hobby that “leads a man to be a systematic naturalist,

and the technological innovations stemming from them, cannot

a virtuoso, or a miser,” he wrote in his autobiography. He never

be stressed enough. Science as a way of knowing is a potent way

really grew out of it. Instead, he just discovered new things to

forward for the understanding of both ourselves and the world

collect. Beetle-collecting was nearly a blood sport with Darwin

we live in, as it has been since the time of Francis Bacon. In its

and many of his fellow students at Cambridge, and after discov¬

modest way, this book seeks to help readers better understand

ering the joys of botanizing with his professor John Stevens

Darwin’s Origin of Species, and thereby better understand the

Henslow, he threw himself into learning the plants of Cam¬

scientific pursuit as a highly successful mode of inquiry into the

bridgeshire with zeal.

natural world.

theology

student,

this

mild-mannered,

good-

Despite initial doubts, he fully intended to take Holy Orders

Before analyzing the Origin itself, however, we must under¬

after finishing at Cambridge. But not before a taste of adventure:

stand where the book came from. By focusing on Darwin’s per¬

Darwin and his fellow naturalists planned a jaunt to Tenerife,

sonal intellectual odyssey and how that was incorporated into

in the Canary Islands. In preparation for this “Canary scheme,”

the Origin’s very structure, we set the stage for our own odyssey

as he put it, Darwin accompanied another Cambridge don. Rev.

through Darwin’s one long argument.

Adam Sedgwick, on a three-week geology expedition to Wales in August 1831. But the Canaries were not to be. Upon returning home from his field excursion with Sedgwick, Darwin found a

The Road to the Origin

letter awaiting him with the offer of a far grander trip—no less than a multiyear voyage around the world! He could barely con¬

Alfred Russel Wallace knew. As the only other person to have

tain his excitement.

stood, with Darwin, gazing upon the grand sweep of life’s diver¬

With the support of his Uncle Josiah, Darwin convinced his

sity with understanding, the brilliant naturalist knew that noth¬

no-nonsense father to permit him to take the journey, complete

THE ANNOTATED ORIGIN

with promises to settle down as a respectable country parson on

try from January 1836, toward the end of the voyage. In New

his return—and assurances that his rather liberal spending hab¬

South Wales, Australia, he recorded these thoughts:

its developed at Cambridge would inevitably be curbed in the confines of a small ship plying the world’s oceans. “I should be

I had been lying on a sunny bank & was reflecting on the strange

deuced clever to spend more than my allowance whilst on board

character of the Animals of this country as compared to the rest of

the Beagle,” Darwin said solicitously. “But they tell me you are

the world. An unbeliever in everything beyond his own reason,

very clever,” came the shrewd reply from his father.

might exclaim “Surely two distinct Creators must have been [at]

Darwin sailed from England in December 1831, companion-

work; their object however has been the same & certainly the end

naturalist to Captain Robert Fitzroy, Commander of HMS

in each case is complete”.— Whilst thus thinking, I observed the

Beagle. Orthodox in his religious views, gentlemanly, well

conical pitfall of a Lion-Ant:— A fly fell in & immediately disap¬

trained in several branches of natural history, amiable, earnest,

peared; then came a large but unwary Ant; his struggles to escape

curious, and, most of the time, seasick, Darwin struck out to see

being very violent, the little jets of sand described by Kirby (Vol. I

the world. Upon his return five years later his father exclaimed,

p. 425) were promptly directed against him.— His fate however

“Why, the shape of his head is quite altered.” It probably ap¬

was better than that of the poor fly’s:— Without a doubt this pre¬

peared so owing to his now-receding hairline. What was inside

dacious Larva belongs to the same genus, but to a different species

that head was altered more profoundly, but not quite in the

from the Europaean one.— Now what would the Disbeliever say

manner commonly supposed. Indeed, it is often mistakenly as¬

to this? Would any two workmen ever hit on so beautiful, so sim¬

sumed that Darwin experienced a revelation of sorts while on

ple & yet so artificial a contrivance? It cannot be thought so.—

the Beagle voyage—a sudden stroke of insight that immediately

The one hand has surely worked throughout the universe.” (Keynes

led him to abandon his belief in the immutability of species. But

2001, pp. 402^03)

there was no “eureka” moment on the voyage. How, then, did this young naturalist, so recently and earnestly able to recite

Here we see young Darwin tentatively testing the waters then

from William Raley’s Evidences of Christianity, and so fully or¬

pulling back. The ideas were gestating and would continue to do

thodox with respect to the question of species immutability and

so for more than a year as he struggled retrospectively to make

the veracity of the Bible, come to embrace the heretical notion

sense of his collections and observations. The data he cited as

of transmutation? Knowing something of Darwin’s intellectual

forming the foundation for his subsequent evolutionary think¬

odyssey is highly instructive on several levels: it expresses the es¬

ing—the relationships between extinct and living species of

sence of the scientific pursuit and underscores the very human¬

South America, and the curiosities of the Galapagos Archipelago

ity of Darwin and his contemporaries. Understanding just how

and their species relationships among those islands as well as

Darwin first glimpsed the reality of transmutation and doggedly

with mainland species—all came together for him later, back in

pursued the idea for some two decades is essential to appreciat¬

England (see Sulloway 1982a,b; 1984). Writing up his notes on

ing Wallace’s prophetic words. Darwin’s intellectual odyssey also

the final leg of the Beagle’s return journey, Darwin seemed to be

gives us insight into the Origin itself Darwin described the book

on the scent, musing,

as “one long argument,” but what is the nature of that argument? First things first.

when 1 see these Islands [Galapagos] in sight of each other and possessed of but a scanty stock of animals, tenanted by these

Darwin had a sharp eye for the natural world. We see what are

[mockingbirds] but slightly differing in structure & filling the

perhaps his earliest musings on the grand philosophical ques¬

same place in Nature, I must suspect they are only varieties. The

tions pertaining to species and centers of creation in a diary en¬

only fact of a similar kind of which I am aware is the constant as-

INTRODUCTION XI

serted difference between the wolf-like Fox of East & West Falk¬

a massive armadillo, a giant sloth, a tank-like armored sloth rel¬

land Isds.— If there is the slightest foundation for these remarks,

ative, a rodent with a skull more than two feet long, dwarfing

the Zoology of Archipelagoes will be well worth examining; for

all living rodents; and a llama so large that Darwin had tenta¬

such facts would undermine the stability of species. (Barlow

tively labeled its bones those of a mastodon! (Herbert 2005,

1963)

pp. 320-324). Something profound was happening, and Owen knew it. Darwin’s finds confirmed the “law of succession,” which

The “stability of species” was a matter that struck at the very

described the replacement of so-called types by related, though

heart of understanding creation. These remarks, penned some¬

distinct, forms through geological time. Lyell was so impressed

time between mid-June and August 1836 (Sulloway 1982b), in¬

by Owen’s conclusions that he devoted his Presidential Ad¬

clude reference to the mockingbirds (Nesomimus) of the Gala¬

dress to the Geological Society to the subject. He pointedly in¬

pagos Islands, a group of birds that exhibits three species, each

vited Darwin to hear his address, given on February 17, 1837:

endemic to one of the southernmost islands in the archipelago,

“These fossils ... establish the fact that the peculiar type of or¬

plus one species that is now regarded as a complex of six sub¬

ganization which is now characteristic of the South American

species, each also confined to one or more of the main islands.

mammalia has been developed on that continent for a long pe¬

Darwin seemed to see the significance of these birds, but he did

riod.” Darwin was impressed, but he was not yet a transmuta-

not yet realize that the finches of those equatorial islands were

tionist.

even more remarkable, nor that the tortoises told a similar story.

Even while Lyell enthused and Darwin marveled over the

Nonetheless, crucial data came to Darwin almost as a revelation

South American fossils, Darwin’s bird collection was being scru¬

when, in London and Cambridge, specialists impressed upon

tinized by the respected ornithologist John Gould at the Zoo¬

him the curious nature of his collections from South America

logical Society. Gould received the birds on January 4,1837, and

and the Galapagos.

began analyzing them soon after. He reported his results at suc¬

Winter and spring of 1837 was a watershed period in Dar¬

cessive meetings of the society: the curious Galapagos finches

win’s thinking. The Beagle arrived home on October 2, 1836,

on January 10 (finding some dozen unique Geospiza species in

and Darwin wasted no time seeing family and friends and mak¬

three subgenera); the raptorial birds on January 24 (including

ing arrangements to distribute his biological collections for

the remarkable Galapagos hawk, which he cited as a “beautiful

study. He took up residence in Cambridge in mid-December, on

intervening link” between the genus Buteo and the mainland ca-

Fitzwilliam Street, where he stayed through March. He twice

racara genus Polyborus); the distinctive mockingbirds of the Ga¬

trekked back to London during this time to attend scientific

lapagos on February 28 (three island-specific species yet so like

meetings and confer with friends and colleagues. Much hap¬

mainland ones); and the South American rheas, a large species

pened in those heady months, particularly in connection with

and a smaller relative, on March 14. Gould’s analysis appears to

his fossil discoveries from South America and the Galapagos

have been the final catalyst precipitating Darwin’s conversion to

avifauna.

transmutation. Darwin met with Gould between March 7 and

Darwin delivered his fossil mammal specimens to Richard

12, 1837 (Sulloway 1982b), and was given a summary of the or¬

Owen in late December or early January, and within a month

nithologist’s findings: more than two-thirds of Darwin’s Gala¬

the great anatomist knew that they were remarkable. Owen said

pagos birds were new species! His finds on the continent were

as much in a letter to Charles Lyell dated January 23, in which

no less interesting. Gould named the small or “petisse” rhea in

he revealed that Darwin had discovered no fewer than five

Darwin’s honor (though the name does not stand today, as dis¬

extinct relatives of mammals found in South America today—

cussed on p. 349). Darwin found it curious that the common

and moreover, that these extinct species were gigantic forms:

large rhea should be geographically replaced by its diminu-

THE ANNOTATED ORIGIN

xii

tive cousin in the south of the continent with no obvious geo¬

but little from that of a warbler.” Darwin is not tipping his hand;

graphical feature demarking the boundary. Why two rheas

he offers only this suggestive comment:

where surely one might suffice? Darwin’s first evolutionary entry in the then-recently re¬ opened notebook, now called the Red notebook, considers just this point: “Speculate on neutral ground of 2. Ostriches; bigger one encroaches on smaller.—change not progressif: produced at one blow . . . Yet new creation affected by Halo of neighboring continent” (Barrett et al. 1987, p. 61). Darwin was increasingly convinced that species change, but how? As this and other early passages suggest, he first toyed with the idea that transmuta¬

I will not here attempt to come to any definite conclusions, as the species have not been accurately examined; but we may infer, that, with the exception of a few wanderers, the organic beings found on this archipelago are peculiar to it; and yet that their general form strongly partakes of an American character . . . The circum¬ stances would be explained, according to the views of some au¬ thors, by saying that the creative power had acted according to the same law over a wide area. (Darwin 1839, p. 474)

tion occurs quickly, perhaps instantaneously. Two pages later he

Darwin soon opened another notebook, in July 1837, dedi¬

muses again about what secret these ostriches hold, and he re¬

cated to transmutation. This is the notebook he was referring to

lates their juxtaposition in space to the temporal juxtaposition

when he wrote in his diary: “In July opened first note Book on

of extinct and living mammals of South America. “Not gradual

‘transmutation of Species’—Had been greatly struck from about

change or degeneration,” he writes on p. 130. “If one species

month of previous March—on character of S. American fos¬

does change into another it must be per saltum [fast, by sudden

sils—& species on Galapagos Archipelago. These facts origin (especially latter) of all my views.” Dubbed the B notebook (af¬

transitions]” (Barrett et al. 1987, p. 63). The important point about Gould’s analysis was not his taxo¬

ter the A notebook on geological matters), running through

nomic assessments per se but what they meant for an under¬

February 1839, it reflects a mind expanding at warp speed—a

standing of species and varieties. As Darwin later wrote in the

riot of ideas, questions, suggestions for experiments, and obser¬

Zoology of the Beagle voyage (1841, pp. 63-64), referring to the

vations. That summer we find him grappling with the rate of

mockingbirds: “I may observe, that as some naturalists may be

transmutation; he had yet to see change in gradualistic terms.

inclined to attribute these differences [among island forms of

We also see him pondering islands, his remarkable Galapagos

the mockingbirds] to local varieties; that if birds so different as

finds fresh in his mind: “According to this view animals, on sep¬

O. trifasciatus and O. parvulus, can be considered as varieties of

arate islands, ought to become different if kept long enough ...

one species, then the experience of all the best ornithologists

Now Galapagos Tortoises, Mocking birds” (Barrett et al. 1987,

must be given up, and whole genera must be blended into one

p. 172). Darwin explored evolutionary ideas and their ramifications

species.” In that book and in the Journal of Researches (1839), Darwin recounted Gould’s remarkable assessment: “In my col¬

over the next months and years. There were doubts, false starts,

lections from these islands, Mr. Gould considers that there are

and blind alleys. He soon gave up the notion of sudden (salta-

twenty-six different species of land birds. With the exception of

tional) evolution, but what was driving gradual change? How

one, all probably are undescribed kinds, which inhabit this ar¬

necessary was isolation? What was the relationship between

chipelago, and no other part of the world.” He goes on to discuss

geological change and species change? Were the formation and

each group, noting, for example, that the finches are the most

extinction of species somehow tied together, with more or less

it is very re¬

constrained lifespans? The essential elements of his theory as ul¬

markable that a nearly perfect gradation of structure in this one

timately published came together over a period of time, some

singular of any [birds] in the archipelago,” that

group can be traced in the form of the beak, from one exceeding

much later than others. In the B notebook (Barrett et al. 1987,

in dimensions that of the largest gros-beak, to another differing

pp. 177,180) we find the first expressions of a genealogical view

INTRODUCTION xiii

of species: first a tentative branching “coral of life,” then, a few

on side projects relevant to it. The Essay and the Sketch were

pages later, a bold ramifying tree-like sketch with species or spe¬

later published by his son Francis as The Foundations of the Ori¬

cies groups branching from a common trunk; this beautiful dia¬

gin of Species (1909).

gram is prefaced with the words “1 think”—a vision that clearly

Why Darwin did not publish his theory as early as the mid-

set Darwin’s conception of transmutation apart from all earlier

1840s has long been a matter of speculation. The time between

attempts.

more or less complete formulation of the theory and its pub¬

In October 1838 Darwin gained insight from re-reading the

lication is sometimes called the “delay years,” but this period

Rev. Thomas Malthus’s Essay on Population. Malthus taught him

probably did not represent a delay at all. Darwin was determined

that the continual “struggle for existence” would lead to differ¬

to make a thoroughly researched and documented case for his

ential survival, and that those individuals with constitutions

theory, knowing full well the scientific world’s extreme skepti¬

best suited to the demands of the struggle would be the most

cism to transmutationist ideas. The very year his Essay was

likely to make it through. Malthus was writing about people, but

penned also saw the pillorying of the then-anonymous au¬

the applicability of his ideas to the natural world immediately

thor of the sensational book Vestiges of the Natural History of

struck Darwin, giving him “a theory by which to work,” as he

Creation, a long transmutationist reverie that embraced an evo¬

wrote in his diary. That same year Darwin apparently realized

lutionary vision of the universe, humanity, and everything in

that domesticated varieties provide a case study, a natural ex¬

between (Secord 2000). Darwin was no metaphysical transmu¬

periment on species and varieties. What is the extent of variabil¬

tationist; he aimed to present a scientific case so well argued and

ity? How are variants propagated and not lost by crossing? His

supported that it would at least have to be given a fair hearing.

reading of the great agricultural breeders of his day—among

Books like Vestiges and even his own grandfather’s Zoonomia,

them Robert Baker, John Sebright (who used the term “selec¬

with its unique brand of fanciful transmutationist speculations,

tion” in describing agricultural breeding), and William Youatt—

were criticized by Darwin and his circle for their unsubstanti¬

underscored how methodical “picking and choosing” gradually

ated notions of transmutation, but owing to their popularity

bends plant and animal breeds to human whims. Darwin coined

with a wide audience, these books paved the way for the broader

the phrase “natural selection” to contrast with this process; the

acceptance of the later idea of organic change. At the time,

earliest appearance of the phrase is in a marginal note he made

though, with transmutationism represented by speculative phi¬

in Youatt’s book on horses in March 1840 (Evans 1984).

losophers on the one hand and the more sober but perhaps

The essential elements of the theory were worked out by 1842,

equally unsubstantiated theories of the brilliant French natural¬

when Darwin sat down to write a “pencil sketch.” Here we see

ist Jean-Baptiste Lamarck on the other, the elite English natural

the domestication analogy, the mechanism of natural selection

philosophers of the mid-nineteenth century would have noth¬

based upon the postulates of heritable variation and struggle for

ing to do with such radical ideas.

existence, and comments on the wide applicability of the theory

During this time, Darwin tentatively reached out to a few

in explaining patterns in diverse departments of natural history.

friends and other correspondents with a view to sharing his

The Sketch, as it is now called, was greatly expanded in 1844 to

ideas. He tested the waters for a certain openness of mind to cast

an Essay of some 230 pages. Darwin sealed a copy in an envelope

an unprejudiced eye on his theory. Lyell passed the test. So did

with a letter to his wife, Emma, asking that she have it published

the botanists Asa Gray and Joseph Dalton Hooker. These natu¬

immediately in the event of his sudden death. He knew that the

ralists became friendly critics and sounding boards as well as

naturalists of the world would wish to know about this theory.

sources of invaluable experience and information as Darwin

But of course Darwin did not die prematurely; despite chronic

continued to develop his theory and struggle with its difficulties.

ill health he continued steadily to work on the theory as well as

In the mid-1840s Hooker commented to Darwin that anyone

THE ANNOTATED ORIGIN

XIV

making pronouncements about the nature of species and varie¬

piece of the puzzle that explained not just transmutation but di¬

ties ought to become expert in some group to show he really

versification (Browne 1980). Darwin later wrote that he always

knew what he was talking about—pay his dues, so to speak.

remembered exactly where he was, riding in his carriage, when

Darwin took this advice to heart and initiated a monographic

this insight came.

study of barnacles, a group rich in species and varieties, and

Into the 1850s there were endless difficulties to work out and

conveniently amenable to analysis in the quiet of his study at

applications of his theory to explore: the nature of sterility and

Down House, Kent, where he had moved his family. Eight years

interfertility, long-distance dispersal, patterns of species appear¬

and four monographs later (the last was published in 1854),

ance and extinction in the fossil record, instinct and its relation¬

Darwin could indeed claim intimate knowledge of the nature of

ship to habit, and on and on. He experimented and collected

species and varieties as manifest in this diverse group, yet the

field data whenever he could, exploring question after question:

barnacles taught him much more besides: he was struck by the

How long could seeds remain viable in sea water? Could aquatic

extreme sexual dimorphism of some species (with males so di¬

invertebrates be transported on the feet of waterfowl? What di¬

minutive they were first thought to be tiny parasites adhering to

versity of plant species can be found in a plot of turf of a given

the female), and the varied modifications of anatomical struc¬

size? What is the rate of seedling mortality? Can simpler ver¬

tures.

sions of even the most complex organs and behaviors be found?

By the 1850s Darwin had amassed a vast amount of informa¬

Darwin took up pigeon breeding in the mid-1850s, using the

tion. He had long since become something of a homebody, fre¬

range of morphological and behavioral variation in this domes¬

quently indisposed by illness but patriarch of a happy and bus¬

ticated group to probe the limits of variability, developmental

tling household. He was far from isolated in his reluctance to

expression of traits, and of course to better understand the pi¬

travel, though; despite his bouts of illness the Darwins enter¬

geon breeder’s art and the power of cumulative selection. He

tained frequently, and he often wrote several letters a day to

was interested in plants, too, initiating in 1854 a study of the

friends, family, scientific correspondents, and others. The sig¬

numbers of species in large vs. small plant genera, a project that

nificance of these letters cannot be overstated: Darwin was at

mushroomed into an exhaustive analysis of the ratio of species

the center of a correspondence maelstrom, an almost frenetic

to varieties in large and small genera based on no fewer than a

give-and-take of information, specimens, books, and critiques

dozen botanical manuals of flora around the world (Stauffer

of his ideas with hundreds of correspondents. Cambridge Uni¬

1977, Browne 1980).

versity Press has made these available in a magisterial set of an¬

During the intervening years, Charles Lyell and Joseph

notated volumes, and more recently has put them online via the

Hooker had become Darwin’s closest friends and confidants.

Darwin Correspondence Project {www.darwinproject.ac.uk/).

Lyell himself was keenly interested in what was called the “spe¬

Darwin thrived in the tranquility of Down House, but the

cies question.” Wallace’s 1855 paper “On the law which has regu¬

family was not insulated from the maladies of the times. Trag¬

lated the introduction of new species,” written from his base in

edy struck in 1851; the loss of the Darwins’ ten-year-old daugh¬

Sarawak, Borneo, prompted Lyell to do two things: initiate a

ter, Annie, to illness seemed to have taken an especially heavy

species notebook of his own (see Wilson 1970); and urge Dar¬

toll on him, and some authors have suggested that this event ex¬

win not to delay publishing his own ideas on species change. Ly¬

tinguished whatever slim remnant of a belief in a personal god

ell recognized the significance of Wallace’s keen insights under¬

Darwin may have retained. Bitter with the loss, he wrote a lov¬

pinning his Sarawak Law: “Every species has come into existence

ing memorial to her and buried himself in his work. Still deep in

coincident both in time and space with a pre-existing closely al¬

his barnacle project at the time, Darwin had a flash of insight in

lied species,” Wallace wrote, something that “connects together

1852, when his “principle of divergence” came to him

and renders intelligible a vast number of independent and hith-

a critical

NTRODUCTION XV

erto unexplained facts.” This statement veritably screams trans¬

word count of about 375,000. It pained Darwin to have to cut it

mutation to a modern reader with the benefit of hindsight, but

down, in part because an abstract deviated from standard pre¬

Darwin was unfazed. The law did not mention natural selection

sentation in scientific books: “My work is now nearly finished,”

nor expound a principle of divergence. Without a mechanism,

he wrote in the Introduction to the Origin, “but as it will take

Darwin felt that Wallace had little to nothing to share. But he

me two or three more years to complete it, and as my health is

had seriously underestimated Wallace’s curiosity and creativity.

far from strong, I have been urged to publish this Abstract.” He

Darwin was deep into his species book (initiated at last in

continues: “I can here give only the general conclusions at which

1856 at Lyell’s urging) when in June 1858 he opened a fateful

1 have arrived, with a few facts in illustration, but which, I hope,

package from Wallace with a letter and a manuscript entitled

in most cases will suffice. No one can feel more sensible than I

“On the tendency of varieties to depart indefinitely from the pa¬

do of the necessity of hereafter publishing in detail all the facts,

rental type.” Posted from the tropical southeast Asian island Ter-

with references, on which my conclusions have been grounded;

nate in the Moluccas some months before, the manuscript had

and I hope in a future work to do this.”

slowly wended its way to Darwin’s study, where the previously

Darwin wanted to make it abundantly clear to readers that he

unfazed Darwin was very fazed indeed. “Your words have come

was presenting a mere abstract, and in fact his first title for the

true with a vengeance that I [should] be forestalled,” he wrote

work was An Abstract of an Essay on the Origin of Species and

Lyell in a restrained but anguished letter. “So all my originality,

Varieties Through Natural Selection. John Murray, his publisher,

whatever it may amount to, will be smashed.” These events and

voiced concerns to Lyell about this reluctant-sounding title.

the subsequent “delicate arrangement” that Lyell and Hooker

Darwin acquiesced: “I am sorry about Murray objecting to term

orchestrated—a joint presentation to the Linnean Society of

abstract as I look at it as only possible apology for not giving

some of Darwin’s priority-establishing writings together with

References 8c facts in full.—but I will defer to him 8c you” {Cor¬

Wallace’s Ternate essay—all unfolded while Wallace remained in

respondence 7: 272). The manuscript was completed in nine

blissful (or tortured?) ignorance in the field. Their papers were

months. One year and four months later, this “abstract” of 490

read on July 1, 1858; Wallace received word much later and was

pages finally appeared, bearing the full title On the Origin of Spe¬

elated to find himself held in high esteem by the eminent scien¬

cies by Means of Natural Selection, or the Preservation of Favoured

tific men of London (Berry 2002).

Races in the Struggle for Life. Darwin was more anxious than an

Darwin’s work was cut out for him. He could not possibly

expectant father, and he dreaded the coming attention and no¬

quickly complete the encyclopedic work over which he had been

toriety. As Janet Browne so compellingly describes in The Power

laboring, so he decided to abstract much of what he had written

of Place (2002), he had retreated to a hydropathic spa in York¬

already and treat the remaining topics in an abbreviated man¬

shire, as much to escape the public eye as to seek treatment for

ner. This was no simple task; by the time Wallace’s package ar¬

his frazzled nerves, skin rashes, and churning stomach. He fired

rived Darwin had written a hefty manuscript of ten and a half

off letter after disarming letter to friends, colleagues, and would-

chapters, a proper scientific treatise brimming with example af¬

be critics, hoping to soften the blow, blunt their resistance: “I

ter example supporting his arguments, backed by tables of data

fear ... that you will not approve of your pupil... If you are in

and numerous citations. In his Introduction to the draft “big

ever so slight a degree staggered (which I hardly expect) on the

species book,” published as Charles Darwin’s Natural Selection,

immutability of species, then I am convinced with further re¬

Stauffer (1975) estimated that, on the basis of the word count of

flexion you will become more 8c more staggered, for this has

the surviving manuscript of eight and a half chapters (some of

been the process through which my mind has gone,” he wrote to

the original was later cannibalized for Variation of Animals and

his old Cambridge mentor Henslow. “I know there will be much

Plants Under Domestication), the final book would have had a

in it, which you object to ... I am very far from expecting to

THE ANNOTATED ORIGIN

XVI

convert you to many of my heresies,” he told Thomas Henry

philosophical work Preliminary Discourse on the Study of Natu¬

Huxley in another letter; “my book will horrify & disgust you,”

ral Philosophy was published in 1831, the same year he was

he fretted to Thomas Eyton. His grimmest expression may have

knighted. (Coincidentally, that was also the year the Beagle sailed

been sent to Hugh Falconer: “Lord how savage you will be . . .

from England with the eager young Darwin aboard; Darwin

and how you will long to crucify me alive! I fear it will produce

met Herschel while on the voyage, far from home in South Af¬

no other effect on you” (Correspondence?: 350,368,370; Browne

rica in 1836.)

2002, p. 84). Darwin steeled himself and resolutely stood by his theory.

In Herschel’s philosophy of science, the case for a vera causa is best made if we can (1) identify the existence of a mechanism,

A letter written to his American confidant Asa Gray nicely ex¬

(2) demonstrate (or persuasively argue for) its adequacy or com¬

presses this stand, as well as his look to the future: “I fully admit

petence in effecting the phenomenon of interest, and (3) show

there are very many difficulties ... but I cannot possibly believe

that this mechanism has wide explanatory power—responsibility

that a false theory would explain so many classes of facts as

for diverse observations. Herschelian responsibility is akin to

I think it certainly does explain.— On these grounds I drop

the vera causa of another prominent philosopher of science in

my anchor, & believe that the difficulties will slowly disappear”

Darwin’s day: William Whewell’s consilience of inductions, a

{Correspondence 7: 369). A copy of the book meanwhile was

“jumping together” of inductive inferences from diverse and

coursing its way to southeast Asia, arriving at Amboina, in the

unrelated fields (Ruse 1979). According to the philosophical

Moluccas, in February 1860. “God knows what the public will

precepts of the day, demonstration of responsibility should be

think,” Darwin said in an accompanying letter. The public was

independent of the cases for existence and adequacy of the

in for a transmutation of its own. Though few realized it at the

mechanism (Hodge 1977,1992; Hull 2003; Waters 2003). These

time, their world would never be the same. Alfred Russel Wallace knew.

ideas provide a road map to the Origin. Darwin opens the book with a chapter on domestication, for viewing domestic varieties as an analog to natural varieties helps argue the existence and adequacy of cases for transmutation by natural selection. Dar¬

The Origin Revealed

win sees selection as the causal agent of change both under do¬

At first glance the Origins narrative structure can be confusing

dogs, say, are genealogically related, developed by the cumula¬

—a seeming hodgepodge of topics. How can a single argument-

tive effects of selective breeding (artificial selection) in a way

cum-narrative unite the entire book? Historians and philoso¬

that parallels the action of natural selection in promoting the

phers of science see consistency in the way Darwin presented his

formation of varieties (and ultimately species) in nature.

mestication and in nature: varieties of domesticated species like

case, beginning with the 1842 Sketch, continuing through the

While chapter I presents the domestication analogy, chapters

1844 Essay, Natural Selection, and the Origin itself (Hodge 1977,

II-V set forth the logical argument for natural selection based

Ruse 1979, Waters 2003). There is a consensus that Darwin

on naturally occurring, heritable variation and differential sur¬

largely followed Herschelian logic in his scientific reasoning, the

vival and reproduction linked to this variation. These complete

tenets for establishing verae causae, true causes, put forth by the

the existence and adequacy cases. Much of the rest of the book is

eminent astronomer and mathematician Sir John Herschel.

aimed at addressing potential problems (chapters VI-VIII) and

Herschel was something of a polymath who made contributions

showing the wide applicability of the theory in explaining sets

to astronomy, light theory, and the development of photogra¬

of observations from paleontology to biogeography to embryol¬

phy. He was conversant in geology, zoology, and botany as well,

ogy (chapters IX-XIII), corresponding to the Herschelian re¬

and achieved renown as a philosopher of science. His important

sponsibility case.

INTRODUCTION XVII

The Origin is not the first place Darwin pursued this logical

ment and clearly reflects how Darwin thought about his the¬

structure. Scholars are essentially agreed that he did so from the

ory from the earliest days. It is supported by an early outline,

earliest formulations of his theory. If we read Darwin’s Sketch of

thought to be written about 1842, found later by his son Francis

1842, Essay of 1844, and the Origin with an eye toward discern¬

(see F. Darwin 1909, p. xviii):

ing the layout and presentation of these ideas, a general bipartite structure is evident in all these works. Francis Darwin divided his edition of the Sketch into Parts 1 and II, following his father’s mention of two divisions; the domestication analogy is immedi¬ ately followed by a case for natural variability, struggle, and nat¬ ural selection. The remainder is largely devoted to applying the idea of transmutation mediated by natural selection to diverse

I. The Principles of Var. in domestic organisms. II. The possible and probable application of these same principles to wild animals and consequently the possible and probable pro¬ duction of wild races, analogous to the domestic ones of plants and animals. III. The reasons for and against believing that such races really have been produced, forming what are called species.

observations. This structure parallels the two overarching argu¬ ments Darwin makes: natural selection as agent of change and the reality of transmutation in general.

This outline may have been intended as the layout for the 1842 Sketch. Comparing it with the three-part structure Darwin

If we delve a bit deeper, it becomes clear that a tripartite struc¬

described in Variation some twenty-five years later, it is clear

ture reflects even better the way Darwin conceptualized his ar¬

that the basic logical structure of his argument did not change

gument. He describes a three-part layout in the Introduction to

appreciably over time. In some respects it had to change, of

his 1868 book Variation of Animals and Plants Under Domestica¬

course, as Darwin’s understanding of the process became more

tion. Significantly, that work was intended as the first of three

nuanced. What is remarkable, though, is the clarity of Darwin’s

books Darwin planned post-Origin, each of which represents a

earliest insight into the evolutionary process, and the fact that

component of the logical argument. To be clear: having been

the conceptual structure he formulated soon after his return

forestalled in completing his big species book and publishing

from the Beagle voyage has stood the test of time.

instead an abstracted form of this work as the Origin, Darwin

It is unfortunate, though, that Darwin did not expressly stick

was determined to publish more lengthy and thoroughly docu¬

to this three-part scheme in all major presentations of his work.

mented versions of his views. He decided this would best be

He had some tough decisions to make, in particular where and

done by presenting each of the three components of his concep¬

how to address difficulties. From early on (1842) Darwin antici¬

tual argument as a book in its own right. Accordingly, the first

pated and offered arguments to defuse certain problems that he

two chapters of the big species book were expanded into the two

knew would loom large in the minds of his critics. He decided to

Variation volumes. In the Introduction to Variation he states

take these on immediately following the case for natural selec¬

that in a second work he will expand his case for variation in

tion and before moving on to the chapters on applications of his

nature, the struggle for existence, natural selection, and other

theory. In the Origin, these “difficulties” treatments correspond

difficulties. “In a third work,” he continues, “I shall try the prin¬

to chapters VI-VIII.

ciple of natural selection by seeing how far it will give a fair ex¬

Most authors take Darwin at his word when, in the opening

planation of the several classes of facts just alluded to”: the re¬

paragraph of chapter VI (“Difficulties on Theory”), he identifies

sponsibility case (Darwin 1868b, pp. 8,9). Darwin never realized

four issues he considered the most important explanatory ob¬

his plan for the two additional works—other books seemed to

stacles faced by his theory and states that he will address the first

get in the way—so we have no such concise trio reflecting the

two in that chapter and the next two in the subsequent chapters,

way he conceptualized his theory and its presentation. Nonethe¬

VII (“Instinct”) and VIII (“Hybridism”), respectively. In fact he

less, the three-part structure is a useful map to Darwin’s argu¬

goes beyond this. Chapter IX, “On the Imperfection of the Geo-

THE ANNOTATED

ORIGIN

xviii

logical Record,” is a treatment of yet another set of difficulties, albeit not those associated with the efficacy of natural selection. Chapter X on patterns of the fossil record (“On the Geological Succession of Organic Beings”) can be seen as paired with IX, presenting the geological evidence favoring his theory to bal¬ ance IX’s geological evidence apparently contradicting his the¬ ory. I believe this is evident in the twO biogeographical chapters as well, though Darwin is not explicit on this point. Chapter XI can be seen as addressing a difficulty in the geographical distri¬ bution of species; namely, by what process did they become dis¬ tributed in different areas across the globe if not created there? Darwin then posits mechanisms facilitating the migration of species, all turning on such means of dispersal as wind, water, and animals, and the geological and climatic cycles that open up migration corridors. In chapter XII he shifts gears, more explic¬ itly presenting his case for empirical observations of species dis¬ tributions that support transmutation; for example, relation¬ ships of species on the same continent both with each other and with extinct forms on that continent, the fact that geography and not habitat or climate best predicts species relationships, and the striking “worlds within worlds” pattern that oceanic ar¬ chipelagos manifest with their unique species that show affini¬ ties with species of the nearest mainland. Comparing these with the other difficulties chapters, we can discern a consistency of approach; present the problem, then of¬ fer solutions. We see this in the treatments of lack of transitional forms and organs of extreme perfection (chapter VI), highly complex instincts and the special conundrum posed by sterile insect castes (chapter VII), and the problem of hybrid sterility and interfertility (chapter VIII). In each case Darwin’s approach is to show that the difficulty is really a non-issue on closer in¬ spection, or can be satisfactorily explained. In this light, chap¬ ters VI-XII fit into the general scheme. Admittedly, however, my scenario of Darwin’s alternating difficulties with solutions seems to break down in chapter XIII, which consists almost entirely of positive arguments in favor of his theory. But this is unsurprising; Darwin saved his most potent arguments and ob¬ servations for last, and this entire chapter has a triumphal mo¬ mentum to it. Indeed, it is no coincidence that he ends his case

with embryology and morphology, fields whose facts alone, he declares, would convince him of the correctness of his theory “even if it were unsupported by other facts or arguments” (Ori¬ gin, p. 458). Now that’s confidence. Regardless of precisely how the Origin is philosophically dis¬ sected, Darwin’s portrayal of the book as “one long argument” is certainly borne out. Whether seen as a unitary work or a work with bi- or tripartite arguments (and it is all of these), it stands as a masterly narrative, bold in its vision and ringing with pas¬ sion. Marcel Proust is credited with saying that “the real voyage of discovery consists not in seeking new landscapes but in having new eyes.” Darwin managed both. His words in the concluding paragraphs of the Origin proved prophetic; “I look with confi¬ dence to the future, to young and rising naturalists,” a new gen¬ eration freer of the prejudices of education (!) and preconceived notions. His following did in fact increase with each generation, albeit often with an irritating (to Darwin) tendency to down¬ play natural selection or to invoke ill-defined metaphysical or supernatural notions like “tendency to complexity” or species senescence. Darwin’s commitment to gradualistic transmuta¬ tion mediated primarily, though not exclusively, by natural se¬ lection remained firm to the end. Time and again he would ap¬ peal to natural law in explaining species birth, death, and of course transmutation. We see this philosophy in the epigraphs Darwin chose to stand opposite his title page; quotes from Whewell and Bacon speaking, respectively, to the Divine acting through natural law, and to the wisdom of reading the works of the creator—the natural world—as attentively as the words of the creator. By the close of Darwin’s century, the concept of evolution by natural selection was inextricably tied to his name. According to the Oxford English Dictionary the word “Darwinism” first ap¬ peared in 1856, but in a novel, and in reference to Charles’s re¬ markable grandfather Erasmus. In 1864 Huxley used the word to describe a new philosophy, that of Charles himself, and it has been linked to evolution by natural selection ever since. We honor the sagacious Alfred Russel Wallace for his own profound

INTRODUCTION XIX

contributions to the field, especially his fully independent for¬ mulation of the theory, but the fact that Wallace chose to entitle his own definitive statement on the subject Darwinism (1889) shows that even he lionized his retiring colleague for his insights (and, by the way, gives lie to the persistent conspiracy theories that he was given short shrift by Darwin and his friends). A de¬ cade earlier Gray had set the precedent for eponymous titles honoring Darwin, titling his evolutionary book Darwiniana: Es¬ says and Reviews Pertaining to Darwinism (1876). The Origin of Species is epochal. Darwin’s identification of a naturalistic explanation for species origins—for our origin— does not mean that his ideas are inherently atheistic, as legions of spiritually minded biologists from Gray on can attest. Yet make no mistake: his ideas are fundamentally incompatible with any literal reading of biblical scripture, and indeed with the cre¬ ation narratives of any religion. Nowadays, biblical literalists (particularly but not exclusively in the United States) constitute a vocal and politically active minority, but the rest of us must be vigilant lest young-earth creationists and the neo-creationist “intelligent design” propagandists manage to legislate their way into science classrooms. Society should not countenance ped¬ dling such ideas under the guise of science—a common tactic recently and decisively repudiated by U.S. District Judge John E. Jones III in the Dover, Pennsylvania, intelligent design case. In the grand journey of self-discovery that started with com¬ prehension of the nature of our planet and star in the seven¬ teenth century, Darwin’s Origin of Species is one of humanity’s crowning achievements. This living document opened windows on grand vistas extending back in Deep Time and sweeping for¬ ward to the uncertain future of the ever-ramifying tree of life— a profound step in self-awareness and self-understanding for a remarkable little primate. 1 share Darwin’s exultation that “there is grandeur in this view of life.” Carl Sagan once described hu¬ mans as “star stuff come alive,” the very universe aware of itself. That awareness underwent a quantum leap on November 24, 1859.

Galapagos Islands and Quito, Ecuador March 2008

THE ANNOTATED ORIGIN

XX

The Annotated Origin

p

V

♦ S3|

.)

■■■

X■

X

s

— VI

v -IV

.nr n

G

H

I

/

K

/

/

L

118 1

In general, this is a “success breeds success” model—wide¬

spread and varying forms will give rise to widespread and vary¬ ing forms—that modern biologists would agree with only in the general sense that large, widespread species may exhibit more variation than smaller, more restricted species by virtue of hav¬ ing more individuals that can vary.

THE ANNOTATED ORIGIN

NATURAL SELECTION,

Chap. IV.

and the tendency to variability is in itself hereditary, consequently they will tend to vary, and generally to vary in nearly the same manner as their parents varied. Moreover, these two varieties, being only slightly modified forms, will tend to inherit those ad¬ vantages which made their common parent (A) more numerous than most of the other inhabitants of the same country ; they will likewise partake of those more general advantages which made the genus to which the parent-species belonged, a large genus in its omti country. And these circumstances we know to be favourable to the production of new varieties. If, then, these two varieties be variable, the most divergent of their variations will generally be preserved during the next thousand generations. And after this interval, variety a’ is supposed in the diagram to have produced variety a^, which will, owing to the principle of divergence, differ more from (A) than did variety a'. Variety m} is supposed to have produced two varieties, namely and differing from each other, and more considerably from their common parent (A). We may continue the process by similar steps for any length of time; some of the varieties, after each thousand generations, producing only a single variety, but in a more and more modified condition, some producing two or three varieties, and some failing to produce any. ► Thus the varieties or modified descendants, proceeding from the common parent (A), will generally go on in¬ creasing in number and diverging in character. In the diagram the process is represented up to the tenthousandth generation, and under a condensed and sim¬ plified foiTQ up to the fourteen-thousandth generation. But I must here remark that I do not suppose that the process ever goes on so regularly as is represented in the diagram, though in itself made somewhat irregular.

Chap. IV.

DIVERGENCE OF CHARACTER.

119

I am far from thinking that the most divergent varieties will invariably prevail and multiply: a medium form may often long endure, and may or may not produce more than one modified descendant; for natural selec¬ tion will always act according to the nature of the places which are either unoccupied or not perfectly occupied by other beings; and this will depend on in¬ finitely complex relations. But as a general rule, the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will be increased. In our diagram the line of succession is broken at regular intervals by small num¬ bered letters marking the successive forms which have become sufficiently distinct to be recorded as varieties. But these breaks are imaginary, and might have been inserted anywhere, after intervals long enough to have allowed the accumulation of a considerable amount of divergent variation. As all the modified descendants from a common and widely-diffused species, belonging to a large genus, will tend to partake of the same advantages which made their parent successful in life, they will generally go on multiplying in number as well as diverging in character: this is represented in the diagram by the several divergent branches proceeding from (A). The modified offspring from the later and more highly im¬ proved branches in the lines of descent, will, it is pro¬ bable, often take the place of, and so destroy, the earlier and less improved branches : this is represented in the diagram by some of the lower branches not reach¬ ing to the upper horizontal lines. In some cases I do not doubt that the process of modification wdl be con¬ fined to a single line of descent, and the number of the descendants will not be increased; although the amount

1 An important point to keep in mind: this is not simply a

model in which offspring forms supplant parental forms. If it were, the pattern of transmutation could be linear, with no branching tree necessarily following. Darwin’s divergence of character principle does entail branching and spreading. Diver¬ gence increases competitive ability against not only parental forms but also other competing forms. Outcompeted forms are supplanted, their place in the economy of nature taken over and occupied. 2 Recall that species A at the bottom left of the diagram is a

“common, widely diffused, and varying species, belonging to a genus large in its own country” (p. 117). Implicitly, B, C, and D are not. These are driven to extinction, literally overtopped by the burgeoning tree that is A’s descendants. Recalling that hori¬ zontal position indicates a certain niche in the economy of na¬ ture, we can see that the positions of B-D become co-opted by the group in lineage m; a perpendicular line dropped from m'-rrf would intersect the bottom horizontal line more or less at their positions. Extinction, then, is a central feature of Dar¬ win’s model.

THE ANNOTATED ORIGIN

120 1

Darwin is here making a distinction between what we now

call anagensis, or evolution within a lineage over time, and dadogenesis, evolution by lineage splitting. The English racehorse or pointer might be examples of anagenetic change, for direc¬ tional selection has “pushed” these organisms steadily further from the parental or ancestral form. This change is indicated in the diagram by the angle at which the line from A to

pro¬

ceeds from the vertical; that is, the lineage is moving away from the form of A. Darwin is careful to build in situations where lit¬ tle or no transmutation is seen in even ancient lineages (pre¬ sumably because they are so well adapted to their environment). Lineage E is an example; this might represent so-called living fossils like coelacanths or horseshoe crabs. 2 It is worth noting that because transmutation and divergence are a slow and continuous process, the question of whether any two forms descended from a common ancestor are considered well-marked varieties or actual species is something of a judg¬ ment call. Competent naturalists can disagree on how much or what kinds of difference would be sufficient to consider a given set of forms species or varieties (see the earlier “doubtful forms” discussion on pp. 16-17). This ambiguity holds true for higher taxonomic levels as well: how much difference is enough to put two species in different genera? Regardless, with continual di¬ vergence, groups of descendant species united by common an¬ cestry and distinct from other such groups naturally form— these become arranged into genera, families, orders, classes, etc.: the entire Linnaean hierarchy. This process is explained fully on p. 123.

NATURAL SELECTION.

Chap. IV.

of divergent modification may have been increased in the (► successive generations. This case would be represented in the diagram, if all the lines proceeding from (A) were removed, excepting that from a' to a’“. In the same way, for instance, the English race-horse and English pointer have apparently both gone on slowly diverging in character from their original stocks, with¬ out either having given off any fresh branches or races. After ten thousand generations, species (A) is supposed to have produced three forms, a"’,/'®, and mwhich, from having diverged in character during the successive generations, will have come to differ largely, but perhaps unequally, from each other and from their common parent. If we suppose the amount of change between each horizontal line in our diagram to be excessively small, these three forms may still be only well-marked varieties; or they may have arrived at the doubtful ^ category of sub-species ; but we have only to suppose the steps in the process of modification to be more numerous or greater in amount, to convert these three forms into weU-defined species: thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species. By continuing the same process for a greater number of generations (as shown in the diagram in a condensed and simplified manner), we get eight species, marked by the letters between a '■* and m ’■*, all descended from (A). Thus, as I believe, species are multiplied and genera are formed. In a large genus it is probable that more than one species would vary. In the diagram I have assumed that a second species (I) has produced, by analogous steps, after ten thousand generations, either two wellmarked varieties and 2'°) or two species, according to the amount of change supposed to be represented be-

THE ANNOTATED ORIGIN

Chap. IV.

DIVERGENCE OF CHARACTER.

121

tween the horizontal lines. After fourteen thousand generations, six new species, marked by the letters to z are supposed to have been produced. In each genus, the species, which are already extremely dif¬ ferent in character, will generally tend to produce the greatest number of modified descendants ; for these wiU have the best chance of filling new and widely different places in the polity of nature: hence in the diagram I have chosen the extreme species (A), and the nearly extreme species (I), as those which have largely varied, and have given rise to new varieties and species. The other nine species (marked by capital letters) of our original genus, may for a long period continue trans¬ mitting unaltered descendants; and this is shown in the diagram by the dotted lines not prolonged far up¬ wards from want of space. But during the process of modification, represented ^ in the diagram, another of our principles, namely that of extinction, will have played an important part. As in each fuUy stocked country natural selection necessarily acts by the selected form having some advantage in the struggle for life over other forms, there wiU be a con¬ stant tendency in the improved descendants of any one species to supplant and exterminate in each stage of descent their predecessors and their original parent. For it should be remembered that the competition will generally be most severe between those forms which are most nearly related to each other in habits, constitu¬ tion, and structure. Hence all the intermediate forms between the earlier and later states, that is between the less and more improved state of a species, as weU as the original parent-species itself, wUl generaUy tend to become extinct. So it probably will be with many whole collateral lines of descent, which wUl be conquered by later and improved lines of descent. If, however, the

T Extinction is, again, an integral part of the divergence pro¬ cess. It was not so very long before the Origin that the occur¬ rence of extinction was still questioned. Some Enlightenment thinkers held that organisms did not go extinct but rather con¬ tinued to exist somewhere. The idea of extinction seemed antithetical to the principle of plenitude, which in some forms dates back to Aristotle and was later incorporated into Christian theology. Plenitude holds that all things exist, or that God realizes all possibilities (Lovejoy 1964). Another Enlightenment thinker, the Comte de Buffon (1707-1788), rejected the plenitude concept and embraced ex¬ tinction as part of his vision of earth’s formation and evolution, set forth in his book Epoques de la Nature (1788). Extinction per se was irrelevant in some biological models, including La¬ marck’s transmutation theory. Lamarck’s brand of evolution held that animals are continually evolving up a supposed scale of organizational complexity, where lower, less complex, levels are constantly replenished by spontaneous generation. Thus all levels of organization are occupied at all times, at least in the contemporary world. Lamarck ran afoul of Georges Cuvier (1769-1832), the leading comparative anatomist of the era, who strongly believed that extinction occurs. Cuvier held that the history of life consisted of episodes of creation and extinction— catastrophes, the most recent of which was the Noachian flood. As late as 1830, in the first edition of Principles of Geology, Lyell speculated that plants and animals of past ages may return once suitable environmental conditions cycle back. For this he was lampooned by Henry de la Beche in the famous “Awful Changes” cartoon, in which Professor Ichthyosaurus lectures to young ichthyosauri on the nature of the extinct animal—humans— whose skull with puny jawbone and teeth are before them (see Rudwick 1992, p. 49). By Darwin’s day the fact of extinction was firmly established. Helping to crystallize this concept in the public’s imagination was the discovery of giant (and apparently quite extinct) reptiles in England in the 1820s—the group Rich¬ ard Owen later named Dinosauria.

THE ANNOTATED ORIGIN

122 1 Tree-pruning; the descendants of A and I are supposed to

have “taken the places of, and thus exterminated, not only their parents ... but likewise some of the original species which were most nearly related to their parents.” Note that only F has persisted, the lineage of the species set A-F that differs most from A.

NATURAL SELECTION.

Chap. IV.

modified offspring of a species get into some distinct country, or become quickly adapted to some quite new station, in which child and parent do not come into competition, both may continue to exist. If then our diagram be assumed to represent a considerable amount of modification, species (A) and all the earlier varieties will have become extinct, having been replaced by eight new species {a to m ; and (I) will have been replaced by six (n*'* to z^*) new species. But we may go further than this. The original species of our genus were supposed to resemble each other in unequal degrees, as is so generally the case in nature ; species (A) being more nearly related to B, C, and D, than to the other species; and species (I) more to G, H, K, L, than to the others. These two species (A) and (I), were also supposed to be very common and widely dif¬ fused species, so that they must originally have had some advantage over most of the other species of the genus. Their modified descendants, fourteen in number at the fourteen-thousandth generation, will probably have inherited some of the same advantages: they have also been modified and improved in a diversified manner at each stage of descent, so as to have become adapted to many related places in the natural economy ► of their country. It seems, therefore, to me extremely probable that they will have taken the places of, and thus exterminated, not only their parents (A) and (I), but likewise some of the original species which were most nearly related to their parents. Hence very few of the original species will have transmitted offspring to the fourteen-thousandth generation. We may suppose that only one (F), of the two species which were least closely related to the other nine original species, has transmitted descendants to this late stage of descent.

THE ANNOTATED ORIGIN

Chap. IV.

DIVERGENCE OF ‘CHARACTER.

123

The new species in our diagram descended from the original eleven species, will now be fifteen in number. Owing to the divergent tendency of natural selection, the extreme amount of difference in character between species a and z ** will be much greater than that between the most different of the original eleven species. The new species, moreover, will be allied to each other in a widely different manner. Of the eight descendants from (A) the three marked a'^, will be nearly related from having recently branched off from a ; b''^ and/'*, from having diverged at an earlier period from a ^ will be in some degree distinct from the three firstnamed species; and lastly, o '*, e '■*, and m **, will be nearly related one to the other, but from having di¬ verged at the first commencement of the process of modification, will be widely different from the other five species, and may constitute a sub-genus or even a distinct genus. The six descendants from (I) will form two sub¬ genera or even genera. But as the original species (I) differed largely from (A), standing nearly at the extreme points of the original genus, the six descendants from (I) will, owing to inheritance, differ considerably from the eight descendants from (A) ; the two groups, more¬ over, are supposed to have gone on diverging in dif¬ ferent directions. The intermediate species, also (and this is a very important consideration), which connected the original species (A) and (I), have all become, ex¬ cepting (F), extinct, and have left no descendants. Hence the six new species descended from (I), and the eight descended from (A), will have to be ranked as very distinct genera, or even as distinct sub-families. Thus it is, as I believe, that two or more genera are produced by descent, with modification, from two or more species of the same genus. And the two or

1 The key word here is “allied.” Branching and rebranching as

lineages diversify produces a nested hierarchy of branches and twigs. This is a nested hierarchy of alliance, or similarity: (a'*, p'*)i

/'*), and (o'*, e'*, m'*) are sets of species alliance,

but the first two (a'*-/'*) may represent a genus distinct from genus (o'*, e'*, m**). Taken together,

could be ranked

as one subfamily or family, and the species descending from I another subfamily or family as described in the follow¬ ing paragraph. 2 In this paragraph it becomes clear that this diagram can be

taken as a random slice of time: A-L are descendent forms themselves, as indicated by the dashed lines beneath them. Note that the lines are drawn such that sets of them will converge on a common ancestor. Later in the book Darwin will take this to its logical conclusion: a universal common ancestor for all life forms!

THE ANNOTATED ORIGIN

124 1 This point is very important: to some critics, if Darwin’s ideas were correct, we would expect to see all manner of intermediate forms linking species—extant, living species. That we do not see a continuum of such linking forms could be cited as evidence against descent with modification, in this view. Here, however, Darwin points out that intermediate forms are to be found not directly between extant species but rather between more general groups or types of species. Note the morphological gap between, say, groups (u'h

p'"*) and (h'h/*'*). The principle of diver¬

gence ensures nested groups of related forms, but the pruning effect of extinction ensures that extant groups cannot be arrayed in a perfect continuum of relationship. 2 Here is a glimpse of the heuristic power of the divergence of character diagram. The horizontal lines represent thousands, even millions, of generations in the discussion so far, but they can also represent geological strata. Darwin seems rather under¬ stated here: looking at the diagram as a set of strata studded with fossil remains does throw light on the affinities of extinct beings, both with other extinct forms and with extant species. This echoes Alfred Russel Wallace’s great insight in his Sarawak Law paper: “Every species has come into existence coincident both in space and time with a pre-existing closely allied species” (Wallace 1855, p. 186). This topic will be discussed in greater de¬ tail in chapter X.

THE ANNOTATED ORIGIN

NATURAL SELECTION.

Chap. IV.

more parent-species are supposed to have descended from some one species of an earlier genus. In our diagram, this is indicated by the broken lines, beneath the capital letters, converging in sub-branches down¬ wards towards a single point; this point representing a single species, the supposed single parent of our several new sub-genera and genera. It is worth wliile to reflect for a moment on the cha¬ racter of the new species f '■*, which is supposed not to have diverged much in character, but to have retained the form of (F), either unaltered or altered only in a slight degree. In this case, its affinities to the other fourteen new species will be of a curious and circuitous nature. Having descended from a form which stood between the two parent-species (A) and (I), now sup¬ posed to be extinct and unknown, it wiU be in some degree intermediate in character between the two ► groups descended from these species. But as these two groups have gone on diverging in character from the type of their parents, the new species (f'^) will not be directly intermediate between them, but rather between types of the two groups; and every naturalist will be able to bring some such case before his mind. ^ In the diagram, each horizontal line has hitherto been supposed to represent a thousand generations, but each may represent a million or hundred million generations, and likewise a section of the successive strata of the earth’s crust including extinct remains. We shall, when we come to our chapter on Geology, have to refer again to this subject, and I think we shall then see that the diagram throws liglit on the affinities of extinct beings, which, though generally belonging to the same orders, or families, or genera, with those now living, yet are often, in some degree, intermediate in character between e>. iting groups; and we can understand this fact, for

Chap. IV.

DIVERGENCE OF CHARACTER,

125

the extinct species lived at very ancient epochs when the brancliing lines of descent had diverged less. I see no reason to limit the process of modification, as now explained, to the formation of genera alone. If, in our diagi'am, we suppose the amount of change repre¬ sented by each successive group of diverging dotted lines to be very great, the forms marked ato jo those marked b and/'^ and those marked o to w’k will form three very distinct genera. We shall also have two very distinct genera descended from (I); and as these latter two genera, both from continued divergence of character and from inheritance from a different parent, will differ widely from the three genera descended from (A), the two little groups of genera will fonn two distinct families, or even orders, according to the amount of divergent modification supposed to be represented in the diagram. And the two new families, or orders, will have descended from tw'o species of the original genus; and these two species are supposed to have descended from one species of a still more ancient and unknown genus. We have seen that in each country it is the species of the larger genera which oftenest present vai'ieties or incipient species. This, indeed, might have been ex¬ pected ; for as natural selection acts through one form having some advantage over other forms in the struggle for existence, it will chiefly act on those which already have some advantage; and the largeness of any group -

GEOLOGICAL SUCCESSION.

Chap. X.

and, on the other hand, by similarity of conditions, for the uniformity of the same types in each during the later tertiary periods. Nor can it be pretended that it is an immutable law that marsupials should have been chiefly or solely produced in Australia; or that Eden¬ tata and other American types should have been solely produced in South America. For we know that Europe in ancient times was peopled by numerous marsupials; and I have shown in the publications above alluded to, that in America the law of distribution of terrestrial mammals was formerly different from what it now is. North America formerly partook strongly of the pre¬ sent character of the southern half of the continent; and the southern half was fonuerly more closely allied, than it is at present, to the northern half. In a similar manner we know from Falconer and Cautley’s dis¬ coveries, that northern India was formerly more closely related in its mammals to Africa than it is at the pre¬ sent time. Analogous facts could be given in relation to the distribution of marine animals. On the theory of descent with modification, the great law of the long enduring, but not immutable, succession of the same types within the same areas, is at once explained; for the inhabitants of each quarter of the world will obviously tend to leave in that quarter, during the next succeeding period of time, closely allied though in some degree modified descendants. If the inhabitants of one continent formerly differed greatly from those of another continent, so will their modified descendants still differ in nearly the same manner and degree. But after very long intervals of time and after great geogi’aphical changes, permitting much inter-migration, the feebler will yield to the more dominant forms, and there will be nothing im¬ mutable in the laws of past and present distribution.

THE ANNOTATED ORIGIN

Chap. X.

SAME TYPES IN SAME AEEAS.

341

It may be asked in ridicule, whether I suppose that the megatherium and other allied huge monsters have left behind them in South America the sloth, armadillo, and anteater, as their degenerate descendants. This cannot for an instant be admitted. These huge ani¬ mals have become wholly extinct, and have left no pro¬ geny. But in tlie caves of Brazil, there are many extinct species which are closely allied in size and in other characters to the species still living in South America; and some of these fossils may be the actual progenitors of living species. It must not be for¬ gotten that, on my theory, all the species of the same genus have descended from some one species; so that if six genera, each having eight species, be found in one geological formation, and in the next succeeding forma¬ tion there be six other allied or representative genera with the same number of species, then we may con¬ clude that only one species of each of the six older genera has left modified descendants, constituting the six new genera. The other seven species of the old genera have all died out and have left no progeny. Or, which would probably be a far commoner case, two or three species of two or three alone of the six older genera will have been tlie parents of the six new genera; the other old species and the other whole genera having become utterly extinct. In failing orders, with the genera and species decreasing in numbers, as apparently is the case of the Edentata of South America, stiU fewer genera and species will have left modified blood-descendants.

-

THE ANNOTATED ORIGIN

GEOGRAPHICAL DISTRIBUTION.

Chap. AT.

continuity of range are so numerous and of so grave a nature, that we ought to give up the belief, rendered probable by general considerations, that each species has been produced within one area,, and has migrated thence as far as it could. It wotdd be hopelessly tedious to discuss all the exceptional cases of the same species, now living at distant and separated points; nor do I for a moment pretend that any explanation coidd be offered of many such cases. But after some preliminary remarks, I will discuss a few of the most striking classes of facts; namely, the existence of the same species on the summits of distant mountain-ranges, and at distant points in the arctic and antarctic regions; and secondly (in the following chapter), the wide distribution of fresh¬ water productions; and thirdly, the occurrence of the same terrestrial species on islands and on the mainland, though separated by hundreds of miles of open sea. If the existence of the same species at distant and isolated points of the earth’s surface, can in many instances be explained on the view of each species having migrated from a single birthplace ; then, considering our ignor¬ ance with respect to former climatal and geograpliical changes and various occasional means of transport, the belief that this has been the universal law, seems to me incomparably the safest. In discussing this subject, we shall be enabled at the same time to consider a point equally important for us, namely, whether the several distinct species of a genus, which on my theory have all descended from a common progenitor, can have migrated (undergoing modification during some part of their migration) from the area inhabited by their progenitor. If it can be shown to be almost invariably the case, that a region, of which most of its inhabitants are closely related to, or belong to the same genera with the species of a second region.

Chap. XI.

SINGLE CENTRES OF CREATION.

355

has probably received at some former period immigrants from this other region, my theory will be strengthened; for we can clearly understand, on the principle of modification, why the inhabitants of a region should be related to those of another region, whence it has been stocked. A volcanic island, for instance, upheaved and formed at the distance of a few hundreds of miles from a continent, would probably receive from it in the course of time a few colonists, and their descendants, though modified, would still be plainly related by inheritance to the inhabitants of the continent. Cases of this nature ^ are common, and are, as we shall hereafter more fuUy see, inexplicable on the theory of independent creation. This view of the relation of species in one region to those in another, does not differ much (by substituting the word variety for species) from that lately advanced in an ingenious paper by Mr. Wallace, in which he coneludes, that “every species has come into existence coincident both in space and time with a pre-existing closely allied species.” And I now know from corre¬ spondence, that this coincidence he attributes to gene¬ ration with modification. The previous remarks on “ single and multiple centres of creation ” do not directly bear on another allied question,—namely whether all the individuals of the same species have descended from a single pair, or single hermaphrodite, or whether, as some authors suppose, from many individuals simultaneously created. With those organic beings which never intercross (if such exist), the species, on my theory, must have de¬ scended from a succession of improved varieties, which will never have blended with other individuals or varie¬ ties, but will have supplanted each other ; so that, at each successive stage of modification and improvement, all the individuals of each variety will have descended from

1 Darwin is explicit here in his overarching point: the patterns

we see are inexplicable on the supposition of special creation, but they are either predicted by, make the most sense under, or are at least consistent with his theory of descent with modifi¬ cation. 2 The ingenious paper that Darwin quotes is Wallace’s Sarawak Law paper, published in September 1855. This paper prompted Charles Lyell to initiate his journals on the species question. Lyell immediately recognized the significance of Wallace’s keen observations, and he advised Darwin to publish his own views. Darwin seemed unfazed: he summed up Wallace’s paper with a marginal note that read, “nothing very new.” The following spring, Darwin wrote to Hooker that he “had good talk with LyeU about my species work, & he urges me strongly to publish something” (Correspondence 6: 106). On May 14, 1856, Darwin got moving at last, recording in his pocket diary that he “began by Lyell’s advice writing Species Sketch.”

THE ANNOTATED ORIGIN

356 1 This is populational thinking: favorable variations spread

through the population by intercrossing. 2 Lyell discussed means of dispersal at great length, dedicating chapters 5-7 of the second volume of Principles of Geology to the subject. He also, like Darwin (see pp. 357-358), conducted ex¬ periments such as the persistence of seeds in salt water (Wilson 1970, p. 52) and recorded accounts of unlikely means of disper¬ sal. For example, in a letter to Darwin in May 1856, Lyell excit¬ edly wrote, “I have just heard from Woodward that his friend Mr C. Prentice of Cheltenham caught that large & most power¬ ful of our water beetles Hydrobius piceus with an ancylus fluviatilis adhering to him! . . . Here is a new light as to the way by which these sedentary mollusks may get transported from one river basin to another—That species of Ancylus seems to have got into Madeira before Man—How far can an Hydrobius fly with a favourable gale?” In this same letter Lyell further reported an instance of a water beetle with the egg case of a water spider

►

GEOGRAPHICAL DISTRIBUTION.

Chap. XI.

a single parent. But in the majority of cases, namely, with all organisms which habitually unite for each birth, or which often intercross, I believe that during the slow process of modification the individuals of the species will have been kept nearly uniform by intercrossing; so that many individuals will have gone on simultaneously changing, and the whole amount of modification will not have been due, at each stage, to descent from a single parent. To illustrate what I mean : our English race¬ horses differ slightly from the horses of every other breed; but they do not owe their difference and supe¬ riority to descent from any single pair, but to continued care in selecting and training many individuals during many generations. Before discussing the three classes of facts, which I have selected as presenting the greatest amount of difficulty on the theory of “ single centres of creation,” I must say a few words on the means of dispersal.

under its wings, musing, “What unexpected means of migration will in time be found out” (Correspondence6: 89).

THE ANNOTATED ORIGIN

Means of Dispersal.—Sir C. Lyell and other authors have ably treated this subject. I can give here only the briefest abstract of the more important facts. Change of climate must have had a powerful influence on migration : a region when its climate was different may have been a high road for migration, but now be impassable ; I shall, however, presently have to discuss this branch of the subject in some detail. Changes of level in the land must also have been highly influ¬ ential : a narrow isthmus now separates two marine faunas ; submerge it, or let it fonnerly have been sub¬ merged, and the two faunas will now blend or may fonnerly have blended : where the sea now extends, land may at a former period have connected islands or possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other.

Chap. XI.

MEANS OF DISPERSAL.

357

No geologist will dispute that great mutations of level, have occurred witliin the period of existing organisms. Edward Forbes insisted that aU the islands in the Atlantic must recently have been connected with Europe or Africa, and Em’ope likewise with America. Other authors have thus hypothetically bridged over every ocean, and have united almost every island to some mainland. If indeed the arguments used by Forbes are to be trusted, it must be admitted that scarcely a single island exists which has not recently been united to some continent. This view cuts the Gordian knot of the dispersal of the same species to the most distant points, and removes many a difficulty : but to the best of my judgment we are not authorized in admitting such enormous geographical changes within the period of existing species. It seems to me that we have abundant evidence of great oscillations of level in our continents ; but not of such vast changes in their position and extension, as to have united them within the recent period to each other and to the several inter¬ vening oceanic islands. I freely admit the foimer ex¬ istence of many islands, now buried beneath the sea, which may have served as halting places for plants and for many animals during their migration. In the coral-producing oceans such sunken islands are now marked, as I believe, by rings of coral or atoUs standing over them. Whenever it is fully admitted, as I believe it will some day be, that each species has proceeded from a single birthplace, and when in the course of time we know something definite about the means of dis¬ tribution, we shall be enabled to speculate with security on the former extension of the land. But I do not believe that it will ever be proved that within the recent period continents which are now quite separate, have been continuously, or almost continuously, united

1 Recall that Darwin, like Lyell, was an adherent of the idea of

long-term subsidence and elevation of continents as a dominant geological process governing earth history. In 1846 Forbes pub¬ lished his “Atlantis theory,” proposing the existence of a vast land bridge in the Miocene, spanning the east Atlantic from the Azores to Ireland. Darwin (and others) rejected this idea, favor¬ ing long-distance dispersal, but many geologists seemed taken with Forbes’s land bridge theory, and “continental extensions” became a popular subject of speculation that continued well into the nineteenth century. Consider Wallace’s conclusion to a letter he wrote to the editor of the magazine Natural Science in 1892: I cannot forget that it has been, and still is with many writers, the practice to assume former continental extensions across the great oceans in order to explain difficulties in the distribution of single genera or families; that geologists of repute have claimed the Dolphin bank in the Atlantic trough as the relic of a chain of mountains comparable with the Andes; that oceanic islands have been recently claimed to be merely the tops of submerged moun¬ tains, which can only be properly compared with the highest points of continents, and that a geological critic so late as 1879 considered the idea that the oceans had always been in their pres¬ ent positions “a funny one.” If such extreme views are now less common than they were, I hope that I may, without presumption, claim to have had some share in bringing about the change in sci¬ entific opinion now in progress. (Wallace 1892, p. 718)

2 Darwin is a bit restrained here; he had little patience with the

continental extension school and was somewhat irked to find that Lyell was more sympathetic to the idea. A long letter to Lyell dated June 25, 1856, begins: “As you say you would like to hear my reasons for being most unwilling to believe in the continen¬ tal extensions of late authors, I gladly write them; as, without I am convinced of my error, I shall have to give them condensed in my Essay, when I discuss single and multiple creation. I shall therefore be particularly glad to have your general opinion on (continued)

THE ANNOTATED ORIGIN

358 (continued) them. I may quite likely have persuaded myself in my wrath that there is more in them than there is” (Correspondence6: 153). Darwin always maintained that the “extensionists” overlooked dispersal in their enthusiasm for the more grandiose notion of lost continents. “I quite agree in admiration of Forbes’s Essay,” Darwin wrote to Lyell in November 1860, “yet, on my life, I think, it has done in some respects as much mischief as good. Those who believe in vast continental extensions will never in¬ vestigate means of distribution. Good Heavens look at Heers map of Atlantis!! 1 thought his division & lines of travel of the British Plants very wild & with hardly any foundation” (Corre¬ spondence 8: 479). “Heer” is Oswald Heer, who proposed a map of Atlantis in 1855. 1 Miles Joseph Berkeley reported the results of his seed im¬

mersion experiments in the Gardeners" Chronicle and Agricul¬ tural Gazette in September 1855. Darwin reported similar ex¬ periments in the same journal throughout that year (Darwin 1855a-d), and he wrote to Berkeley in February 1856 asking if he could combine Berkeley’s findings with his own for a paper. Berkeley consented, and Darwin wrote “On the action of sea¬ water on the germination of seeds,” read before the Linnean So¬ ciety that May (and the following year published in the Society’s Journal—-see Darwin 1857b). The significance of such studies for Darwin’s theories is obvi¬ ous: committed to long-distance dispersal as a key element of biogeography, he sought to establish that plants and animals could plausibly survive transoceanic passage. Hence this discus¬ sion of long-distance seed dispersal extends another six pages! Darwin documents myriad seed-dispersal observations and studies in his notebooks. In one passage in the B notebook, he even speculates about a double benefit of seed dispersal by sea: “It would be curious experiment to know whether soaking seeds in salt water 8cc has any tendency to form varieties?” (Barrett et al. 1987, p. 200).

THE ANNOTATED ORIGIN

►

GEOGRAPHICAL DISTRIBUTION.

ClIAl’. XI.

with each other, and with the many existing oceanic islands. Several facts in distribution,—such as the great difference in the marine faunas on the opposite sides of almost every continent,—the close relation of the tertiary inhabitants of several lands and even seas to their present inhabitants,—a certain degree of relation (as we shall hereafter see) between the distribution of mammals and the depth of the sea,-—these and other sucli facts seem to me opposed to tlie admission of such prodigious geographical revolutions within the recent period, as are necessitated on the view advanced by Forbes and admitted by his many followers. The nature and relative proportions of the inhabitants of oceanic islands likewise seem to me opposed to the belief of their former continuity with continents. Nor does their almost universally volcanic composition favour the admission that they are the wrecks of sunken continents;—if they had originally existed as mouutain-ranires on the land, some at least of the islands would have been formed, like other mountain-summits, of granite, metamorphic schists, old fossiliferous or other such rocks, instead of consisting of mere piles of volcanic matter. I must now say a few words on what are called acci¬ dental means, but which more properly might be called occasional means of distribution. I shall here confine myself to plants. In botanical works, this or that plant is stated to be ill adapted for wide dissemination; but for transport across the sea, the greater or less facilities may bo said to be almost Avholly unknown. Until I tried, with Mr. Berkeley’s aid, a few experiments, it was not even known how far seeds could resist the inju¬ rious action of sea-water. To my surprise I found that out of 87 kinds, 04 genninated after an immersion of 28 days, and a few survived an immersion of 137 days.

Chap. XI.

ME.\NS OF DISPERSAL.

359

For convenience sake I chiefly tried small seeds, Avithout the capsule or fruit; and as all of these sank in a few days, they could not be floated across wide spaces of the sea, whether or not they were injured by the salt-water. Afterwards I tried some larger fruits, capsules, &c., and some of these floated for a long time. It is well known what a difference there is in the buoyancy of green and seasoned timber; and it occurred to me that floods might wash down plants or branches, and that these might be dried on the banks, and then by a fresh rise in the stream be washed into the sea. Hence I was led to dry stems and branches of 94 plants with ripe fruit, and to place them on sea water. The majority sank quickly, but some which whilst green floated for a very short time, when dried floated much longer; for instance, ripe hazel-nuts sank immediately, but when dried, they floated for 90 days and afterwards when planted they germinated; an asparagus plant with ripe berries floated for 23 days, when dried it floated for 85 days, and the seeds afterwards germi¬ nated : the ripe seeds of Helosciachum sank in two days, when dried they floated for above 90 days, and after¬ wards germinated. Altogether out of the 94 dried plants, 18 floated for above 28 days, and some of the 18 floated for a veiy much longer period. So that as f ^ seeds germinated after an immersion of 28 days ; and as plants with ripe fruit (but not all the same species as in the foregoing experiment) floated, after being dried, for above 28 days, as far as we may infer anything from these scanty facts, we may conclude that the seeds of t^oo country might be floated by sca-cur- -erish. The mountains would become covered with snow and ice, and their former Alpine inhabitants would descend to the plains. By the time that the eold had reached its maximum, we should have a uniform arctic fauna and flora, covering the central ptrts of Euroj^, as far

Chav. XI.

DURING THE GLACIAL PERIOD.

367

south as the Alps and Pyrenees, and even stretching into Spain. The now temperate regions of the United States would likewise be covered by arctic plants and animals, and these would be nearly the same with those of Europe; for the present circumpolar inhabitants, which we suppose to have everywhere travelled south¬ ward, are remarkably uniform round the world. We may suppose that the Glacial period came on a little earlier or later in North America than in Europe, so will the southern migration there have been a little earlier or later; but this will make no difference in the final result. As the warmth returned, the arctic forms would re¬ treat northward, closely followed up in their retreat by the productions of the more temperate regions. And as the snow melted from the bases of the mountains, the arctic forms would seize on the cleared and thawed ground, always ascending higher and higher, as the warmth increased, whilst their brethren were pursuing their northern journey. Hence, when the warmth had fully returned, the same arctic species, which had lately lived in a body together on the lowlands of the Old and New Worlds, would be left isolated on distant mountainsummits (having been exterminated on all lesser heights) and in the arctic regions of both hemispheres. Thus we can understand the identity of many plants at points so immensely remote as on the mountains of the United States and of Europe. We can thus also understand the fact that the Alpine plants of each mountain-range are more especially related to the arctic forms living due north or nearly due north of them: for the migration as the cold came on, and the re-migration on the returning warmth, will generally have been due south and north. The Alpine plants, for example, of Scotland, as remarked by Mr. H. C. Watson,

1 The alpine flora of Europe and North America are still thought to be glacial relicts, though not all taxa responded in precisely the same way to glacial cycles. The plants of different mountain ranges were affected in different ways and degrees by glaciation, depending on the geography of the range in terms of latitude and orientation (see Nagy et al. 2003).

THE ANNOTATED ORIGIN

368 1 The mountaineer and naturalist Louis Francois Elisabeth Ra-

mond was renowned for his exploration of the high Pyrenees. He published several books and papers on the mountains be¬ tween 1789 and 1825, the last being the botanical work Sur I’etat de la vegetation au sommet du Pic du Midi (On the Condition of the Vegetation on the Summit of the Pic du Midi). 2 Gnathodon is a brackish water bivalve, abundant along the American Gulf Coast. There is one extant North American spe¬ cies, G. cuneatus (now Rangia cuneata), fossils of which are known from the Miocene. In the memoir of his second visit to the United States, Lyell commented on roads in New Orleans constructed of Gnathodon cuneatus shells, and noted extensive beds of these fossils as much as twenty miles inland of the Port of Mobile (Lyell 1849). Fossils of G. cuneatushsve been found as far north as the New Jersey coast, and its former northerly dis¬ tribution is probably what led Darwin to mention its presence as an indication of past warmer conditions.

THE ANNOTATED ORIGIN

GEOGRAPHICAL DISTRIBUTIO:^,

Cjiap. XI.

(► and those of the Pyrenees, as remarked by Ramond, are more especially allied to the plants of northern Scan¬ dinavia ; those of the United States to Labrador; those of the mountains of Siberia to the arctic regions of that country. These views, grounded as they are on the perfectly well-ascertained occurrence of a former Glacial period, seem to me to explain in so satisfactory a manner the present distribution of the Alpine and Arctic productions of Europe and America, that when in other regions we find the same species on distant moun¬ tain-summits, we may almost conclude without other evidence, that a colder climate permitted their former migration across the Ioav intervening tracts, since be¬ come too warm for their existence. If the climate, since the Glacial period, has ever been in any degree warmer than at present (as some geo¬ logists in the United States believe to have been the ^ case, chiefly from the distribution of the fossil Gnatho¬ don), then the arctic and temperate productions will at a very late period have marched a little further north, and subsequently have retreated to their present homes ; but I have met with no satisfactory evidence with respect to this intercalated slightly wanner period, since the Glacial period. The arctic forms, during their long southern migra¬ tion and re-migration northward, will have been exposed to nearly the same climate, and, as is especially to be noticed, they will have kept in a body together; con¬ sequently their, mutual relations will not have been much disturbed, and, in accordance with the principles inculcated in this volume, they will not have been liable to much modification. But with our Alpine productions, left isolated from the moment of the returning warmth, first at the bases and ultimately on the summits of the mountains, the case will have been somewhat dif-

Chap. XI.

DURING THE GLACIAL PERIOD.

369

ferent; for it is not likely that all the same arctic spe¬ cies will have been left on mountain ranges distant from each other, and have survived there ever since; they will, also, in all probability have become mingled with ancient Alpine species, which must have existed on the mountains before the commencement of the Glacial epoch, and which during its coldest period will have been temporarily driven down to the plains; they will, also, have been exposed to somewhat different climatal influences. Their mutual relations will thus have been in some degree disturbed ; consequently they will have been liable to modification; and this we find has been the case ; for if we compare the present Alpine plants and animals of the several great European mountainranges, though very many of the species are identically the same, some present varieties, some are ranked as doubtful forms, and some few are distinct yet closely allied or representative species. In illustrating what, as I believe, actually took place during the Glacial period, I assumed that at its com¬ mencement the arctic productions were as uniform round the polar regions as they are at the present day. But the foi’egoing remarks on distribution apply not only to strictly arctic forms, but also to many sub-arctic and to some few northern temperate forms, for some of these are the same on the lower mountains and on the plains of North America and Europe; and it may be reasonably asked how I account for the necessary de¬ gree of uniformity of the sub-arctic and northern tem¬ perate forms round the world, at the commencement of the Glacial period. At the present day, the sub-arctic and northern temperate productions of the Old and New Worlds are separated from each other by the Atlantic Ocean and by the extreme northern part of the Pacific. During the Glacial period, when the in-

1 This passage is a good reminder of Darwin’s thinking on the

generation of variability. He attributes the species differences between plants atop different mountains to variation generated during the glacial cycles, when the plants were exposed to differ¬ ent environmental conditions and comingled with lowland spe¬ cies. Note his language: these plants were “liable to modifica¬ tion” as a result of disturbance of their “mutual relations” (i.e., ecology). The modern take on these same species differences is that population isolation leads to divergence, with or without natural selection. Note, too, that spatial isolation plays a role: these alpine productions are stranded atop mountains. Is Dar¬ win suggesting in the final sentence of the paragraph that the species of different mountaintops differ from one another, each showing some variants in isolation of the others?

THE ANNOTATED ORIGIN

370 1 In current thinking, intermigration occurred even during the glacial maxima; an ice-free corridor along Beringia, the Ber¬ ing land bridge, linked Siberia and North America. During gla¬ ciation, sea level was 300 or more feet lower than present lev¬ els, exposing land approximately 1,000 miles across its widest point.

!►

THE ANNOTATED ORIGIN

GEOGRAPHICAL DISTRIBUTION,

Chap. XI.

habitants of the Old and New Worlds lived further southwards than at present, they must have been still more completely separated by wider spaces of ocean. I believe the above difficulty may be surmounted by looking to still earlier changes of climate of an opposite nature. We have good reason to believe that during the newer Pliocene period, before the Glacial epoch, and whilst the majority of the inhabitants of the world were specifically the same as now, the climate was Manner than at the present day. Hence vfe may sup¬ pose that the organisms now living under the climate of latitude 60°, during the Pliocene period lived further north under the Polar Circle, in latitude 66°-67°; and that the strictly arctic productions then lived on the broken land still nearer to the pole. Now if we look at a globe, we shall see that under the Polar Circle there is almost continuous land from western Europe, through Siberia, to eastern America. And to tliis continuity of the circumpolar land, and to the consequent freedom for intermigration under a more favourable climate, I attribute the necessary amount of uniformity in the sub-arctic and northern temperate productions of the Old and New Worlds, at a period anterior to the Glacial epoch. Believing, from reasons before alluded to, that our continents have long remained in nearly the same rela¬ tive position, though subjected to large, but partial oscillations of level, I am strongly inclined to extend the above view, and to infer that during some earlier and still warmer period, such as the older Pliocene period, a large number of the same plants and animals inhabited the almost continuous circumpolar land; and that these plants and animals, both in the Old and New Worlds, began slowdy to migrate southwards as the climate became less warm, long before the com-

Chap. XI.

DURING THE GLACIAL PERIOD.

371

mencement of the Glacial period. We now see, as I believe, their descendants, mostly in a modified con¬ dition, in the central parts of Europe and the United States. On tliis view Ave can understand the relation¬ ship, with very little identity, between the productions of North America and Europe,—a relationship which is most remarkable, considering the distance of the two areas, and their separation by the Atlantic Ocean. We can further understand the singular fact remarked on by several observers, that the productions of Europe and America during the later tertiary stages were more closely related to each other than they are at the pre¬ sent time ; for during these warmer periods the northern parts of the Old and New Worlds will have been almost continuously united by land, serving as a bridge, since rendered impassable by cold, for the inter-migration of their inhabitants. During the slowly decreasing warmth of the Pliocene period, as soon as the species in common, which inhabited the New and Old Worlds, migrated south of the Polar Circle, they must have been completely cut off from each other. This separation, as far as the more temperate pro¬ ductions are concerned, took place long ages ago. And as the plants and animals migrated southward, they will have become mingled in the one great region with the native American productions, and have had to compete with them; and in the other great region, with those of the Old World. Consequently we have here everj'thing favourable for much modification,—for far more modification than with the Alpine productions, left isolated, within a much more recent period, on the several mountain-ranges and on the arctic lands of the two Worlds. Hence it has come, that when we compare the now living productions of the temperate regions of the New and Old Worlds, we find very few identical

1

Darwin is right for the wrong reasons here: Eurasia and North

America were indeed linked into a continuous land mass, but during the cold periods when sea level dropped, not during the warm interglacials when Beringia was flooded (as today). The time period to which Darwin is referring—“later Tertiary stages”—corresponds to the last epochs of the Tertiary, the Mio¬ cene and Pliocene (named by Lyell in 1833). We now know that these stages span a far longer stretch of time than the Victorians imagined, from about twenty-three million years ago to nearly two million years ago. “Tertiary” is no longer used by geolo¬ gists and is now divided into the Paleogene and the Neogene pe¬ riods. 2 Since the time of Linnaeus, botanists have noticed the affini¬

ties of Old World and New World flora. Asa Gray was especially intrigued by this distribution pattern, and in 1846 he produced a paper titled Analogy between the flora of Japan and that of the United States. A decade later, Gray’s work on a list of North American alpine species for Darwin snowballed into his semi¬ nal paper Statistics of the flora of the northern United States (1856-1857), and then an even more detailed memoir on the botany of Japan (Gray 1859). Gray’s Harvard colleague W. G. Farlow called this paper “a masterpiece” in his remembrance of Gray read before the National Academy of Sciences on April 17, 1889. The close relationship between the flora (as well as arthro¬ pods and fungi) of eastern Asia and eastern North America is highly striking: the flora of eastern North America is more simi¬ lar to that of eastern China and Japan than to that of western North America, with more than sixty-five genera exhibiting the disjunction. It is believed that this pattern is a relict of the exten¬ sive north-temperate forests of the Neogene, dating to the Mio¬ cene Epoch -23.8 to 5.3 million years ago (Boufford and Spongberg 1983; Wen 1999).

THE ANNOTATED ORIGIN

372 1

James Dwight Dana was a geologist for the U.S. Exploring Ex¬

pedition of 1838-1842 and subsequently worked on the crusta¬ ceans collected on the expedition. 2 Relationship without identity is an important point: Darwin is underscoring the puzzle of having related yet distinct species living under essentially identical conditions.

(►

^

THE ANNOTATED ORIGIN

GEOGRAPHICAL DISTRIBUTIOX,

Clixp. XI.

species (thoufrJi Asa Gray has lately shown that more plants are identical than ivas formerly supposed), hut Ave find in every great class many forms, Ailiich some naturalists rank as geographical races, and others as dis¬ tinct species; and a host of closely allied or represen¬ tative forms which are ranked by all naturalists as specifically distinct. As on the land, so in the waters of the sea, a slow southern migration of a marine fauna, Avhich during the Pliocene or even a somewhat earlier period, was nearly uniform along the continuous shores of the Polar Circle, will account, on the theory of modification, for many closely allied forms now living in areas completely sundered. Thus, I think, we can understand the pre¬ sence of many existing and tertiary representative fomrs on tlie eastern and western shores of temperate North America; and the still more striking case of many closely allied crustaceans (as described in Dana’s ad¬ mirable work), of some fish and other marine animals, in the Mediterranean and in the seas of Japan,—areas now separated bj' a continent and by nearly a hemi¬ sphere of equatorial ocean. These cases of relationship, ivithout identity, of the inhabitants of seas now disjoined, and likeivise of the past and present inhabitants of the temperate lands of North America and Europe, are inexplicable on the theory of creation. M"e cannot say that they have been created alike, in correspondence with the nearly similar physical conditions of the areas; for if we com¬ pare, for instance, certain parts of South America with the southern continents of the Old World, we see countries closely corresponding in all their physical conditions, but with their inhabitants utterly dissimilar. But we must return to our more immediate subject, the Glacial period. I am convinced that Forbes’s view

Chap. XI.

DURING THE GLACIAL PERIOD.

373

may be largely extended. In Etiropo we liave the plainest evidence of the cold period, from the western shores of Britain to the Oural range, and southward to the Pyrenees. We may infer, from the frozen mammals and nature of the motinttdn vegetation, that Siberia was similarly affected. Along the Himalaya, at points 900 miles apart, glaciers liave left the marks of their former low descent; and in Sikkim, Dr. Hooker .saw maize growing on gigantic ancient moraines. South of the equator, we have some direct evidence of former glacial action in New Zealand; and the same jilants, found on widely separated mountains in this island, tell the same stoiy. If one account which has been published can be tnisted, we have direct evidence of glacial action in the south-eastern corner of Australia. Looking to America ; in the northern half, ice-borne fragments of rock have been observed on the eastern side as far south as lat. 36°-37°, and on the shores of the Pacific, where the climate is now so different, as far south as lat. 46°; erratic boulders have, also, been noticed on the Kocky Mountains. In the Cordillera of Equatorial South America, glaciers once extended far below their present level. In central Chile I was astonished at the structure of a vast mound of detritus, about 800 feet in height, crossing a valley of the Andes; and this I now feel convinced was a gigantic moraine, left far below any existing glacier. Further south on both sides of the continent, from lat. 41° to the southernmost extremity, we have the clearest evidence of former glacial action, in huge boulders transported far from their parent source. We do not know that the Glacial epoch was strictly simultaneous at these several far distant points on o^iposite sides of the world. But we have good evidence in almost every case, that the ejioch was included within

1

Hooker led an expedition to the Himalayas in 1847-1850,

during which he and his party were taken prisoner for several weeks by the Rajah of Sikkim. He recounted this and other ad¬ ventures, along with vivid descriptions of the geology and bot¬ any of the region, in his book Himalayan Journal (1854), which he dedicated to Darwin. Darwin’s information on New Zealand was also provided by Hooker, from an earlier voyage. Hooker was assistant surgeon and naturalist on the HMS Erebus, which with its sister ship HMS Terror explored the southern oceans from 1839 to 1841. The botanical work from this voyage made Hooker’s reputation: his collections became one of the two Flora Antarctica volumes (1844-1847), and he subsequently published the acclaimed books Flora Novae-Zelandiae (1853-1855) and Flora Tasmaniae (1855-1S60). 2 There is no explicit mention of this detritus mound in either

the Voyage of the Beagle or Geological Observations on South America, but Darwin was much struck by conical mounds and terraces of unconsolidated material throughout the Chilean val¬ leys. He interpreted these as evidence of incursions of the sea. In March 1835, for example, he inspected the valley of the Maypu River in Chile; “All the main valleys in the Cordillera are charac¬ terized by ... a fringe or terrace of shingle and sand, rudely stratified, and generally of considerable thickness... and which were undoubtedly deposited when the sea penetrated Chile” (Voyage, chapter XV). In chapter IX of Voyage, Darwin reported finding immense erratic boulders, which he interpreted as iceberg-borne. After his conversion to the glacial theory, Darwin realized that the ter¬ races, detritus piles, and erratic boulders of South America were deposited by glaciers. His iceberg theory was not unreasonable: he had observed gigantic icebergs loaded with rocky debris in the Straits of Magellan: “In [Eyre’s] Sound, about fifty icebergs were seen at one time floating outwards... Some of the icebergs were loaded with blocks of no inconsiderable size, of granite and other rocks, different from the clay-slate of the surrounding mountains” (Voyage, chapter XI).

THE ANNOTATED ORIGIN

374 1 Darwin conjectures that the glacial period was a time of

global cooling, not just localized drops in temperature. He sup¬ poses that such cooling would homogenize conditions world¬ wide and allow for the southerly migration of now northerly adapted species. With warming of the climate, some of these plants would be extirpated locally, but others would retreat to high-elevation sites as the lowlands became too warm. The sce¬ nario of worldwide cooling is supported both by recent data in¬ dicating that the past two northern and southern hemisphere glacial advances (22,000 and 150,000 years ago) occurred si¬ multaneously (Kaplan et al. 2004), and by pollen records from northern and southern hemisphere glacial sites (Moreno et al.

2001). 2 Darwin was keenly interested in Hooker’s botanical findings in Tierra del Fuego. He wrote to him in November 1843, soon

►

after Hooker’s return to England: 1 have long thought that some general sketch of the Flora of [Tierra del Fuego], stretching so far into the southern seas, would be very curious.—Do make comparative remarks on the species allied to the European species, for the advantage of Botanical Ignoramus’es like myself. It has always struck me as a curious point to find out, whether there are many European genera in T. del Fuego, which are not found along the ridge of the Cordillera; the separation in such cases wd be so enormous. {Correspondence 2: 408)

3 The English botanical explorer George Gardner gave an ac¬ count of the botany of the Organ Mountains, Brazil, in Travels in the Interior of Brazil (1846). 4 The nearly 3,000-meter-high Silla de Caracas is a mountain in the coastal range of Venezuela, near the city of Caracas. Alex¬ ander von Humboldt and Aime Bonpland ascended the moun¬ tain in the course of their explorations in South America, and Darwin read about the mountain’s botany in volume III of Humboldt’s Personal Narrative.

THE ANNOTATED ORIGIN

>

GEOGKAPHICAL DISTRIBUTION,

Chap. XI.

the latest geological period. We have, also, excellent evidence, that it endured for an enormous time, as measured by years, at each point. The cold may have come on, or have ceased, earlier at one point of the globe than at another, but seeing that it endured for long at each, and that it was contemporaneous in a geological sense, it seems to me probable that it was, during a part at least of the period, actually simidtaneous throughout the world. Without some distinct evidence to the contrary, we may at least admit as probable that the glacial action was simultaneous on the eastern and Avestern sides of North America, in the Cordillera under the equator and under the wanner temperate zones, and on both sides of the southern extremity of the continent. If this be admitted, it is difficult to avoid believing that the temperature of the Avhole world was at this period simultaneously cooler. But it would suffice for my purpose, if the temperature Avas at the same time lower along certain broad belts of longitude. On this vieAv of the whole world, or at least of broad longitudinal belts, having been simultaneously colder from pole to pole, much light can be throAvm on the present distribution of identical and allied species. In America, Dr. Hooker has shoAvn that between forty and fifty of the flowering plants of Tierra del Fuego, forming no inconsiderable part of its scanty flora, are common to Europe, enormously remote as these tAvo points are ; and there are many closely allied species. On the lofty mountains of equatorial America a host of peculiar species belonging to European genera occur. On the highest mountains of Brazil, some few European genera were found by Gardner, which do not exist in the wide intervening hot countries. So on the Silla of Caraccas the illustrious Humboldt long ago found species belong-

Chap. XI.

DURING THE GLACIAL PERIOD.

375

ing to genera characteristic of the Cordillera. On the mountains of Abyssinia, several European forms and some few representatives of the peculiar flora of the Cape of Good Hope occur. At the Cape of Good Hope a very few European species, believed not to have been introduced by man, and on the mountains, some few representative European forms are found, which have not been discovered in the intertropical parts of Africa. On the Himalaya, and on the isolated mountain-ranges of the peninsula of India, on the heights of Ceylon, and on the volcanic cones of Java, many plants occur, either identically the same or representing each other, and at the same time representing plants of Europe, not found in the intervening hot lowlands. A list of the genera collected on the loftier peaks of Java raises a picture of a collection made on a hill in Europe! Still more striking is the fact that southern Australian forms are clearly represented by plants growing on the summits of the mountains of Borneo. Some of these Australian forms, as I hear from Dr. Hooker, extend along the heights of the peninsula of Malacca, and are thinly scattered, on the one hand over India and on the other as far north as Japan. On the southern mountains of Australia, Dr. F. ^ Muller has discovered several European species; other species, not introduced by man, occur on the lowlands ; and a long list can be given, as I am informed by Dr. Hooker, of European genera, found in Australia, but not in the intermediate torrid regions. In the admir¬ able ‘Introduction to the Flora of New Zealand,’ by Dr. Hooker, analogous and striking facts are given in regard to the plants of that large island. Hence we see that throughout the world, the plants growing on the more lofty mountains, and on the temperate lowlands of the northern and southern hemispheres, are sometimes

1

Ferdinand von Muller published several books on the flora

of Australia, among them Fragmenta Phytographica Australiae (1862-1881) and Plants of Victoria (1860-1865). The “Introduc¬ tion” Darwin refers to by Hooker is a thirty-nine-page introduc¬ tory essay to his Flora Novae-Zelandiae (Flora of New Zealand) in 1853. Intended as a prefatory work, this essay became an ac¬ claimed treatment of botanical geography in its own right. 2 This is a significant point: the plants at high elevations in these far-flung locales are usually “specifically distinct”—i.e., different species—and only in a few cases “identically the same,” i.e., the same species. They are related in “a most remarkable manner” because they are very close relatives yet live so far apart. The supposition is that these close relatives descended from a common ancestor in the recent past, during or prior to the last glacial period. This scenario is largely accepted today, with at least three key differences from Darwin’s era: the time scales are longer than the Victorians imagined; we now recognize multiple glacial cycles opening and closing migration corridors; and con¬ tinents are now known to move, adding another factor to the dynamic of species movements in deep time.

THE ANNOTATED ORIGIN

376

ous appearances. James Dwight Dana was cited on p. 372, as here, in connection with his work on the crustaceans of the U.S. Exploring Expedition. The Scottish naturalist and arctic ex¬ plorer John Richardson wrote Fauna Boreali-Americana. Hooker we have met several times. He did not work extensively on algae, and would also have been intimately familiar with the work of

►

the Irish phycologist William Henry Harvey, who described Darwin’s algal collections from the Beagle voyage. The plant biogeographer Hewett Cottrell Watson, finally, treated British alpine plants in Cybele Britannica (1847). 2 Arctic, temperate, and tropical plants are imagined to respond to the steadily cooling climate of the glacial period in different ways. This long paragraph describes what happens to the tem¬ perate and tropical species. Here is a summary: Darwin sup¬ poses that the tropical species have stricter thermal require¬ ments and experience much extinction, with surviving species retreating to the narrow remaining suitable areas within the tropics. The temperate-adapted species, in contrast, are imag¬ ined to be more physiologically flexible and thus ultimately be¬ come geographically displaced to a much greater degree. As a result of this migration, these plants experience altered environ¬ mental conditions and in some cases comingle with the more cool-tolerant species of the tropics. The net effect of this pro¬ cess, Darwin imagines, is to engender variations that selection may act upon to adapt the temperate species to their new homes, and new species are formed. Later, when the climate warms up again, these northern-affinity species are driven back north out of the tropics, except for some representatives that move upward to the mountain peaks instead of northward. Those that had made it beyond the equatorial zone into the southern hemi¬ sphere get driven still further southward, to the south temperate zone, as the tropical species expand out of their refugia and once again dominate the tropical landscape. Thus we are left with northern-affinity species in both hemispheres, especially in the cool climates of the high elevations. The species of these hemi¬ spheres differ but are closely related.

THE ANNOTATED ORIGIN

Chap. XI,

identically the same ; but they are much oftener speci¬ fically distinct, though related to each other in a most

1 The three authorities cited on this page have all made previ¬

but he did collect many algal species on the voyage of the Erebus

GEOGRAPHICAL DISTRIBUTION,

^

remarkable manner. This brief abstract applies to plants alone: some strictly analogous facts could be given on the distribu¬ tion of terrestrial animals. In marine productions, similar cases occur; as an example, I may quote a remark by the highest authority. Prof. Dana, that “ it is certainly a wonderful fact that New Zealand should have a closer resemblance in its Crustacea to Great Britain, its antipode, than to any other part of the world.” Sir J. Richardson, also, speaks of the re¬ appearance on the shores of New Zealand, Tasmania, &c., of northern forms of fish. Dr. Hooker informs me that twenty-five species of Algae are common to New Zealand and to Europe, but have not been found in the intermediate tropical seas. It should be observed that the northern species and forms found in the southern parts of the southern hemi¬ sphere, and on the mountain-ranges of the intertropical regions, are not arctic, but belong to the northern tem¬ perate zones. As Mr. H. C. Watson has recently re¬ marked, “ In receding from polar towards equatorial latitudes, the Alpine or mountain floras really become less and less arctic.” Many of the forms living on the mountains of the warmer regions of the earth and in the southern hemisphere are of doubtful value, being ranked by some naturalists as specifically distinct, by others as varieties; but some are certainly identical, and many, though closely related to northern forms, must be ranked as distinct species. Now let us see what light can be thrown on the fore¬ going facts, on the belief, supported as it is by a large body of geological evidence, that the whole world, or a large part of it, was during the Glacial period simulta-

Chap. XI.

DURING THE GLACIAL PERIOD.

377

neously much colder than at present. The Glacial period, as measured by years, must have been very long; and when we remember over what vast spaces some naturalised plants and animals have spread ^vithin a few centuries, this period will have been ample for any amount of migration. As the cold came slowly on, all the tropical plants and other productions vdll have retreated from both sides towards the equator, followed in the rear by the temperate productions, and these by the arctic ; but with the latter we are not now con¬ cerned. The tropical plants probably suffered much extinction; how much no one can say; perhaps for¬ merly the tropics sujiported as many species as we see at the present day crowded together at the Cape of Good Hope, and in parts of temperate Australia. As we know that many tropical plants and animals can withstand a considerable amount of cold, many might have escaped extermination during a moderate fall of temperature, more especially by escaping into the warmest spots. But the great fact to bear in mind is, that all tropical productions will have suffered to a cer¬ tain extent. On the other hand, the temperate pro¬ ductions, after migrating nearer to the equator, though they ivill have been placed under somewhat new con¬ ditions, will have suffered less. And it is certain that many temperate plants, if protected from the inroads of competitors, can withstand a much warmer climate than their own. Hence, it seems to me possible, bearing in mind that the tropical productions were in a suffering state and could not have presented a firm front against intruders, that a certain number of the more vigorous and dominant temperate forms might have penetrated the native ranks and have reached or even crossed the equator. The invasion would, of course, have been greatly favoured by high land, and perhaps

THE ANNOTATED ORIGIN

378 1 The commonalities between the flora of Tierra del Fuego and

that of Europe were first described by Hooker in Flora Antarc¬ tica, but he did not specifically address Fuegian plants that were also present in Europe. This is important to Darwin because he opposes continental extensions and multiple creations; oversea and overland dispersal are the only mechanisms he accepts. We have already seen the energy Darwin expended on dispersal by sea. Here he explores the possibility of overland dispersal made possible by corridors created during cool periods. Darwin suggested to Gray in May 1856 that it would be useful “to compare the list of European plants in Tierra del Fuego (in

►

Hooker) with those in N. America; for without multiple cre¬ ation, I think we must admit that all now in T. del Fuego, must have traveled through N. America” (Correspondence 6: 92). Two months later, still puzzling over the Patagonia-Europe connec¬ tion, he asked Gray if the “Alleghenies” (Appalachians) could provide a migration route to the southern part of North Amer¬ ica; “[Are the] Alleghenies ... sufficiently continuous so that the plants could travel from the north in the course of ages thus far south? I remember Bartram makes the same remark with re¬ spect to several trees on the Occone Mts.” (Correspondence 6: 182). Darwin read the Travels of William Bartram, the Ameri¬ can botanist who collected in the southern Appalachians in the 1770s. “Occone Mts.” refers to Station Mountain in upstate South Carolina. In October 1856 Darwin asked Hooker to comment on a draft manuscript containing material that was to become much of this chapter. This was part of a months-long exchange be¬ tween the two, in which Darwin tried to convince Hooker of his hypothesis of pathways of species migration during the cooler epochs. Hooker was wedded to the idea of continental exten¬ sions to explain the distribution of the southern hemisphere flora, a notion that Darwin found anathema. 2 Hooker’s Himalayan Journals (1854—1855) offered accounts of the landscape and its people and natural history. Hooker comments on the plant families and their distribution with an especially keen eye. In Natural Selection (Stauffer 1975, p. 549), Darwin related Hooker’s account of the vegetation found at the base of the Himalayas, “where true Tropical form are mingled with such northern forms as Birches, Maples, whortle-berries, strawberries, &c.”

^

GEOGRAPHICAL DISTRIBUTION,

Chap. XI.

by a diy climate; for Dr. Falconer informs me that it is the damp with the heat of the tropics which is so destructive to perennial plants from a temperate cli¬ mate. On the other hand, the most humid and hottest districts Mull have afforded an asylum to the tropical natives. The mountain-ranges north-west of the Hima¬ laya, and the long line of the Cordillera, seem to have afforded tu'O great lines of invasion: and it is a striking fact, lately communicated to me by Dr. Hooker, that all the flowering plants, about forty-six in number, common to Tierra del Fuego and to Europe still exist in North America, M'hich must have lain on the line of march. But I do not doubt that some temperate productions entered and crossed even the lowlands of the tropics at the period when the cold was most intense,—M’hen arctic forms had migrated some twenty-five degrees of latitude from their native countiy and covered the land at the foot of the Pyrenees. At this period of ex¬ treme cold, I believe that the climate under the equator at the level of the sea was about the same with that now felt there at the height of six or seven thousand feet. During this the coldest period, I suppose that large spaces of the tropical lowlands M'ere clothed with a mingled tropical and temperate vegetation, like that now groM'ing with strange luxuriance at the base of the Himalaya, as graphically described by Hooker. Thus, as I believe, a considerable number of plants, a few terrestrial animals, and some marine productions, migrated during the Glacial period from the northern and southern temperate zones into the intertropical re¬ gions, and some even crossed the equator. As the warmth returned, these temperate forms would naturally ascend the higher mountains, being exterminated on the low¬ lands ; those which had not reached the equator, would re-migrate northward or southward towards their former

Chap. XI.

DURING THE GLACIAL PERIOD.

379

homes; but the forms, chiefly northern, which had crossed the equator, would travel still further from their homes into tlie more temperate latitudes of the opposite hemisphere. Although we have reason to believe from geological evidence that the whole body of arctic shells underwent scarcely any modification during their long southern migration and re-migration northward, the case may have been wholly dilferent with those intruding forms which settled themselves on the intertropical mountains, and in the southern hemisphere. These being surrounded by strangers will have had to compete noth many new forms of life; and it is probable that selected modifications in their structure, habits, and con¬ stitutions will have profited them. Thus many of these wanderers, though still plainly related by inheritance to their brethren of the northern or southern hemispheres, now exist in their new homes as well-marked varieties or as distinct species. It is a remarkable fact, strongly insisted on by Hooker in regard to America, and by Alph. de Candolle in regard to Australia, that many more identical plants and allied forms have apparently migrated from the north to the south, than in a reversed direction. We see, however, a few southern vegetable forms on the mountains of Borneo and Abyssinia. I suspect that this preponderant migration from north to south is due to the greater extent of land in the north, and to the northern forms having existed in their o^vn homes in greater numbers, and having consequently been ad¬ vanced through natural selection and competition to a higher stage of perfection or dominating power, than the southern forms. And thus, wlien they became com¬ mingled during the Glacial period, the northern forms were enabled to beat the less powerful southern forms. Just in the same manner as we see at the present day.

1 This is a key concluding statement, to which the argument of

the previous several pages has been headed. 2 Hooker observed in the introduction to Flora of New Zealand that a far greater number of northern species are represented in the south than are southern species in the north; Alphonse de Candolle made the same point in 1855 in his important work Geographie Botanique Raisonnee. Darwin called this a “curious difficulty” and a “singular fact” in Natural Selection (Stauffer 1975, pp. 558,559). His explanation is instructive: he relates this finding back to land area, population size, and competitive abil¬ ity (perhaps borrowing from de Candolle, who, he pointed out, reported that species in the Russian Empire have larger ranges than their relatives from the same botanical families at the southern tip of Africa). Recall Darvv^in’s idea that large popu¬ lation sizes lead to greater variability, which in turn leads to greater responsiveness to selection pressures. Such populations thus become better and better adapted, while small populations languish. The greater land area of the northern hemisphere is supposed to lead to larger species ranges. Species from areas with larger ranges are thus imagined to be better competitors than those from limited land areas, so when the two come to¬ gether and compete, the northern species tend to displace the southern rather than vice versa. Hooker wrote a famous review of the Geographie Botanique in which he defended transmutation, an idea de Candolle vigor¬ ously opposed. By then Hooker had been let in on Darwin’s thinking. Though he was skeptical about the theory, he acknowl¬ edged that it was an intriguing hypothesis worthy of consider¬ ation. Darwin commented to Hooker: “I have read half your Review & like it very much. D.C. ought to be very much pleased; but I suppose the sugar is at the top & the sour at the bottom” (Correspondence6: 203).

THE ANNOTATED ORIGIN

380 1 This passage, too, gives insight into Darwin’s thinking. Islands

also have limited land areas; recall from chapter IV (see pp. 81, 105-107) his explanation for why species on islands are so easily outcompeted and displaced by invaders from continental areas: small land area means smaller population, less variability, and poorer competitiveness.

His explanation

for why we see

northern-affinity species on the mountains of the tropics turns on precisely this point. “A mountain is an island on the land,” and so the endemic alpine species of the tropical mountains, limited in land area and so poor competitors, have been dis¬ placed by northern-affinity rather than southern-affinity spe¬ cies.

THE ANNOTATED ORIGIN

GEOGRAPHICAL DISTRIBUTION,

Chap. XI,

that very many European productions cover the ground in La Plata, and in a lesser degree in Australia, and have to a certain extent beaten the natives; whereas extremely few southern forms have become naturalised in any part of Europe, though hides, wool, and other objects likely to carry seeds have been largely im¬ ported into Europe during the last two or three cen¬ turies from La Plata, and during the last thirty or forty years from Australia. Something of the same kind must have occurred on the iutertropical mountains: no doubt before the Glacial period they were stocked with endemic Alpine forms ; but these have almost eveiywhere largely yielded to the more dominant forms, generated in the larger areas and more efficient work¬ shops of the north. In many islands the native pro¬ ductions are nearly equalled or even outnumbered by the naturalised ; and if the natives have not been actu¬ ally exterminated, their numbers have been greatly reduced, and this is the first stage towards extinction. A mountain is an island on the land; and the intertropical mountains before the Glacial period must have been completely isolated; and I believe that the pro¬ ductions of these islands on the land yielded to those produced within the larger areas of the north, just in the same way as the productions of real islands have everywhere lately yielded to continental forms, natu¬ ralised by man’s agency. I am far from supposing that all difficulties are re¬ moved on the view here given in regard to the range and affinities of the allied species which live in the northern and southern temperate zones and on the mountains of the iutertropical regions. Veiy nianv difficulties remain to be solved. I do not pretend to indicate the exact lines and means of migration, or the reason why certain species and not others have migrated;

Chap. XI.

DUHING THE GLACIAL PERIOD.

381

why certain species have been modified and have given rise to new groups of foirnis, and others have remained unaltered. AVe cannot hope to explain such facts, until we can say why one species and not another be¬ comes naturalised by man’s agency in a foreign land; why one ranges twice or thrice as far, and is twice or thrice as common, as another species wdthin their own homes. I have said that many difficulties remain to be solved : -4 some of the most remarkable are stated wdth admirable clearness by Dr. Hooker in his botanical w'orks on the antarctic regions. These cannot be here discussed. I will only say that as far as regards the occurrence of identical species at points so enormously remote as Kerguelen Land, New Zealand, and Fuegia, I believe that tow^ards the close of the Glacial period, icebergs, as suggested by Lyell, have been largely concerned in their dispersal. But the existence of several quite distinct species, belonging to genera exclusively confined to the south, at these and other distant points of the southern hemisphere, is, on my theory of descent with modification, a far more remarkable case of difficulty. For some of these species are so distinct, that we cannot suppose that there has been time since the commence¬ ment of the Glacial period for their migration, and for their subsequent modification to the necessary degree. The facts seem to me to indicate that pe- 4 culiar and very distinct species have migrated in radi ating lines from some common centre ; and I am in¬ clined to look in the southern, as in the northern hemi¬ sphere, to a former and wanner period, before the com¬ mencement of the Glacial period, w'hen the antaictic lauds, now covered with ice, supported a highly peculiar and isolated flora. I suspect that before this flora was exterminated by the Glacial epoch, a few forms were

1

Botanical geography of the southern hemisphere was given

its first detailed treatment by Hooker in Flora Antarctica (18441847). Two facts about plant distribution in the far south had to be explained. On the one hand, identical species exist in widely separated areas, and on the other, sets of species—even genera— in these same areas are exclusively southern. The regions Dar¬ win mentions are circum-Antarctic: Tierra del Fuego at the tip of South America, New Zealand, and Kerguelen Land (a large volcanic island in the south Indian Ocean, discovered in 1772 by the French explorer Yves Joseph de Kerguelen-Tremarec, who named it Desolation Island). Darwin repeats his reliance on icebergs to explain the first ob¬ servation, whereas Hooker was an adherent of the idea that an extensive land mass linked South America, Australia, New Zea¬ land, and several islands in the southern oceans. The second ob¬ servation is a real puzzle. Why should a sizable number of plant species, genera, and even families be found exclusively in the southern hemisphere, with no affinity to northern forms? This suggests transmutation over vast time periods, yet the northto-south plant migration during the glacial epoch that Darwin proposed in this chapter is a comparatively recent phenomenon, so the flora would not have nearly enough time post-glacial period to become so strikingly differentiated. In Natural Selec¬ tion Darwin refers to this mystery as the most “extraordinary [case] as yet known” concerning the distribution of plants in the southern hemisphere (Stauffer 1975, p. 560). 2 To explain the unique southern flora, Darwin posited an an¬

cient warm period in which forests covered the Antarctic region. The onset of the glacial period would have exterminated much of this flora, but vestiges of it are seen in the circum-Antarctic distribution: “According to all analogy,” he wrote, “this Antarctic vegetation from its isolation would have been very peculiar, but would have been in some degree related to that of the two near¬ est continents, America and Australia” (Stauffer 1975, p. 579). It would have delighted Darwin to learn that Antarctica once sup¬ ported an extensive flora and fauna related to those of South America and Australia (Riffenburgh 2007). The explanation, (continued)

THE ANNOTATED ORIGIN

382

however, would have amazed him: these continents were once united as part of the Pangean supercontinent. The “continental extension” school Darwin railed against was a bit closer to the truth than were the strict dispersalists. In 1851 Hooker told Dar¬ win, “I am becoming slowly more convinced of the probability of the southern flora being a fragmentary one—all that remains envisioned a continent spanning the south polar region, one that upon its disintegration and subsidence left fragments of land scattered throughout the region. In modern understand¬ ing, there was no land extension linking the continents of the southern hemisphere; rather, the continents themselves were adjoined and through the mechanism of plate tectonics gradu¬ ally moved apart. 1 The passage Darwin mentions here may come from chapter

43 of the eighth (1850) edition of Principles of Geology; note Lyell’s reference to the “great cycle of climate”: It will follow ... that as often as the climates of the globe are pass¬ ing from the extreme of heat to that of cold—from the summer to the winter of the great year . . . the migratory movement [of spe¬ cies] will be directed constantly from the poles towards the equa¬ tor ... But when, on the contrary, a series of changes in the physi¬ cal geography of the globe, or any other supposed cause, occasions an elevation of the general temperature,—when there is a passage from the winter to one of the vernal or summer seasons of the great cycle of climate,—then the order of the migratory move¬ ment is inverted. The different species of animals and plants direct their course from the equator towards the poles.

2 This is Darwin at his lyrical best. The image of an ebb and flow of whole communities of species in response to climatic changes captures his grand scenario for worldwide species dis¬ tributions, especially the anomalous northern-affinity species “stranded” in the southern hemisphere and atop the loftiest mountains. Darwin’s analogy with the “savage races of man” able to hang on only in remote mountain fastnesses is very much of his colonialist time. His analogy makes the point that these remnant populations provide a clue to the past.

THE ANNOTATED ORIGIN

Chap. XI.

widely dispersed to various points of the southern hemisphere by occasional means of transport, and by the aid, as halting-places, of existing and now sunken islands, and perhaps at the commencement of the Glacial period, by icebergs. By these means, as I be¬ lieve, the southern shores of America, Australia, New Zealand have become slightly tinted by the same pecu¬

(continued)

of a great Southern Continent” (Correspondence 5: 67). Hooker

GEOGRAPHICAL DISTRIBUTION.

►

liar forms of vegetable life. Sir C. Lyell in a striking passage has speculated, in language almost identical with mine, on the effects of great alternations of climate on geographical distri¬ bution. I believe that the world has recently felt one of his great cycles of change; and that on this view, combined with modification through natural selection, a multitude of facts in the present distribution both of the same and of allied forms of bfe can be ex¬ plained. The living waters may be said to have flowed during one short period from the north and from the south, and to have crossed at the equator; but to have flowed with greater force from the north so as to have freely inundated the south. As the tide leaves its drift in horizontal lines, though rising higher on the shores where the tide rises highest, so have the living waters left their living drift on our mountainsummits, in a line gently rising from the arctic low¬ lands to a great height under the equator. The various beings thus left stranded may be compared with savage races of man, driven up and surviving in the mountainfastnesses of almost every land, w'hich serve as a record, full of interest to us, of the former inhabitants of the surrounding lowlands.

Chap. XU.

FRESH-WATER PRODUCTIONS.

383 The previous chapter addressed the apparent anomalies of geo¬ graphical dispersion, offering scenarios of migration during ice ages and mechanisms of long-distance dispersal in answer to the difficulties such peculiarities present. Much of this chap¬

CHAPTEE XIL

ter, in contrast, presents biogeographical evidence in support of common descent. Patterns of relationship among island flora

Geographical Distribution—continued.

and fauna with respect to species of the nearest mainland, Distribution of fresli-water productions — On tbe inhabitants of

as well as the peculiarities of island species themselves, are all

oceanic islands — Absence of Batracliians and of terrestrial Mam¬ mals — On the relation of the inhabitants of islands to those of

strongly suggestive of transmutation in splendid isolation. As early as 1836, musing on the distribution and relationships of

the nearest mainland— On colonisation from the nearest source with subsequent modification — Summary of the last and pre¬

the curious birds and tortoises of the Galapagos, Darwin noted

sent chapters.

As lakes and river-systems are separated from each other by barriers of land, it might have been thought that fresh-water productions would not have ranged widely within the same country, and as the sea is ap¬ parently a still more impassable barrier, that they never would have extended to distant countries. But the case is exactly the reverse. Not only have many fresh-water species, belonging to quite different classes, an enormous range, but allied species prevail in a remarkable manner thi’ougliout the w^orld. I well re¬ member, when first collecting in the fresh waters of Brazil, feeling much surprise at the similarity of the fresh-water insects, shells, &c., and at the dissimilarity of the surrounding terrestrial beings, compared with those of Britain. But this power in fresh-water productions of ranging widely, though so unexpected, can, I think, in most cases be explained by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream; and liability to wide dispersal would follow from this capacity as an almost necessary consequence. ^Ve can here consider only a few cases. In regard to

in his diary that “the zoology of Archipelagoes will be well worth examining; for such facts [would] undermine the stabil¬ ity of Species” (see Barlow 1963). Bear in mind that Darwin’s thinking about the importance of spatial isolation changed in important ways over time. In the transmutation notebooks of the late 1830s through the Essay of 1844, he placed great emphasis on geographical separation in the formation of new species, but his discovery of the princi¬ ple of divergence (see chapter IV) in 1852 led him to place in¬ creasing emphasis on ecological and what he termed “partial” isolation—forms of isolation more compatible with his idea of the importance of large continuous areas. It is sometimes as¬

-4

serted that Darwin abandoned geographical isolation as a fac¬ tor in speciation, but he did not so much drop isolation as re¬ fine it by supplementing ecological with geographical barriers. He always maintained that complete isolation was necessary for the development of reproductive incompatibility, which is the point of no return with regard to species distinctness (Sulloway 1979). Still, it is true that spatial isolation, which is con¬ sidered essential today, took on secondary importance in Dar¬ win’s thinking.

1 This similarity may be superficial, reflecting the general mor¬

phological conservatism of these invertebrate groups.

THE ANNOTATED ORIGIN

384 1 Another mechanism to consider is stream capture, as drain¬

ages evolve by erosion. 2 “Inosculation” is a term coined in connection with plants, re¬

ferring to the merging or growing together of different individ¬ uals. Here Darwin uses the term in a similar vein to describe the merging of river systems. Elsewhere, however, he and others in his time used the word in a different sense. William Sharp Macleay referred to “inosculating” (merging) species in describing his ill-fated Quinarian System of classification. Darwin some¬ times used “inosculate” in the sense of one species changing into another or passing from one form to another.

THE ANNOTATED ORIGIN

ts

GEOGRAPHICAL DISTRIBUTION.

Chap. XII.

fish, I believe that the same species never occur in the fresh waters of distant continents. But on the same continent the species often range widely and almost capriciously; for two river-systems will have some fish in common and some different. A few facts seem to favour the possibility of their occasional trans¬ port by accidental means; like that of the live fish not rarely dropped by whirlwinds in India, and the vitality of their ova when removed from the water. But I am inclined to attribute the dispersal of fresh-water fish mainly to slight changes within the recent period in the level of the land, having caused rivers to flow into each other. Instances, also, could be given of this having occurred during floods, without any change of level. We have evidence in the loess of the Rhine of considerable changes of level in the land within a veiy recent geological period, and when the surface wus peopled by existing laud and fresh-water shells. The wide difference of the fish on opposite sides of con¬ tinuous mountain-ranges, which from an early period must have parted river-systems and completely pre¬ vented their inosculation, seems to lead to this same conclusion. With respect to allied fresh-water fish occurring at very distant points of the world, no doubt there are many cases which cannot at present be ex¬ plained: but some fresh-water fish belong to very ancient forms, and in such cases there will have been ample time for great geographical changes, and con¬ sequently time and means for much migration. In the second place, salt-water fish can with care be slowly accustomed to live in fresh water; and, according to Valenciennes, there is hardly a single group of fishes confined exclusively to fresh water, so that we may imagine that a marme member of a fresh-water group might travel far along the shores of the sea, and subse¬

Chap. XII.

FEESH-WATER PRODUCTIONS.

385

quently become modified and adapted to the fresh waters of a distant land. Some species of fresh-water shells have a very wide range, and allied species, which, on my theoiy, are de¬ scended from a common parent and must have proceeded from a single source, prevail throughout the world. Their distribution at first perplexed me much, as their ova are not likely to be transported by birds, and they are immediately killed by sea water, as are the adults. I could not even understand how some naturalised species have rapidly spread throughout the same country. But two facts, wliicli I have observed—and no doubt many others remain to be observed—throw some light on this subject. Wlien a duck suddenly emerges from a pond covered with duck-weed, I have twice seen these little plants adhering to its back; and it has happened to me, in removing a little duck¬ weed from one aquarium to another, that I have quite unintentionally stocked the one with fresh-water shells from the other. But another agency is perhaps more effectual: I suspended a duck’s feet, which might -

CLASSIFICATION.

Chap. XIII.

have inherited to a certain extent their characters. This natural arrangement is shown, as far as is possible on paper, in the diagram, but in much too simple a manner. If a branching diagram had not been used, and only the names of the groups had been written in a linear series, it would have been still less possible to have given a natural arrangement; and it is notoriously not possible to represent in a series, on a flat surface, the affinities which we discover in nature amongst the beings of the same group. Thus, on the view which I hold, the natural system is genealogical in its arrange¬ ment, like a pedigree ; but the degrees of modification which the different groups have undergone, have to be expressed by ranking them under different so-called genera, sub - families, families, sections, orders, and classes. It may be worth while to illustrate this view of classi¬ fication, by taking the case of languages. If we pos¬ sessed a perfect pedigree of mankind, a genealogical arrangement of the races of man would afford the best classification of the various languages now spoken throughout the world ; and if all extinct languages, and all intermediate and slowly changing dialects, had to be included, such an arrangement would, I think, be the only possible one. Yet it might be that some very ancient language had altered little, and had given rise to few new languages, whilst others (owing to the spreading and subsequent isolation and states of civilisa¬ tion of the several races, descended from a common race) had altered much, and had given rise to many new languages and dialects. The various degrees of differ¬ ence in the languages from the same stock, would have to be expressed by groups subordinate to groups; but the proper or even only possible arrangement would still be genealogical; and this would be strictly natural, as

Chap, XIII.

CLASSIFICATION.

423

it would connect together all languages, extinct and modern, by the closest affinities, and would give the filiation and origin of each tongue. In confirmation of this view, let us glance at the classification of varieties, which are believed or known to have descended from one species. These are grouped under species, with sub-varieties under varieties; and with om- domestic productions, several -4 other grades of difference are requisite, as we have seen with pigeons. The origin of the existence of groups subordinate to groups, is the same with varieties as with species, namely, closeness of descent with various degrees of modification. Nearly the same rules are fol¬ lowed in classifying varieties, as with species. Authors have insisted on the necessity of classing varieties on a natural instead of an artificial system; we are cau¬ tioned, for instance, not to class two varieties of the pine-apple together, merely because their fruit, thougli the most important part, happens to be nearly identical; no one puts the Swedish and common turnips together, though the esculent and thickened stems are so similar. Whatever part is found to be most constant, is used in classing varieties: thus the great agriculturist Marshall says the horns are very useful for this puiqiose with cattle, because they are less variable than the shape or colour of the body, &c.; whereas with sheep the horns are much less serviceable, because less con¬ stant. In classing varieties, I apprehend if we had a real pedigree, a genealogical classification would be universally preferred; and it has been attempted by some authors. For we might feel sm-e, whether there had been more or less modification, the principle of inheritance would keep the forms together which were allied in the greatest number of points. In tumbler pigeons, though some sub-varieties differ from the others

1 The production of domesticated varieties by artificial selec¬

tion provided a potent analogy for natural selection, as we saw in chapter I. In this paragraph that analogy is continued: people have intuitively classified domesticated varieties genealogically, Darwin suggests, as opposed to grouping them by similarity of traits, which can arise by convergence. Turnips are a case in point. The Swedish turnip, or rutabaga (Brassica napobrassica), originated as a cross between cabbage (Brassica oleracea) and the common turnip (Brassica rapa). In a discussion of the ori¬ gins and variability of Brassica vegetables in the first volume of Variation, Darwin mentions turnip-like convergence among species: “In the production of large, fleshy, turnip-like stems, we have a case of analogous variation in three forms which are gen¬ erally considered as distinct species. But scarcely any modifica¬ tion seems so easily acquired as a succulent enlargement of the stem or root—that is, a store of nutriment laid up for the plant’s own future use” (Darwin 1883, ch. IX). 2 Darwin read several books by William Marshall. A discussion

of horns is given in a book Darwin did not record in his reading list, but it nicely expresses Marshall’s views on the use of horns in identifying cattle breeds: The doctrine of horns has long appeared to me ... as a craft con¬ venient to leading-breeders, in establishing their respective sys¬ tems. . . . The horn has been mentioned as a permanent specific character of cattle. Hence in varieties it may have its use as a crite¬ rion. Thus supposing a male and female of superior form and flesh, and with horns resembling each other (as nearly as the horns of males and females of the same variety naturally do), no matter whether short or long, sharp or clubbed, rising or falling; and sup¬ posing a variety to be established from this parentage, it is highly probable that the horns of the parents would continue for a while to be a characteristic of the true breed, and might by inferior judges be depended upon, in some degree, as a criterion. (Marshall 1788, pp. 189-190; emphases in original)

THE ANNOTATED ORIGIN

424 1 “Hottentot” was the name given to the pastoral Khoikhoi people of southwestern Africa by the European colonists; it is considered offensive today. The Khoikhoi are one of several eth¬ nically distinct southern African groups, along with the Nguni, Basotho, Zulu, Khoisan, and Xhosa. The Khoikhoi possess sev¬ eral distinctive physical characteristics, including steatopygia (significant fat deposits in and around the buttocks) and elon¬ gated labia minora. In Darwin’s day these features led natural¬ ists to identify the Khoikhoi as a distinct “race” and to speculate as to their relationship to the more widespread Bantu ethnolinguistic group, which white Europeans collectively referred to as “negro.” This is the context for the idea of the Hottentot “descend[ing] from the Negro.” (This sentence was dropped from the fifth and sixth editions.) 2 This is a curious line of argument: one demonstration that true affinity and not mere outward similarity is the guiding principle of classification is given by cases in which individuals differ morphologically (sexually dimorphic species, larval vs. adult forms, etc.) yet are recognized as members of the same species. This is also true of animals that alternate between mor¬ phological forms in their development—dubbed “alternation of generations” in 1845 by Johannes Japetus Steenstrup. Darwin is suggesting that these cases are, in essence, an intuitive acknowl¬ edgment of the primacy of descent, insofar as the distinctive morphs are by definition related by common ancestry. Alternation of generations also occurs in plants, as do cases of such strikingly different flower morphs that early naturalists as¬ sumed they belonged to different taxa. The case of the orchids given near the bottom of this page is interesting because Darwin played a role in establishing that these three “genera” are really floral morphs of a single species. In 1854 the English explorer Sir Robert Schomburgk reported finding flowers of all three oc¬ curring on a single plant! The name Catasetum had taxonomic priority, so the others became synonyms of that genus. Darwin became keenly interested in this floral polymorphism, and in 1861 (the same year his orchid book came out) he read a paper to the Linnean Society announcing that the three morphs were in fact male, female, and hermaphroditic flowers.

THE ANNOTATED ORIGIN

►

CLASSIFICATION.

Chap. XIII,

in the important character of having a longer beak, yet all are kept together from having the common habit of tumbling; but the short-faced breed has nearly or quite lost this habit; nevertheless, without any reasoning or thinking on the subject, these tumblers are kept in the same group, because allied in blood and alike in some other respects. If it could be proved that the Hottentot had descended from the Negro, I think he would be classed under the Negro group, however much he might differ in colour and other important characters from negroes. With species in a state of nature, every naturalist has in fact brought descent into his classification; for he includes in his lowest grade, or that of a species, the two sexes; and how enormously these sometimes differ in the most important charactei’s, is known to every naturalist; scarcely a single fact can be predicated in common of the males and hermaphrodites of certain cirripedes, when adult, and yet no one dreams of sepa¬ rating them. The naturalist includes as one species the several larval stages of the same individual, however much they may differ from each other and from the adult; as he likewise includes the so- called alternate generations of Steenstrup, which can only in a technical sense be considered as the same individual. He in¬ cludes monsters; he includes varieties, not solely be¬ cause they closely resemble the parent-form, but because they are descended from it. He who believes that the cowslip is descended from the primrose, or conversely, ranks them together as a single species, and gives a single definition. As soon as three Orchidean forms (Monochanthus, Myanthus, and Catasetum), wliich had previously been ranked as three distinct genera, were known to be sometimes produced on the same spike, they were immediately included as a single species.

Chap. XIII.

CLASSIFICATION,

425

But it may be asked, what ought we to do, if it could be proved that one species of kangaroo had been produced, by a long course of modification, from a bear ? Ought we to rank this one species with bears, and what should we do with the other species? The supposition is of course preposterous; and I might answer by the argumentum ad hominemy and ask what should be done if a perfect kangaroo were seen to come out of the womb of a bear ? According to all analogy, it would be ranked with bears; but then assuredly all the other species of the kangaroo family would have to be classed under the bear genus. The whole case is preposterous; for where there has been close descent in common, there will certainly be close resemblance or affinity. As descent has universally been used in classing to¬ gether the individuals of the same species, though the males and females and larvse are sometimes extremely difiTerent; and as it has been used in classing varieties which have undergone a certain, and sometimes a con¬ siderable amount of modification, may not this same element of descent have been unconsciously used in grouping species under genera, and genera under higher groups, though in these cases the modification has been greater in degree, and has taken a longer time to com¬ plete ? I believe it has thus been unconsciously used; and only thus can I understand the several rules and guides which have been followed by our best systematists. We have no written pedigrees; we have to make out community of descent by resemblances of any kind. Therefore we choose those characters which, as far as we can judge, are the least likely to have been modified in relation to the conditions of life to which each species has been recently exposed. Rudimentary structures on tliis view are as good as, or even some¬ times better than, other parts of the organisation. We

--

century. The idea originated with the German naturalist Lorenz Oken, who presented his theory in an address entitled “Signifi¬ cation of the bones of the skull” in 1807. Owen later champi¬ oned the idea in his book On the Archetype and Homologies of the Vertebrate Skeleton (London, 1848).

^

3 Horticulturists were familiar with “monstrous plants,” or sports (e.g., cases in which a leaf develops where a petal or thorn should be). Sporting is so common that some later critics pro¬ posed a now-defunct theory of saltational evolution (from Latin saltus, “jumping”) based on mutations. The most famous ex¬ ponent of this idea was the Dutch botanist Hugo de Vries (1848-1935), author of Species and Varieties, Their Origin by Mutation (Chicago, 1905). De Vries thought that species arose not gradually but by radical shifts in morphology brought about by mutations. His treatise concludes with the memorable state¬ ment: “Natural selection may explain the survival of the fittest, but it cannot explain the arrival of the fittest.”

THE ANNOTATED ORIGIN

^

MOKPHOLOGY.

Chap. XIII.

suppose that their common progenitor had an upper lip, mandibles, and two pair of maxillte, these parts being perhaps very simple in form ; and then natural selection will account for the infinite diversity in structure and function of the mouths of insects. Nevertheless, it is couceivable that the general pattern of an organ might become so much obscured as to be finally lost, by the atrophy and ultimately by the complete abortion of cer¬ tain parts, by the soldering together of other parts, and by the doubling or multiplication of others,—variations which we know to be within the limits of possibility. In the paddles of the extinct gigantic sea-lizards, and in the mouths of certain suctorial crustaceans, the general pattern seems to have been thus to a certain extent obscured. There is another and equally curious branch of the present subject; namely, the comparison not of the same part in different members of a class, but of the different parts or organs in the same individual. Most physio¬ logists believe that the bones of the skull are homo¬ logous with—that is correspond in number and in re¬ lative connexion with—the elemental parts of a certain number of vertebrm. The anterior and posterior limbs in each member of the vertebrate and articulate classes are plainly homologous. We see the same law in comparing the wonderfully complex jaws and legs in crustaceans. It is familiar to almost every one, that in a flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intel¬ ligible on the view that they consist of metamorphosed leaves, arranged in a spire. In monstrous plants, we often get direct evidence of the possibility of one organ being transformed into another; and we can actually see in embryonic crustaceans and in many other ani¬ mals, and in flowers, that organs, which when mature

MORPHOLOGY.

become extremely different, are at an early stage of growth exactly alike. How inexplicable are these facts on the ordinary view of creation! Why should the brain be enclosed in a box composed of such numerous and such extra¬ ordinarily shaped pieces of bone? As Owen has re¬ marked, the benefit derived from the yielding of the separate pieces in the act of parturition of mammals, will by no means explain the same construction in the'skulls of birds. Why should similar bones have been created in the formation of the wing and leg of a bat, used as they are for such totally different purposes? Why should one crustacean, which has an extremely complex mouth formed of many parts, consequently always have fewer legs; or conversely, those with many legs have simpler mouths? Why should the sepals, petals, sta¬ mens, and pistils in any individual flower, though fitted for such widely different purposes, be all constructed on the same pattern ? On the theory of natural selection, we can satisfactorily answer these questions. In the vertebrata, we see a series of inteiTial vertebrae bearing certain processes and appen¬ dages ; in the articulata, we see the body divided into a series of segments, bearing external appendages ; and in flowering plants, we see a series of successive spiral whorls of leaves. An indefinite repetition of the same part or organ is the common characteristic (as Owen has observed) of all low or little-modified forms; there¬ fore we may readily believe that the unknown progenitor of the vertebrata possessed many vertebra?; the unknown progenitor of the articulata, many segments; and the unknown progenitor of flowering plants, many spiral whorls of leaves. We have formerly seen that parts many times repeated are eminently liable to vary in number and structure; consequently it is quite probable that

1

In this lyrical passage Darwin touches on an apparent diffi¬

culty for those inclined to see evidence of design in organisms. The skull sutures of mammals—perhaps especially humans— might be seen as a sign of benevolent design, permitting the compression of the skull during birth. As even Owen was aware, however, this feature could not have been designed for human parturition since such sutures are also found in birds and rep¬ tiles, which are born from eggs. Darwin refers to Owen’s remark in his treatise On the Archetype and Homologies of the Vertebrate Skeleton: We may admit that the multiplied points of ossification in the skull of the human foetus facilitate, and were designed to facilitate, childbirth; yet something more than such a final purpose lies be¬ neath the fact, that most of those osseous centres represent perma¬ nently distinct bones in the cold-blooded vertebrates. The cra¬ nium of the bird, which is composed in the adult of a single bone, is ossified from the same number of points as in the human em¬ bryo, without the possibility of a similar purpose being subserved thereby, in the extrication of the chick from the fractured egg¬ shell. (Owen 1848b, p. 73)

Owen also considered the homology of repeated flower parts, writing in 1855 of the “law of vegetative or irrelative repetition,” which explained the “multiplication of organs performing the same function.” Darwin heavily annotated this and other works by Owen.

THE ANNOTATED ORIGIN

438 1 Many mollusks are so modified that identifying homologies is very difficult. Bivalves and gastropods posed a special chal¬ lenge: bivalves (clams, mussels, etc.) have no obvious head and tail end, and gastropods (snail, slugs, etc.) exhibit torsion in de¬ velopment, which involves a 180° twisting of the mantle and vis¬ cera such that the posterior end becomes situated above the head facing forward. Darwin found them perplexing. Huxley sent him a copy of an article he wrote attempting to establish molluskan homologies; Darwin wrote back in gratitude; “Many thanks for your paper ... I have read it with attention & interest, for I had often wondered how a gasteropod [sic], a bivalve, & cephalopod [could] be brought to same type.... An acephalous Mollusc has always looked to me a complete mystery, & I really know no more about it, than a man does, who has only eat oys¬ ter patties; the relation of the animal to the shell & crust being about the same in my eyes” (Correspondence 5: 281). 2 We saw on p. 436 that Owen, after Goethe and Oken, argued for the homology of cranial plates and vertebrae. Huxley, who squabbled endlessly with Owen, took issue with the idea and at¬ tacked it at length in his Croonian Lecture On The Theory of the Vertebrate Skull. Huxley argued on the basis of embryology that the two were not homologous. Owen’s philosophy, Huxley said, was “introduc[ing] the phraseology and mode of thought of an obsolete and scholastic realism into biology.” He continued: “The fallacy involved in the vertebral theory of the skull is like that which . . . infested our notions of the relations between fishes and mammals. The mammal was imagined to be a modi¬ fied fish, whereas, in truth, fish and mammal start from a com¬ mon point, and each follows its own road thence” (Huxley 1858, p. 432). This is what Darwin is getting at when he speaks of skull and vertebrae having been metamorphosed from a “common element.”

THE ANNOTATED ORIGIN

MOEPHOLOGY.

Cjiap. XIII.

natural selection, during a long-continued course of mod ification, should have seized on a certain number of the primordially simdar elements, many times repeated, and have adapted them to the most diverse purposes. And as the wliole amount of modification will have been effected by slight successive steps, we need not wonder at discovering in such parts or organs, a certain degree of fundamental resemblance, retained by the strong principle of inheritance. In the great class of molluscs, though we can homologise the parts of one species with those of another and distinct species, we can indicate but few serial homo¬ logies ; that is, we are seldom enabled to say that one part or organ is homologous with another in the same individual. And we can understand this fact; for in molluscs, even in the lowest members of the class, we do not find nearly so much indefinite repetition of any one part, as we find in the other great classes of the ani¬ mal and vegetable kingdoms. Naturalists frequently speak of the skull as formed of metamorphosed vertebrm : the jaws of crabs as meta¬ morphosed legs; the stamens and pistils of flowers as metamorphosed leaves; but it would in these cases pro¬ bably be more correct, as Professor Huxley has remarked, to speak of both skull and vertebrae, both jaws and legs, &c.,—as having been metamorphosed, not one from the other, but from some common element. Naturalists, however, use such language only in a metaphorical sense; they are far from meaning that during a long course of descent, primordial organs of any kind—verte¬ brae in the one case and legs in the other—have actually been modified into skulls or jaws. Yet so strong is the appearance of a modification of this nature having oc¬ curred, that naturalists can hardly avoid employing language having this plain signification. On ray view

Chap. XIII.

EMBRYOLOGY.

439

these terms may be used literally; and the wonderful fact of the jaws, for instance, of a crab retaining nume¬ rous characters, which they would probably have retained through inheritance, if they had really been metamor¬ phosed during a long course of descent from true legs, or from some simple appendage, is explained.

1 Darwin may have singled out the crab example because crabs exhibit a series of appendages from walking legs to mouthparts, making the homology of mouthparts with walking legs espe¬ cially obvious. Decapods, the order of crabs and lobsters, have an eight-segmented thorax. The first three thoracic segments (thoracomeres) are fused with the head segments to form a cephalothorax. Anterior to the thoracomeres, on the head proper,

Embryology.—It has already been casually remarked that certain organs in the individual, which when mature become widely different and serve for different purposes, are in the embryo exactly alike. The embryos, also, of distinct animals within the same class are often strikingly similar: a better proof of this cannot be given, than a circumstance mentioned by Agassiz, namely, that having forgotten to ticket the embryo of some vertebrate ani¬ mal, he cannot now tell whether it be that of a mammal, bird, or reptile. The vermiform larvae of moths, flies, beetles, &c., resemble each other much more closely than do the mature insects; but in the case of larvae, the embryos are active, and have been adapted for spe¬ cial lines of life. A trace of the law of embryonic re¬ semblance, sometimes lasts till a rather late age; thus birds of the same genus, and of closely allied genera, often resemble each other in their first and second plumage; as we see in the spotted feathers in the thrush group. In the cat tribe, most of the species are striped or spotted in lines; and stripes can be plainly distingui.shed in the whelp of the lion. We occasion¬ ally though rarely see something of this kind in plants: thus the embryonic leaves of the ulex or furze, and the first leaves of the phyllodineous acaceas, are pinnate or divided like the ordinary leaves of the leguminosae. The points of structure, in which the embryos of widely different animals of the same class resemble each other, often have no direct relation to their condi-

are the mandibles and maxillae. Posterior to the thoracomeres are the five independent thoracic segments. All segments of the thorax have a pair of appendages; the thoracomeres each bear maxillipeds, and the independent segments each bear walking legs (the name Decapoda, “ten feet,” alludes to these five pairs of legs). The maxillipeds, then, are mouthparts that are literally

■

EMBRYOLOGY.

Chap. XIII.

young. But there was one remarkable exception to this rule, for the young of the short-faced tumbler differed from the young of the wild rock-pigeon and of the other breeds, in all its proportions, almost exactly as much as in the adult state. The two principles above given seem to me to explain these facts in regard to the later embryonic stages of our domestic varieties. Fanciers select their horses, dogs, and pigeons, for breeding, when they are nearly grown up ; they are indifferent whether the desired qualities and structures have been acquired earlier or later in life, if the full-grovui animal possesses them. And the cases just given, more especially that of pigeons, seem to show that the characteristic differences which give value to each breed, and which have been accumulated by man’s selection, have not generally first appeared at an early period of life, and have been in¬ herited by the offspring at a corresponding not earlv period. But the case of the short-faced tumbler, vdiich when twelve hours old had acquired its proper propor¬ tions, proves that this is not the universal rule; for here the characteristic differences must either have appeared at an earlier period than usual, or, if not so, the differ¬ ences must have been inherited, not at the corresponding, but at an earlier age. Now let us apply these facts and the above two principles—which latter, though not proved true, can be shown to be in some degree probable—to species in a state of nature. Let us take a genus of birds, descended on my theory from some one parent-species, and of which the several new species have become modified through natural selection in accordance with their diverse habits. Then, from the many slight suc¬ cessive steps of variation having supervened at a rather late age, and having been inherited at a corresponding

Chap. XIII.

EMBRYOLOGY.

447

age, the young of the new species of our supposed genus will manifestly tend to resemble each other much more closely than do the adults, just as we have seen in the case of pigeons. We may extend this view to whole families or even classes. The fore-limbs, for instance, which served as legs in the parent-species, may be¬ come, by a long course of modification, adapted in one descendant to act as hands, in another as paddles, in another as wings; and on the above two principles— namely of each successive modification supervening at a rather late age, and being inherited at a corre¬ sponding late age—the fore-limbs in the embryos of the several descendants of the parent-species will still resemble each other closely, for they will not have been modified. But in each individual new species, the embryonic fore-limbs will differ greatly from the fore-limbs in the mature animal; the limbs in the latter having undergone much modification at a I’ather late period of life, and having thus been con¬ verted into hands, or paddles, or wings. Whatever influence long-continued exercise or use on the one hand, and disuse on the other, may have in modi¬ fying an organ, such influence wall mainly affect the mature animal, which has come to its full powers of activity and has to gain its own living; and the effects thus produced will be inherited at a corresponding mature age. W^hereas the young will remain unmodified, or be modified in a lesser degree, by the effects of use and disuse. In certain cases the successive steps of variation might supervene, from causes of which we are wholly ignorant, at a very early period of life, or each step might be inherited at an earlier period than that at which it first appeared. In either case (as with the short-faced tumbler) the young or embryo w'ould closely

1 Shades of Lamarck: in this sentence we see Darwin explicitly

admit the possibility of inheritance of acquired characteristics. He suggests that use and disuse affect the adults more than the juveniles. Older animals are imagined to have had more time (and need) to accumulate modifications of some structure com¬ pared with young animals, so this is perhaps another reason the modifications appear later rather than earlier in development.

THb ANNOTATED ORIGIN

448 1

Darwin put his finger on the explanation that is accepted

today. Note the use of “causality” language so characteristic of Darwin’s century. Scientists no longer speak of final causation per se, but the nearest thing to it in the modern scientific lexicon is “ultimate” causation. The evolutionary biologist Ernst Mayr

►

first used the terms “ultimate” and “proximate” causation in the context of, respectively, evolutionary vs. more immediate mech¬ anistic explanations for biological phenomena (Mayr 1961). 2 This rather vague statement about further explanation was fi¬ nally dropped in the fourth edition. 3 There are many examples of “retrograde” development in in¬ sects, particularly in cases where the juveniles need to be mobile while the adults do not. The barnacle-like scale insects, fam¬ ily Coccidae, are a good example: these sap-sucking insects can form sizable encrustations on plants, and many are serious agri¬

^

cultural and ornamental plant pests. The immatures form the primary dispersal stage, while the sessile females develop a shell¬ like carapace that becomes cemented to the plant. Under the carapace the insects tap their proboscis into the plant. The eyes, wings, antennae, etc., of such females have been lost or are greatly reduced, while the immatures have the fully developed eyes and legs necessary for dispersal. Bagworm moths, family Psychidae, are another familiar example. Caterpillars of both sexes are active leaf feeders with fully developed sensory and locomotory appendages. The caterpillars construct structures of silk and leaf debris that they carry around with them—conve¬ nient shelters into which to withdraw in times of danger. The female caterpillar “degenerates” in metamorphosis to a grublike form devoid of all external organs and never leaves her bag. Winged males locate females via sex pheromones and mate with them from outside the structure. Which is “higher” or more complex? In traditional terms the question is moot.

^

EMBRYOLOGY.

Chap. XIII.

resemble the mature parent-form. We have seen that this is the rule of development in certain whole groups of animals, as with cuttle-fish and spiders, and with a few members of the great class of insects, as with Aphis. With re.spect to the final cause of the young in these cases not undergoing any metamorphosis, or closely resembling their parents from their earliest age, we can see that this would result from the two following contingencies; firstly, from the young, during a course of modification carried on for many generations, having to provide for their own w'auts at a very early stage of development, and secondly, from their following exactly the same habits of life with their parents ; for in this case, it would be indispen.sable for the existence of the species, that the child should be modified at a very early age in the same manner with its parents, in accordance with their similar habits. Some further explanation, however, of the embryo not undergoing any metamorphosis is perhaps requisite. If, on the other hand, it profited the young to follow habits of life in any degree different from those of their parent, and conse¬ quently to be constructed in a slightly different manner, then, on the principle of inheritance at corresponding ages, the active young or larvaj might easily be ren¬ dered by natural selection different to any conceivable extent from their parents. Such differences might, also, become correlated with successive stages of deve¬ lopment ; so that the larvm, in the first stage, might differ greatly from the larvse in the second stage, as w'e have seen to be the case with cirripedes. The adult might become fitted for sites or habits, in which organs of locomotion or of the senses, &c., would be useless; and in this case the final metamorphosis would be said to be retrograde. As all the organic beings, extinct and recent, which

THE ANNOTATED ORIGIN

Chap. XIII.

EMBRYOLOGY.

449

have ever lived on this earth have to be classed together, and as all have been connected by the finest gradations, the best, or indeed, if our collections were nearly perfect, the only possible arrangement, would be genealogical. Descent being on my view the hidden bond of con¬ nexion which naturalists have been seeking under the term of the natural system. On this view we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult. For the embryo is the animal in its less modified state ; and in so far it reveals the structure of its progenitor. In two groups of animal, however much they may at present differ from each other in structure and habits, if they pass through the same or similar embryonic stages, we may feel assured that they have both descended from the same or nearly similar parents, and are there¬ fore in that degree closely related. Thus, community in embryonic structure reveals community of descent. It will reveal this community of descent, however much the structure of the adult may have been modified and obscured ; we have seen, for instance, that cirripedes can at once be recognised by their larvre as belonging to the great class of crustaceans. As the embryonic state of each species and group of species partially shows us the structure of their less modified ancient progeni¬ tors, we can clearly see why ancient and extinct forms of life should resemble the embryos of their descend¬ ants,—our existing species. Agassiz believes this to be a law of nature; but I am bound to confess that I only hope to see the law hereafter proved true. It can be proved true in those cases alone in which the ancient state, now supposed to be represented in many embryos, has not been obliterated, either by the successive varia¬ tions in a long course of modification having super-

1 Although Darwin suggests here and elsewhere in the Origin that the best classification should be based on genealogy, he did not follow this approach in his work on barnacle taxonomy, de◄J

spite having fully worked out his theory by then. Perhaps do¬ ing so would have tipped his hand prematurely (Padian 1999). The important point here is that embryology helps reveal this genealogical arrangement—revealing more of affinities than do the adults, embryos provide a window into that “hidden bond of connexion” that is common descent. 2 Darwin’s embryology was essentially that of Karl Ernst von

Baer, who is credited with establishing the techniques and con¬ ceptual framework of modern embryology. Although von Baer was an adherent of the archetype concept, his recognition that development consists of embryonic differentiation from a com¬ mon form or body plan set the stage for naturalists like Darwin to see branching differentiation in an evolutionary context. 3 What does Darwin mean by ancient and extinct forms resem¬ bling the embryos of their descendants? He is referring to Agas¬ siz’s idea of a threefold parallelism among individual develop¬ ment (embryology), relationships within a type (phylogeny), and geological history (paleontology). In his work on fossil fish (1844), for example, Agassiz stated that “the successive creations [of life on earth] have passed through phases of development analogous to those through which the embryo passes during its growth.” He went on to argue that “the embryo of the fish during its development, the class of existing fishes in its numer¬ ous families, and the fish type in its planetary history traverse in all respects analogous phases” (quoted in Ospovat 1976). One could interpret parallelism in terms of common descent, as Dar¬ win did. This exciting idea was one of those Darwin shared with Lyell in 1856. “Darwin thinks that Agassiz’s embryology has something in it, or that the order of development in individuals 8c of similar types in time may be connected,” Lyell reported in his journal (Wilson 1970, p. 54).

THE ANNOTATED ORIGIN

450 1

Darwin is revolutionizing the science of embryology here!

The field had already shifted from viewing development in terms of recapitulation of a linear chain of being (in its most extreme form) to the idea of branching development from com¬ mon types. To strict recapitulationists of the early nineteenth century, the embryo literally manifests a sequence of organisms lower down in its series as it develops. They supposed that each stage was created, but to adherents of the archetype concept (Owen, Agassiz, et al.) this was dangerously close to transmuta-

(►

tionist thinking. They rejected strict recapitulation, arguing in¬ stead that the animal kingdom was divided into four completely separate types. Organisms showed fundamental affinity to their type in early development but bore no relationship to other types. These scientists advocated the embryology of von Baer, who argued in 1828 that, within types, embryonic development represents divergence from a common form—the archetype. Darwin rejects “chain of being” thinking and the archetype idea, though development from common forms is closer to his de¬ scent theory. He reinterpreted von Baerian embryology, pro¬

BUDIMENTAHY ORGAN'S.

Chap. XIII.

vened at a very earlv age, or by the variations having been inherited at an earlier period than that at which they first appeared. It should also be borne in mind, that the supposed law of resemblance of ancient forms of life to the embryonic stages of recent forms, may be true, but yet, owing to the geological record not extend¬ ing far enough back in time, may remain for a long period, or for ever, incapable of demonstration. Thus, as it seems to me, the leading facts in embrjmlogy, which are second in importance to none in natural history, are explained on the principle of slight modifi¬ cations not appearing, in the many descendants from some one ancient progenitor, at a very early period in the life of each, though perhaps caused at the earliest, and being inherited at a corresponding not early period. Embryology rises greatly in interest, when we thus look at the embryo as a picture, more or less obscured, of the common parent-form of each great class of animals.

phetically closing this section with the comment that embryol¬ ogy “rises greatly in interest” when seen in the light of descent (see Ospovat 1976). 2 All the examples of “rudimentary” organs given in this para¬ graph are fascinating. In the case of the “bastard-wing” (alula) of birds, Darwin predicted to Lyell in 1859 that the structure would be confirmed as a reduced digit if early bird fossils were found. He received exciting news in January 1863, when Fal¬ coner wrote to him about the discovery of a nearly complete fossil Archaeopteryx skeleton. Falconer suggested that it was just the sort of linking form Darwin had predicted. Darwin wrote back: “I much wish to hear ... which digits are developed; when examining birds two or three years ago, I distinctly remember writing to Lyell that some day a fossil bird would be found with ... the bastard wing and other part both well developed” (Cor¬ respondence 11; 11). Indeed, the alula is well developed in this and other dinosaur-bird transitional fossils.

THE ANNOTATED ORIGIN

^

Rudimentary, atrophied, or aborted organs.—Organs or parts in this strange condition, bearing the stamp of inutility, are extremely common throughout nature. For instance, rudimentary mammse are very general in the males of mammals : I presume that the “ bastard-wing ” in birds may be safely considered as a digit in a rudi¬ mentary state: in very many snakes one lobe of the lungs is rudimentary ; in other snakes there are rudiments of the pelvis and hind limbs. Some of the cases of rudi¬ mentary organs are extremely curious ; for instance, the presence of teeth in foetal whales, which when grown up have not a tooth in their heads; and the presence of teeth, which never cut through the gums, in the upper jaws of our unborn calves. It has even been stated on good authority that rudiments of teeth can be detected

Chap. XIII.

RUDIMENTARY ORGANS.

451

in the beaks of certain embryonic birds. Nothing can be plainer than that wings are formed for flight, yet in how many insects do we see wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing-cases, firmly soldered together! The meaning of rudimentary organs is often quite unmistakeable: for instance there are beetles of the same genus (and even of the same species) resembling each other most closely in all respects, one of which will have full-sized wings, and another mere rudiments of membrane; and here it is impossible to doubt, that the rudiments represent wings. Kudimentary organs some¬ times retain their potentiality, and are merely not deve¬ loped : this seems to be the case with the mammie of male mammals, for many instances are on record of these organs having become well developed in full-grown males, and having secreted milk. So again there are normally four developed and two rudimentary teats in the udders of the genus Bos, but in our domestic cows the two sometimes become developed and give milk. In individual plants of the same species the petals some¬ times occur as mere rudiments, and sometimes in a welldeveloped state. In plants with separated sexes, the male flowers often have a rudiment of a pistil; and Kolreuter found that by crossing such male plants with an hermaphrodite species, the rudiment of the pistil in the hybrid offspring was much increased in size ; and this shows that the rudiment and the perfect pistil are essentially abke in nature. An organ serving for two purposes, may become rudimentary or utterly aborted for one, even the more important purpose; and remain perfectly efficient for the other. Thus in plants, the office of the pistil is to allow the pollen-tubes to reach the ovules protected in the ovarium at its base. The pistil consists of a stigma

1 The “unmistakeable” meaning of rudimentary organs is that

they are in fact derived from functional versions of the same or¬ gan. The cases given in this paragraph demonstrate variability in these organs, a necessary ingredient if they are to become gradually modified. Note that in some cases the organs vary be¬ tween species of the same genus, and in others they vary among individuals of the same species. The significance of rudimentary organs did not escape the notice of Alfred Russel Wallace. In his Sarawak Law paper, Wal¬ lace wrote: Another important series of facts, quite in accordance with, and even necessary deductions from, the law now developed, are those of rudimentary organs ... To every thoughtful naturalist the ques¬ tion must arise. What are these for? What have they to do with the great laws of creation? Do they not teach us something of the sys¬ tem of Nature? If each species has been created independently, and without any necessary relations with pre-existing species, what do these rudiments, these apparent imperfections mean? There must be a cause for them; they must be the necessary results of some great natural law. (Wallace 1855, p. 195)

2 Today it is thought that gene families evolve in precisely the

same way. Genes can become duplicated through various mech¬ anisms (e.g., unequal crossing over, gene conversion, etc.), and once redundancy occurs, the extra copies may be modified by natural selection. Globin genes are a good example of this. There are five alpha-globin gene loci (plus two non-functional— vestigial!—pseudogenes) on chromosome 16. Chromosome 11

^

bears six beta-globin loci plus one pseudogene. The related myoglobin gene is found on chromosome 22 (Ohta 2003).

THE ANNOTATED ORIGIN

452 1 “Compositae” is an obsolete botanical name now replaced by

family Asteraceae. These plants, which include daisies, asters, and sunflowers, are stUl called composites, however, because the “flower” is actually a composite of many tiny florets. These are often differentiated into disk florets with undeveloped petals in the center, and ray florets with one long petal along the margin. Collectively, the ray florets give the appearance of a single flower encircled by petals. Darwin undertook a lengthy study of flower form and male-female flower differentiation that was published as a book in 1877. 2 Darwin’s point is well taken here, but the modern under¬ standing of swimbladders and their relationship to lungs differs from his. This topic was discussed on p. 190; see also essay seven in Gould (1993). 3 The discovery of rudimentary teeth in the dental grooves of fetal baleen whales (Balaenidae) is attributed to Geoffroy St. Hilaire but was described most fully after the Origin was pub¬ lished, notably by the Danish cetologists D. F. Eschricht and J. Reinhart, who published a monograph on the right whale (then called the Greenland whale) in 1861. Baleen whales were known as “whalebone whales,” an ironic name since “whale¬ bone” (baleen) is not bone tissue but keratin, the same material as hair and nails.

THE ANNOTATED ORIGIN

►

EUDIMENTARY ORGANS.

Chap. XIII.

supported on tlie style; but in some Compositte, the male florets, which of course cannot be fecundated, have a pistil, which is in a rudimentary state, for it is not crowned with a stigma; but the style remains well de¬ veloped, and is clothed with hairs as in other compo¬ sitae, for the purpose of brushing the pollen out of the surrounding anthers. Again, an organ may become rudimentary for its proper purpose, and be used for a distinct object: in certain fish the swim-bladder seems to be rudimentary for its proper function of giving buoy¬ ancy, but has become converted into a nascent breathing organ or lung. Other similar instances could be given. Rudimentary organs in the individuals of the same species are very liable to vary in degree of development and in other respects. Moreover, in closely allied species, the degree to which the same organ has been rendered rudimentary occasionally differs much. This latter fact is well exemplified in the state of the wings of the female moths in certain groups. Eudimentary' organs may be utterly aborted; and this implies, that we find in an animal or plant no trace of an organ, which analogy would lead us to expect to find, and which is occasionally found in monstrous individuals of the species. Thus in the snapdragon (antirrhinum) we generally do not find a rudiment of a fifth stamen; but this may sometimes be seen. In tracing the homologies of the same part in different members of a class, nothing is more common, or more necessary, than the use and discovery of rudiments. This is well shown in the drawings given by Owen of the bones of the leg of the horse, ox, and rhinoceros. It is an important fact that rudimentary organs, such as teeth in the upper jaws of whales and ruminants, can often be detected in the embryo, but afterwards wholly disappear. It is also, I believe, a universal

Chap. XIII.

RUDIMENTARY ORGANS.

453

rule, that a rudimentary part or organ is of greater size relatively to the adjoining parts in the embryo, than in the adult; so that the organ at this early age is less rudimentary, or even cannot be said to be in any degree rudimentary. Hence, also, a rudimentary organ in the adult, is often said to have retained its embryonic condition. I have now given the leading facts with respect to rudimentary organs. In reflecting on them, every one must be struck with astonishment: for the same reasonmg power which tells us plainly that most parts and organs are exquisitely adapted for certain purposes, tells us with equal plainness that these rudimentary or atrophied organs, are imperfect and useless. In works on natural history rudimentary organs are gene¬ rally said to have been created “ for the sake of sym¬ metry,” or in order “ to complete the scheme of natui’e but this seems to me no explanation, merely a re- l . led».-'"a

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