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| author | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:16:43 -0700 |
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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:16:43 -0700 |
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diff --git a/1228-h/1228-h.htm b/1228-h/1228-h.htm new file mode 100644 index 0000000..547dcb6 --- /dev/null +++ b/1228-h/1228-h.htm @@ -0,0 +1,16951 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" +"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> +<head> +<meta http-equiv="content-type" content="text/html; charset=UTF-8" /> +<meta http-equiv="Content-Style-Type" content="text/css" /> +<title>The Project Gutenberg eBook of On the Origin of Species, by Charles Darwin</title> + +<style type="text/css"> + +body { margin-left: 20%; + margin-right: 20%; + text-align: justify;} + +h1, h2, h3, h4, h5 {text-align: center; font-style: normal; font-weight: +normal; line-height: 1.5; margin-top: .5em; margin-bottom: .5em;} + +h1 {font-size: 300%; + margin-top: 0.6em; + margin-bottom: 0.6em; + letter-spacing: 0.12em; + word-spacing: 0.2em; + text-indent: 0em;} +h2 {font-size: 150%; margin-top: 2em; margin-bottom: 1em;} +h3 {font-size: 150%; margin-top: 2em;} +h4 {font-size: 120%;} +h5 {font-size: 110%;} + +.no-break {page-break-before: avoid;} /* for epubs */ + +hr {width: 80%; margin-top: 2em; margin-bottom: 2em;} + +div.chapter {page-break-before: always; margin-top: 4em;} + +p {text-indent: 1em; + margin-top: 0.25em; + margin-bottom: 0.25em; } + +.p2 {margin-top: 2em;} + +p.letter {text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.noindent {text-indent: 0% } + +p.center {text-align: center; + text-indent: 0em; + margin-top: 1em; + margin-bottom: 1em; } + +p.right {text-align: right; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +sup { vertical-align: top; font-size: 0.6em; } + +a:link {color:blue; text-decoration:none} +a:visited {color:blue; text-decoration:none} +a:hover {color:red} + +</style> + +</head> + +<body> +<div>*** START OF THE PROJECT GUTENBERG EBOOK 1228 ***</div> + +<p class="center"> +There are several editions of this ebook in the Project Gutenberg collection. +Various characteristics of each ebook are listed to aid in selecting the +preferred file.<br /> +Click on any of the filenumbers below to quickly view each ebook. +</p> + + +<table summary="" style="margin-right: auto; margin-left: auto" cellpadding="4" border="3"> + +<tr><td> + <b><a href="http://www.gutenberg.org/files/1228/1228-h/1228-h.htm"> +1228</a> </b> </td><td>1859, First Edition +</td></tr> + +<tr><td> + <b><a href="http://www.gutenberg.org/files/22764/22764-h/22764-h.htm"> +22764</a></b></td><td>1860, Second Edition +</td></tr> + +<tr><td> + <b><a href="http://www.gutenberg.org/files/2009/2009-h/2009-h.htm"> +2009</a></b> </td><td>1872, Sixth Edition, considered the definitive edition. +</td></tr> + +</table> + + +<h1>On<br />the Origin of Species</h1> + +<h4>BY MEANS OF NATURAL SELECTION,</h4> + +<h3>OR THE<br />PRESERVATION OF FAVOURED RACES IN THE STRUGGLE FOR LIFE.</h3> + +<h2 class="no-break">By Charles Darwin, M.A.,</h2> + +<h4>Fellow Of The Royal, Geological, Linnæan, Etc., Societies;<br /> +Author Of ‘Journal Of Researches During H.M.S. Beagle’s Voyage +Round The World.’<br /><br /><br /> +LONDON:<br />JOHN MURRAY, ALBEMARLE STREET.<br />1859.</h4> + +<hr /> + +<p class="letter"> +“But with regard to the material world, we can at least go so far as +this—we can perceive that events are brought about not by insulated +interpositions of Divine power, exerted in each particular case, but by the +establishment of general laws.” +</p> + +<p class="right"> +W. W<small>HEWELL</small>: <i>Bridgewater Treatise</i>. +</p> + +<p class="letter"> +“To conclude, therefore, let no man out of a weak conceit of sobriety, or +an ill-applied moderation, think or maintain, that a man can search too far or +be too well studied in the book of God’s word, or in the book of +God’s works; divinity or philosophy; but rather let men endeavour an +endless progress or proficience in both.” +</p> + +<p class="right"> +B<small>ACON</small>: <i>Advancement of Learning</i>. +</p> + +<p class="letter"> +<i>Down, Bromley, Kent,<br /> + October</i>, 1<i>st</i>, 1859. +</p> + +<hr /> + +<h2>Contents</h2> + +<table summary="" style="margin-left: auto; margin-right: auto"> + +<tr> +<td> <a href="#chap00">INTRODUCTION.</a></td> +</tr> + +<tr> +<td> <a href="#chap01">1. VARIATION UNDER DOMESTICATION.</a></td> +</tr> + +<tr> +<td> <a href="#chap02">2. VARIATION UNDER NATURE.</a></td> +</tr> + +<tr> +<td> <a href="#chap03">3. STRUGGLE FOR EXISTENCE.</a></td> +</tr> + +<tr> +<td> <a href="#chap04">4. NATURAL SELECTION.</a></td> +</tr> + +<tr> +<td> <a href="#chap05">5. LAWS OF VARIATION.</a></td> +</tr> + +<tr> +<td> <a href="#chap06">6. DIFFICULTIES ON THEORY.</a></td> +</tr> + +<tr> +<td> <a href="#chap07">7. INSTINCT.</a></td> +</tr> + +<tr> +<td> <a href="#chap08">8. HYBRIDISM.</a></td> +</tr> + +<tr> +<td> <a href="#chap09">9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.</a></td> +</tr> + +<tr> +<td> <a href="#chap10">10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.</a></td> +</tr> + +<tr> +<td> <a href="#chap11">11. GEOGRAPHICAL DISTRIBUTION.</a></td> +</tr> + +<tr> +<td> <a href="#chap12">12. GEOGRAPHICAL DISTRIBUTION—<i>continued</i>.</a></td> +</tr> + +<tr> +<td> <a href="#chap13">13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY:</a></td> +</tr> + +<tr> +<td> <a href="#chap14">14. RECAPITULATION AND CONCLUSION.</a></td> +</tr> + +<tr> +<td> <a href="#chap15">INDEX</a></td> +</tr> + +</table> + +<h3>DETEAILED CONTENTS. ON THE ORIGIN OF SPECIES.</h3> + +<p class="noindent"> +<a href="#chap00">INTRODUCTION.</a><br /><br /> +</p> + +<p class="noindent"> +<a href="#chap01">CHAPTER I. VARIATION UNDER DOMESTICATION.</a><br /> +<br /> + Causes of Variability.<br /> + Effects of Habit.<br /> + Correlation of Growth.<br /> + Inheritance.<br /> + Character of Domestic Varieties.<br /> + Difficulty of distinguishing between Varieties and Species.<br /> + Origin of Domestic Varieties from one or more Species.<br /> + Domestic Pigeons, their Differences and Origin.<br /> + Principle of Selection anciently followed, its Effects.<br /> + Methodical and Unconscious Selection.<br /> + Unknown Origin of our Domestic Productions.<br /> + Circumstances favourable to Man’s power of Selection.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap02">CHAPTER 2. VARIATION UNDER NATURE.</a><br /> +<br /> + Variability.<br /> + Individual Differences.<br /> + Doubtful species.<br /> + Wide ranging, much diffused, and common species vary most.<br /> + Species of the larger genera in any country vary more than the species + of the smaller genera.<br /> + Many of the species of the larger genera resemble varieties in being + very closely, but unequally, related to each other, and in having + restricted ranges.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap03">CHAPTER 3. STRUGGLE FOR EXISTENCE.</a><br /> +<br /> + Bears on natural selection.<br /> + The term used in a wide sense.<br /> + Geometrical powers of increase.<br /> + Rapid increase of naturalised animals and plants.<br /> + Nature of the checks to increase.<br /> + Competition universal.<br /> + Effects of climate.<br /> + Protection from the number of individuals.<br /> + Complex relations of all animals and plants throughout nature.<br /> + Struggle for life most severe between individuals and varieties of the + same species; often severe between species of the same genus.<br /> + The relation of organism to organism the most important of all relations.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap04">CHAPTER 4. NATURAL SELECTION.</a><br /> +<br /> + Natural Selection: its power compared with man’s selection, its power + on characters of trifling importance, its power at all ages and on + both sexes.<br /> + Sexual Selection.<br /> + On the generality of intercrosses between individuals of the same + species.<br /> + Circumstances favourable and unfavourable to Natural Selection,<br /> + namely, intercrossing, isolation, number of individuals.<br /> + Slow action.<br /> + Extinction caused by Natural Selection.<br /> + Divergence of Character, related to the diversity of inhabitants of + any small area, and to naturalisation.<br /> + Action of Natural Selection, through Divergence of Character and<br /> + Extinction, on the descendants from a common parent.<br /> + Explains the Grouping of all organic beings.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap05">CHAPTER 5. LAWS OF VARIATION.</a><br /> +<br /> + Effects of external conditions.<br /> + Use and disuse, combined with natural selection; organs of flight and + of vision.<br /> + Acclimatisation.<br /> + Correlation of growth.<br /> + Compensation and economy of growth.<br /> + False correlations.<br /> + Multiple, rudimentary, and lowly organised structures variable.<br /> + Parts developed in an unusual manner are highly variable: specific + characters more variable than generic: secondary sexual characters + variable.<br /> + Species of the same genus vary in an analogous manner.<br /> + Reversions to long-lost characters.<br /> + Summary.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap06">CHAPTER 6. DIFFICULTIES ON THEORY.</a><br /> +<br /> + Difficulties on the theory of descent with modification.<br /> + Transitions.<br /> + Absence or rarity of transitional varieties.<br /> + Transitions in habits of life.<br /> + Diversified habits in the same species.<br /> + Species with habits widely different from those of their allies.<br /> + Organs of extreme perfection.<br /> + Means of transition.<br /> + Cases of difficulty.<br /> + Natura non facit saltum.<br /> + Organs of small importance.<br /> + Organs not in all cases absolutely perfect.<br /> + The law of Unity of Type and of the Conditions of Existence embraced + by the theory of Natural Selection.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap07">CHAPTER 7. INSTINCT.</a><br /> +<br /> + Instincts comparable with habits, but different in their origin.<br /> + Instincts graduated.<br /> + Aphides and ants.<br /> + Instincts variable.<br /> + Domestic instincts, their origin.<br /> + Natural instincts of the cuckoo, ostrich, and parasitic bees.<br /> + Slave-making ants.<br /> + Hive-bee, its cell-making instinct.<br /> + Difficulties on the theory of the Natural Selection of instincts.<br /> + Neuter or sterile insects.<br /><br /> + Summary.<br /> +</p> + +<p class="noindent"> +<a href="#chap08">CHAPTER 8. HYBRIDISM.</a><br /> +<br /> + Distinction between the sterility of first crosses and of hybrids.<br /> + Sterility various in degree, not universal, affected by close + interbreeding, removed by domestication.<br /> + Laws governing the sterility of hybrids.<br /> + Sterility not a special endowment, but incidental on other + differences.<br /> + Causes of the sterility of first crosses and of hybrids.<br /> + Parallelism between the effects of changed conditions of life and + crossing.<br /> + Fertility of varieties when crossed and of their mongrel offspring not + universal.<br /> + Hybrids and mongrels compared independently of their fertility.<br /> + Summary.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap09">CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.</a><br /> +<br /> + On the absence of intermediate varieties at the present day.<br /> + On the nature of extinct intermediate varieties; on their number.<br /> + On the vast lapse of time, as inferred from the rate of deposition and + of denudation.<br /> + On the poorness of our palæontological collections.<br /> + On the intermittence of geological formations.<br /> + On the absence of intermediate varieties in any one formation.<br /> + On the sudden appearance of groups of species.<br /> + On their sudden appearance in the lowest known fossiliferous strata.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap10">CHAPTER 10. ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.</a><br /> +<br /> + On the slow and successive appearance of new species.<br /> + On their different rates of change.<br /> + Species once lost do not reappear.<br /> + Groups of species follow the same general rules in their appearance + and disappearance as do single species.<br /> + On Extinction.<br /> + On simultaneous changes in the forms of life throughout the world.<br /> + On the affinities of extinct species to each other and to living species.<br /> + On the state of development of ancient forms.<br /> + On the succession of the same types within the same areas.<br /> + Summary of preceding and present chapters.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap11">CHAPTER 11. GEOGRAPHICAL DISTRIBUTION.</a><br /> +<br /> + Present distribution cannot be accounted for by differences in + physical conditions.<br /> + Importance of barriers.<br /> + Affinity of the productions of the same continent.<br /> + Centres of creation.<br /> + Means of dispersal, by changes of climate and of the level of the + land, and by occasional means.<br /> + Dispersal during the Glacial period co-extensive with the world.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap12">CHAPTER 12. GEOGRAPHICAL DISTRIBUTION—<i>continued</i>.</a><br /> +<br /> + Distribution of fresh-water productions.<br /> + On the inhabitants of oceanic islands.<br /> + Absence of Batrachians and of terrestrial Mammals.<br /> + On the relation of the inhabitants of islands to those of the nearest + mainland.<br /> + On colonisation from the nearest source with subsequent modification.<br /> + Summary of the last and present chapters.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap13">CHAPTER 13. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: EMBRYOLOGY: + RUDIMENTARY ORGANS.</a><br /> +<br /> + CLASSIFICATION, groups subordinate to groups.<br /> + Natural system.<br /> + Rules and difficulties in classification, explained on the theory of + descent with modification.<br /> + Classification of varieties.<br /> + Descent always used in classification.<br /> + Analogical or adaptive characters.<br /> + Affinities, general, complex and radiating.<br /> + Extinction separates and defines groups.<br /> + MORPHOLOGY, between members of the same class, between parts of the + same individual.<br /> + EMBRYOLOGY, laws of, explained by variations not supervening at an + early age, and being inherited at a corresponding age.<br /> + RUDIMENTARY ORGANS; their origin explained.<br /> + Summary.<br /><br /> +</p> + +<p class="noindent"> +<a href="#chap14">CHAPTER 14. RECAPITULATION AND CONCLUSION.</a><br /> +<br /> + Recapitulation of the difficulties on the theory of Natural Selection.<br /> + Recapitulation of the general and special circumstances in its favour.<br /> + Causes of the general belief in the immutability of species.<br /> + How far the theory of natural selection may be extended.<br /> + Effects of its adoption on the study of Natural history.<br /> + Concluding remarks.<br /> +</p> + +<div class="chapter"> + +<h2>ON THE ORIGIN OF SPECIES.</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap00"></a>INTRODUCTION.</h2> + +<p> +<a name="Page1"></a> +When on board H.M.S. ‘Beagle,’ as naturalist, I was much struck +with certain facts in the distribution of the inhabitants of South America, and +in the geological relations of the present to the past inhabitants of that +continent. These facts seemed to me to throw some light on the origin of +species—that mystery of mysteries, as it has been called by one of our +greatest philosophers. On my return home, it occurred to me, in 1837, that +something might perhaps be made out on this question by patiently accumulating +and reflecting on all sorts of facts which could possibly have any bearing on +it. After five years’ work I allowed myself to speculate on the subject, +and drew up some short notes; these I enlarged in 1844 into a sketch of the +conclusions, which then seemed to me probable: from that period to the present +day I have steadily pursued the same object. I hope that I may be excused for +entering on these personal details, as I give them to show that I have not been +hasty in coming to a decision. +</p> + +<p> +My work is now nearly finished; but as it will take me two or three more years +to complete it, and as my health is far from strong, I have been urged to +publish this Abstract. I have more especially been induced to do this, as Mr. +Wallace, who is now studying the +<a name="Page2"></a> +natural history of the Malay archipelago, has arrived at almost exactly the +same general conclusions that I have on the origin of species. Last year he +sent to me a memoir on this subject, with a request that I would forward it to +Sir Charles Lyell, who sent it to the Linnean Society, and it is published in +the third volume of the Journal of that Society. Sir C. Lyell and Dr. Hooker, +who both knew of my work—the latter having read my sketch of +1844—honoured me by thinking it advisable to publish, with Mr. +Wallace’s excellent memoir, some brief extracts from my manuscripts. +</p> + +<p> +This Abstract, which I now publish, must necessarily be imperfect. I cannot +here give references and authorities for my several statements; and I must +trust to the reader reposing some confidence in my accuracy. No doubt errors +will have crept in, though I hope I have always been cautious in trusting to +good authorities alone. I can here give only the general conclusions at which I +have arrived, with a few facts in illustration, but which, I hope, in most +cases will suffice. No one can feel more sensible than I do of the necessity of +hereafter publishing in detail all the facts, with references, on which my +conclusions have been grounded; and I hope in a future work to do this. For I +am well aware that scarcely a single point is discussed in this volume on which +facts cannot be adduced, often apparently leading to conclusions directly +opposite to those at which I have arrived. A fair result can be obtained only +by fully stating and balancing the facts and arguments on both sides of each +question; and this cannot possibly be here done. +</p> + +<p> +I much regret that want of space prevents my having the satisfaction of +acknowledging the generous assistance which I have received from very many +naturalists, some of them personally unknown to me. I cannot, however, +<a name="Page3"></a> +let this opportunity pass without expressing my deep obligations to Dr. Hooker, +who for the last fifteen years has aided me in every possible way by his large +stores of knowledge and his excellent judgment. +</p> + +<p> +In considering the Origin of Species, it is quite conceivable that a +naturalist, reflecting on the mutual affinities of organic beings, on their +embryological relations, their geographical distribution, geological +succession, and other such facts, might come to the conclusion that each +species had not been independently created, but had descended, like varieties, +from other species. Nevertheless, such a conclusion, even if well founded, +would be unsatisfactory, until it could be shown how the innumerable species +inhabiting this world have been modified, so as to acquire that perfection of +structure and coadaptation which most justly excites our admiration. +Naturalists continually refer to external conditions, such as climate, food, +etc., as the only possible cause of variation. In one very limited sense, as we +shall hereafter see, this may be true; but it is preposterous to attribute to +mere external conditions, the structure, for instance, of the woodpecker, with +its feet, tail, beak, and tongue, so admirably adapted to catch insects under +the bark of trees. In the case of the misseltoe, which draws its nourishment +from certain trees, which has seeds that must be transported by certain birds, +and which has flowers with separate sexes absolutely requiring the agency of +certain insects to bring pollen from one flower to the other, it is equally +preposterous to account for the structure of this parasite, with its relations +to several distinct organic beings, by the effects of external conditions, or +of habit, or of the volition of the plant itself. +</p> + +<p> +The author of the ‘Vestiges of Creation’ would, I presume, say +that, after a certain unknown number of +<a name="Page4"></a> +generations, some bird had given birth to a woodpecker, and some plant to the +misseltoe, and that these had been produced perfect as we now see them; but +this assumption seems to me to be no explanation, for it leaves the case of the +coadaptations of organic beings to each other and to their physical conditions +of life, untouched and unexplained. +</p> + +<p> +It is, therefore, of the highest importance to gain a clear insight into the +means of modification and coadaptation. At the commencement of my observations +it seemed to me probable that a careful study of domesticated animals and of +cultivated plants would offer the best chance of making out this obscure +problem. Nor have I been disappointed; in this and in all other perplexing +cases I have invariably found that our knowledge, imperfect though it be, of +variation under domestication, afforded the best and safest clue. I may venture +to express my conviction of the high value of such studies, although they have +been very commonly neglected by naturalists. +</p> + +<p> +From these considerations, I shall devote the first chapter of this Abstract to +Variation under Domestication. We shall thus see that a large amount of +hereditary modification is at least possible, and, what is equally or more +important, we shall see how great is the power of man in accumulating by his +Selection successive slight variations. I will then pass on to the variability +of species in a state of nature; but I shall, unfortunately, be compelled to +treat this subject far too briefly, as it can be treated properly only by +giving long catalogues of facts. We shall, however, be enabled to discuss what +circumstances are most favourable to variation. In the next chapter the +Struggle for Existence amongst all organic beings throughout the world, which +inevitably follows from their high geometrical powers of +<a name="Page5"></a> +increase, will be treated of. This is the doctrine of Malthus, applied to the +whole animal and vegetable kingdoms. As many more individuals of each species +are born than can possibly survive; and as, consequently, there is a frequently +recurring struggle for existence, it follows that any being, if it vary however +slightly in any manner profitable to itself, under the complex and sometimes +varying conditions of life, will have a better chance of surviving, and thus be +<i>naturally selected</i>. From the strong principle of inheritance, any +selected variety will tend to propagate its new and modified form. +</p> + +<p> +This fundamental subject of Natural Selection will be treated at some length in +the fourth chapter; and we shall then see how Natural Selection almost +inevitably causes much Extinction of the less improved forms of life and +induces what I have called Divergence of Character. In the next chapter I shall +discuss the complex and little known laws of variation and of correlation of +growth. In the four succeeding chapters, the most apparent and gravest +difficulties on the theory will be given: namely, first, the difficulties of +transitions, or in understanding how a simple being or a simple organ can be +changed and perfected into a highly developed being or elaborately constructed +organ; secondly the subject of Instinct, or the mental powers of animals, +thirdly, Hybridism, or the infertility of species and the fertility of +varieties when intercrossed; and fourthly, the imperfection of the Geological +Record. In the next chapter I shall consider the geological succession of +organic beings throughout time; in the eleventh and twelfth, their geographical +distribution throughout space; in the thirteenth, their classification or +mutual affinities, both when mature and in an embryonic condition. In the last +chapter I shall give a +<a name="Page6"></a> +brief recapitulation of the whole work, and a few concluding remarks. +</p> + +<p> +No one ought to feel surprise at much remaining as yet unexplained in regard to +the origin of species and varieties, if he makes due allowance for our profound +ignorance in regard to the mutual relations of all the beings which live around +us. Who can explain why one species ranges widely and is very numerous, and why +another allied species has a narrow range and is rare? Yet these relations are +of the highest importance, for they determine the present welfare, and, as I +believe, the future success and modification of every inhabitant of this world. +Still less do we know of the mutual relations of the innumerable inhabitants of +the world during the many past geological epochs in its history. Although much +remains obscure, and will long remain obscure, I can entertain no doubt, after +the most deliberate study and dispassionate judgment of which I am capable, +that the view which most naturalists entertain, and which I formerly +entertained—namely, that each species has been independently +created—is erroneous. I am fully convinced that species are not +immutable; but that those belonging to what are called the same genera are +lineal descendants of some other and generally extinct species, in the same +manner as the acknowledged varieties of any one species are the descendants of +that species. Furthermore, I am convinced that Natural Selection has been the +main but not exclusive means of modification. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page7"></a><a name="chap01"></a>CHAPTER I.<br /> +VARIATION UNDER DOMESTICATION.</h2> + +<p class="letter"> +Causes of Variability. Effects of Habit. Correlation of Growth. Inheritance. +Character of Domestic Varieties. Difficulty of distinguishing between Varieties +and Species. Origin of Domestic Varieties from one or more Species. Domestic +Pigeons, their Differences and Origin. Principle of Selection anciently +followed, its Effects. Methodical and Unconscious Selection. Unknown Origin of +our Domestic Productions. Circumstances favourable to Man’s power of +Selection. +</p> + +<p> +When we look to the individuals of the same variety or sub-variety of our older +cultivated plants and animals, one of the first points which strikes us, is, +that they generally differ much more from each other, than do the individuals +of any one species or variety in a state of nature. When we reflect on the vast +diversity of the plants and animals which have been cultivated, and which have +varied during all ages under the most different climates and treatment, I think +we are driven to conclude that this greater variability is simply due to our +domestic productions having been raised under conditions of life not so uniform +as, and somewhat different from, those to which the parent-species have been +exposed under nature. There is, also, I think, some probability in the view +propounded by Andrew Knight, that this variability may be partly connected with +excess of food. It seems pretty clear that organic beings must be exposed +during several generations to the new conditions of life to cause any +appreciable amount of variation; and that when the organisation has once begun +to vary, it generally continues to vary for many generations. +<a name="Page8"></a> +No case is on record of a variable being ceasing to be variable under +cultivation. Our oldest cultivated plants, such as wheat, still often yield new +varieties: our oldest domesticated animals are still capable of rapid +improvement or modification. +</p> + +<p> +It has been disputed at what period of life the causes of variability, whatever +they may be, generally act; whether during the early or late period of +development of the embryo, or at the instant of conception. Geoffroy St. +Hilaire’s experiments show that unnatural treatment of the embryo causes +monstrosities; and monstrosities cannot be separated by any clear line of +distinction from mere variations. But I am strongly inclined to suspect that +the most frequent cause of variability may be attributed to the male and female +reproductive elements having been affected prior to the act of conception. +Several reasons make me believe in this; but the chief one is the remarkable +effect which confinement or cultivation has on the functions of the +reproductive system; this system appearing to be far more susceptible than any +other part of the organisation, to the action of any change in the conditions +of life. Nothing is more easy than to tame an animal, and few things more +difficult than to get it to breed freely under confinement, even in the many +cases when the male and female unite. How many animals there are which will not +breed, though living long under not very close confinement in their native +country! This is generally attributed to vitiated instincts; but how many +cultivated plants display the utmost vigour, and yet rarely or never seed! In +some few such cases it has been found out that very trifling changes, such as a +little more or less water at some particular period of growth, will determine +whether or not the plant sets a seed. I cannot here enter on the copious +details which I have collected on +<a name="Page9"></a> +this curious subject; but to show how singular the laws are which determine the +reproduction of animals under confinement, I may just mention that carnivorous +animals, even from the tropics, breed in this country pretty freely under +confinement, with the exception of the plantigrades or bear family; whereas, +carnivorous birds, with the rarest exceptions, hardly ever lay fertile eggs. +Many exotic plants have pollen utterly worthless, in the same exact condition +as in the most sterile hybrids. When, on the one hand, we see domesticated +animals and plants, though often weak and sickly, yet breeding quite freely +under confinement; and when, on the other hand, we see individuals, though +taken young from a state of nature, perfectly tamed, long-lived, and healthy +(of which I could give numerous instances), yet having their reproductive +system so seriously affected by unperceived causes as to fail in acting, we +need not be surprised at this system, when it does act under confinement, +acting not quite regularly, and producing offspring not perfectly like their +parents or variable. +</p> + +<p> +Sterility has been said to be the bane of horticulture; but on this view we owe +variability to the same cause which produces sterility; and variability is the +source of all the choicest productions of the garden. I may add, that as some +organisms will breed most freely under the most unnatural conditions (for +instance, the rabbit and ferret kept in hutches), showing that their +reproductive system has not been thus affected; so will some animals and plants +withstand domestication or cultivation, and vary very slightly—perhaps +hardly more than in a state of nature. +</p> + +<p> +A long list could easily be given of “sporting plants;” by this +term gardeners mean a single bud or offset, which suddenly assumes a new and +sometimes very different character from that of the rest of the plant. +<a name="Page10"></a> +Such buds can be propagated by grafting, etc., and sometimes by seed. These +“sports” are extremely rare under nature, but far from rare under +cultivation; and in this case we see that the treatment of the parent has +affected a bud or offset, and not the ovules or pollen. But it is the opinion +of most physiologists that there is no essential difference between a bud and +an ovule in their earliest stages of formation; so that, in fact, +“sports” support my view, that variability may be largely +attributed to the ovules or pollen, or to both, having been affected by the +treatment of the parent prior to the act of conception. These cases anyhow show +that variation is not necessarily connected, as some authors have supposed, +with the act of generation. +</p> + +<p> +Seedlings from the same fruit, and the young of the same litter, sometimes +differ considerably from each other, though both the young and the parents, as +Müller has remarked, have apparently been exposed to exactly the same +conditions of life; and this shows how unimportant the direct effects of the +conditions of life are in comparison with the laws of reproduction, and of +growth, and of inheritance; for had the action of the conditions been direct, +if any of the young had varied, all would probably have varied in the same +manner. To judge how much, in the case of any variation, we should attribute to +the direct action of heat, moisture, light, food, etc., is most difficult: my +impression is, that with animals such agencies have produced very little direct +effect, though apparently more in the case of plants. Under this point of view, +Mr. Buckman’s recent experiments on plants seem extremely valuable. When +all or nearly all the individuals exposed to certain conditions are affected in +the same way, the change at first appears to be directly due to such +conditions; but in some cases it can be shown that quite opposite conditions +produce +<a name="Page11"></a> +similar changes of structure. Nevertheless some slight amount of change may, I +think, be attributed to the direct action of the conditions of life—as, +in some cases, increased size from amount of food, colour from particular kinds +of food and from light, and perhaps the thickness of fur from climate. +</p> + +<p> +Habit also has a decided influence, as in the period of flowering with plants +when transported from one climate to another. In animals it has a more marked +effect; for instance, I find in the domestic duck that the bones of the wing +weigh less and the bones of the leg more, in proportion to the whole skeleton, +than do the same bones in the wild-duck; and I presume that this change may be +safely attributed to the domestic duck flying much less, and walking more, than +its wild parent. The great and inherited development of the udders in cows and +goats in countries where they are habitually milked, in comparison with the +state of these organs in other countries, is another instance of the effect of +use. Not a single domestic animal can be named which has not in some country +drooping ears; and the view suggested by some authors, that the drooping is due +to the disuse of the muscles of the ear, from the animals not being much +alarmed by danger, seems probable. +</p> + +<p> +There are many laws regulating variation, some few of which can be dimly seen, +and will be hereafter briefly mentioned. I will here only allude to what may be +called correlation of growth. Any change in the embryo or larva will almost +certainly entail changes in the mature animal. In monstrosities, the +correlations between quite distinct parts are very curious; and many instances +are given in Isidore Geoffroy St. Hilaire’s great work on this subject. +Breeders believe that long limbs are almost always accompanied by an elongated +head. Some instances of correlation are quite whimsical; thus +<a name="Page12"></a> +cats with blue eyes are invariably deaf; colour and constitutional +peculiarities go together, of which many remarkable cases could be given +amongst animals and plants. From the facts collected by Heusinger, it appears +that white sheep and pigs are differently affected from coloured individuals by +certain vegetable poisons. Hairless dogs have imperfect teeth; long-haired and +coarse-haired animals are apt to have, as is asserted, long or many horns; +pigeons with feathered feet have skin between their outer toes; pigeons with +short beaks have small feet, and those with long beaks large feet. Hence, if +man goes on selecting, and thus augmenting, any peculiarity, he will almost +certainly unconsciously modify other parts of the structure, owing to the +mysterious laws of the correlation of growth. +</p> + +<p> +The result of the various, quite unknown, or dimly seen laws of variation is +infinitely complex and diversified. It is well worth while carefully to study +the several treatises published on some of our old cultivated plants, as on the +hyacinth, potato, even the dahlia, etc.; and it is really surprising to note +the endless points in structure and constitution in which the varieties and +sub-varieties differ slightly from each other. The whole organisation seems to +have become plastic, and tends to depart in some small degree from that of the +parental type. +</p> + +<p> +Any variation which is not inherited is unimportant for us. But the number and +diversity of inheritable deviations of structure, both those of slight and +those of considerable physiological importance, is endless. Dr. Prosper +Lucas’s treatise, in two large volumes, is the fullest and the best on +this subject. No breeder doubts how strong is the tendency to inheritance: like +produces like is his fundamental belief: doubts have been thrown on this +principle by theoretical writers alone. When a +<a name="Page13"></a> +deviation appears not unfrequently, and we see it in the father and child, we +cannot tell whether it may not be due to the same original cause acting on +both; but when amongst individuals, apparently exposed to the same conditions, +any very rare deviation, due to some extraordinary combination of +circumstances, appears in the parent—say, once amongst several million +individuals—and it reappears in the child, the mere doctrine of chances +almost compels us to attribute its reappearance to inheritance. Every one must +have heard of cases of albinism, prickly skin, hairy bodies, etc., appearing in +several members of the same family. If strange and rare deviations of structure +are truly inherited, less strange and commoner deviations may be freely +admitted to be inheritable. Perhaps the correct way of viewing the whole +subject, would be, to look at the inheritance of every character whatever as +the rule, and non-inheritance as the anomaly. +</p> + +<p> +The laws governing inheritance are quite unknown; no one can say why the same +peculiarity in different individuals of the same species, and in individuals of +different species, is sometimes inherited and sometimes not so; why the child +often reverts in certain characters to its grandfather or grandmother or other +much more remote ancestor; why a peculiarity is often transmitted from one sex +to both sexes or to one sex alone, more commonly but not exclusively to the +like sex. It is a fact of some little importance to us, that peculiarities +appearing in the males of our domestic breeds are often transmitted either +exclusively, or in a much greater degree, to males alone. A much more important +rule, which I think may be trusted, is that, at whatever period of life a +peculiarity first appears, it tends to appear in the offspring at a +corresponding age, though sometimes earlier. In many cases this could +<a name="Page14"></a> +not be otherwise: thus the inherited peculiarities in the horns of cattle could +appear only in the offspring when nearly mature; peculiarities in the silkworm +are known to appear at the corresponding caterpillar or cocoon stage. But +hereditary diseases and some other facts make me believe that the rule has a +wider extension, and that when there is no apparent reason why a peculiarity +should appear at any particular age, yet that it does tend to appear in the +offspring at the same period at which it first appeared in the parent. I +believe this rule to be of the highest importance in explaining the laws of +embryology. These remarks are of course confined to the first <i>appearance</i> +of the peculiarity, and not to its primary cause, which may have acted on the +ovules or male element; in nearly the same manner as in the crossed offspring +from a short-horned cow by a long-horned bull, the greater length of horn, +though appearing late in life, is clearly due to the male element. +</p> + +<p> +Having alluded to the subject of reversion, I may here refer to a statement +often made by naturalists—namely, that our domestic varieties, when run +wild, gradually but certainly revert in character to their aboriginal stocks. +Hence it has been argued that no deductions can be drawn from domestic races to +species in a state of nature. I have in vain endeavoured to discover on what +decisive facts the above statement has so often and so boldly been made. There +would be great difficulty in proving its truth: we may safely conclude that +very many of the most strongly-marked domestic varieties could not possibly +live in a wild state. In many cases we do not know what the aboriginal stock +was, and so could not tell whether or not nearly perfect reversion had ensued. +It would be quite necessary, in order to prevent the effects of intercrossing, +that only a +<a name="Page15"></a> +single variety should be turned loose in its new home. Nevertheless, as our +varieties certainly do occasionally revert in some of their characters to +ancestral forms, it seems to me not improbable, that if we could succeed in +naturalising, or were to cultivate, during many generations, the several races, +for instance, of the cabbage, in very poor soil (in which case, however, some +effect would have to be attributed to the direct action of the poor soil), that +they would to a large extent, or even wholly, revert to the wild aboriginal +stock. Whether or not the experiment would succeed, is not of great importance +for our line of argument; for by the experiment itself the conditions of life +are changed. If it could be shown that our domestic varieties manifested a +strong tendency to reversion,—that is, to lose their acquired characters, +whilst kept under unchanged conditions, and whilst kept in a considerable body, +so that free intercrossing might check, by blending together, any slight +deviations of structure, in such case, I grant that we could deduce nothing +from domestic varieties in regard to species. But there is not a shadow of +evidence in favour of this view: to assert that we could not breed our cart and +race-horses, long and short-horned cattle, and poultry of various breeds, and +esculent vegetables, for an almost infinite number of generations, would be +opposed to all experience. I may add, that when under nature the conditions of +life do change, variations and reversions of character probably do occur; but +natural selection, as will hereafter be explained, will determine how far the +new characters thus arising shall be preserved. +</p> + +<p> +When we look to the hereditary varieties or races of our domestic animals and +plants, and compare them with species closely allied together, we generally +perceive in each domestic race, as already remarked, less uniformity of +character than in true species. Domestic races of +<a name="Page16"></a> +the same species, also, often have a somewhat monstrous character; by which I +mean, that, although differing from each other, and from the other species of +the same genus, in several trifling respects, they often differ in an extreme +degree in some one part, both when compared one with another, and more +especially when compared with all the species in nature to which they are +nearest allied. With these exceptions (and with that of the perfect fertility +of varieties when crossed,—a subject hereafter to be discussed), domestic +races of the same species differ from each other in the same manner as, only in +most cases in a lesser degree than, do closely-allied species of the same genus +in a state of nature. I think this must be admitted, when we find that there +are hardly any domestic races, either amongst animals or plants, which have not +been ranked by some competent judges as mere varieties, and by other competent +judges as the descendants of aboriginally distinct species. If any marked +distinction existed between domestic races and species, this source of doubt +could not so perpetually recur. It has often been stated that domestic races do +not differ from each other in characters of generic value. I think it could be +shown that this statement is hardly correct; but naturalists differ most widely +in determining what characters are of generic value; all such valuations being +at present empirical. Moreover, on the view of the origin of genera which I +shall presently give, we have no right to expect often to meet with generic +differences in our domesticated productions. +</p> + +<p> +When we attempt to estimate the amount of structural difference between the +domestic races of the same species, we are soon involved in doubt, from not +knowing whether they have descended from one or several parent-species. This +point, if it could be cleared up, would be interesting; if, for instance, it +could be shown that the greyhound, +<a name="Page17"></a> +bloodhound, terrier, spaniel, and bull-dog, which we all know propagate their +kind so truly, were the offspring of any single species, then such facts would +have great weight in making us doubt about the immutability of the many very +closely allied and natural species—for instance, of the many +foxes—inhabiting different quarters of the world. I do not believe, as we +shall presently see, that all our dogs have descended from any one wild +species; but, in the case of some other domestic races, there is presumptive, +or even strong, evidence in favour of this view. +</p> + +<p> +It has often been assumed that man has chosen for domestication animals and +plants having an extraordinary inherent tendency to vary, and likewise to +withstand diverse climates. I do not dispute that these capacities have added +largely to the value of most of our domesticated productions; but how could a +savage possibly know, when he first tamed an animal, whether it would vary in +succeeding generations, and whether it would endure other climates? Has the +little variability of the ass or guinea-fowl, or the small power of endurance +of warmth by the rein-deer, or of cold by the common camel, prevented their +domestication? I cannot doubt that if other animals and plants, equal in number +to our domesticated productions, and belonging to equally diverse classes and +countries, were taken from a state of nature, and could be made to breed for an +equal number of generations under domestication, they would vary on an average +as largely as the parent species of our existing domesticated productions have +varied. +</p> + +<p> +In the case of most of our anciently domesticated animals and plants, I do not +think it is possible to come to any definite conclusion, whether they have +descended from one or several species. The argument mainly relied on by those +who believe in the multiple origin +<a name="Page18"></a> +of our domestic animals is, that we find in the most ancient records, more +especially on the monuments of Egypt, much diversity in the breeds; and that +some of the breeds closely resemble, perhaps are identical with, those still +existing. Even if this latter fact were found more strictly and generally true +than seems to me to be the case, what does it show, but that some of our breeds +originated there, four or five thousand years ago? But Mr. Horner’s +researches have rendered it in some degree probable that man sufficiently +civilized to have manufactured pottery existed in the valley of the Nile +thirteen or fourteen thousand years ago; and who will pretend to say how long +before these ancient periods, savages, like those of Tierra del Fuego or +Australia, who possess a semi-domestic dog, may not have existed in Egypt? +</p> + +<p> +The whole subject must, I think, remain vague; nevertheless, I may, without +here entering on any details, state that, from geographical and other +considerations, I think it highly probable that our domestic dogs have +descended from several wild species. In regard to sheep and goats I can form no +opinion. I should think, from facts communicated to me by Mr. Blyth, on the +habits, voice, and constitution, etc., of the humped Indian cattle, that these +had descended from a different aboriginal stock from our European cattle; and +several competent judges believe that these latter have had more than one wild +parent. With respect to horses, from reasons which I cannot give here, I am +doubtfully inclined to believe, in opposition to several authors, that all the +races have descended from one wild stock. Mr. Blyth, whose opinion, from his +large and varied stores of knowledge, I should value more than that of almost +any one, thinks that all the breeds of poultry have proceeded from the common +wild +<a name="Page19"></a> +Indian fowl (Gallus bankiva). In regard to ducks and rabbits, the breeds of +which differ considerably from each other in structure, I do not doubt that +they all have descended from the common wild duck and rabbit. +</p> + +<p> +The doctrine of the origin of our several domestic races from several +aboriginal stocks, has been carried to an absurd extreme by some authors. They +believe that every race which breeds true, let the distinctive characters be +ever so slight, has had its wild prototype. At this rate there must have +existed at least a score of species of wild cattle, as many sheep, and several +goats in Europe alone, and several even within Great Britain. One author +believes that there formerly existed in Great Britain eleven wild species of +sheep peculiar to it! When we bear in mind that Britain has now hardly one +peculiar mammal, and France but few distinct from those of Germany and +conversely, and so with Hungary, Spain, etc., but that each of these kingdoms +possesses several peculiar breeds of cattle, sheep, etc., we must admit that +many domestic breeds have originated in Europe; for whence could they have been +derived, as these several countries do not possess a number of peculiar species +as distinct parent-stocks? So it is in India. Even in the case of the domestic +dogs of the whole world, which I fully admit have probably descended from +several wild species, I cannot doubt that there has been an immense amount of +inherited variation. Who can believe that animals closely resembling the +Italian greyhound, the bloodhound, the bull-dog, or Blenheim spaniel, +etc.—so unlike all wild Canidæ—ever existed freely in a state of +nature? It has often been loosely said that all our races of dogs have been +produced by the crossing of a few aboriginal species; but by crossing we can +get only forms in some degree intermediate between their parents; and if we +<a name="Page20"></a> +account for our several domestic races by this process, we must admit the +former existence of the most extreme forms, as the Italian greyhound, +bloodhound, bull-dog, etc., in the wild state. Moreover, the possibility of +making distinct races by crossing has been greatly exaggerated. There can be no +doubt that a race may be modified by occasional crosses, if aided by the +careful selection of those individual mongrels, which present any desired +character; but that a race could be obtained nearly intermediate between two +extremely different races or species, I can hardly believe. Sir J. Sebright +expressly experimentised for this object, and failed. The offspring from the +first cross between two pure breeds is tolerably and sometimes (as I have found +with pigeons) extremely uniform, and everything seems simple enough; but when +these mongrels are crossed one with another for several generations, hardly two +of them will be alike, and then the extreme difficulty, or rather utter +hopelessness, of the task becomes apparent. Certainly, a breed intermediate +between <i>two very distinct</i> breeds could not be got without extreme care +and long-continued selection; nor can I find a single case on record of a +permanent race having been thus formed. +</p> + +<p> +<i>On the Breeds of the Domestic Pigeon</i>.—Believing that it is always +best to study some special group, I have, after deliberation, taken up domestic +pigeons. I have kept every breed which I could purchase or obtain, and have +been most kindly favoured with skins from several quarters of the world, more +especially by the Honourable W. Elliot from India, and by the Honourable C. +Murray from Persia. Many treatises in different languages have been published +on pigeons, and some of them are very important, as being of considerable +antiquity. I have associated with several eminent fanciers, and have been +permitted to join two +<a name="Page21"></a> +of the London Pigeon Clubs. The diversity of the breeds is something +astonishing. Compare the English carrier and the short-faced tumbler, and see +the wonderful difference in their beaks, entailing corresponding differences in +their skulls. The carrier, more especially the male bird, is also remarkable +from the wonderful development of the carunculated skin about the head, and +this is accompanied by greatly elongated eyelids, very large external orifices +to the nostrils, and a wide gape of mouth. The short-faced tumbler has a beak +in outline almost like that of a finch; and the common tumbler has the singular +and strictly inherited habit of flying at a great height in a compact flock, +and tumbling in the air head over heels. The runt is a bird of great size, with +long, massive beak and large feet; some of the sub-breeds of runts have very +long necks, others very long wings and tails, others singularly short tails. +The barb is allied to the carrier, but, instead of a very long beak, has a very +short and very broad one. The pouter has a much elongated body, wings, and +legs; and its enormously developed crop, which it glories in inflating, may +well excite astonishment and even laughter. The turbit has a very short and +conical beak, with a line of reversed feathers down the breast; and it has the +habit of continually expanding slightly the upper part of the oesophagus. The +Jacobin has the feathers so much reversed along the back of the neck that they +form a hood, and it has, proportionally to its size, much elongated wing and +tail feathers. The trumpeter and laugher, as their names express, utter a very +different coo from the other breeds. The fantail has thirty or even forty +tail-feathers, instead of twelve or fourteen, the normal number in all members +of the great pigeon family; and these feathers are kept expanded, and are +carried so erect that in good birds the head and tail +<a name="Page22"></a> +touch; the oil-gland is quite aborted. Several other less distinct breeds might +have been specified. +</p> + +<p> +In the skeletons of the several breeds, the development of the bones of the +face in length and breadth and curvature differs enormously. The shape, as well +as the breadth and length of the ramus of the lower jaw, varies in a highly +remarkable manner. The number of the caudal and sacral vertebræ vary; as does +the number of the ribs, together with their relative breadth and the presence +of processes. The size and shape of the apertures in the sternum are highly +variable; so is the degree of divergence and relative size of the two arms of +the furcula. The proportional width of the gape of mouth, the proportional +length of the eyelids, of the orifice of the nostrils, of the tongue (not +always in strict correlation with the length of beak), the size of the crop and +of the upper part of the oesophagus; the development and abortion of the +oil-gland; the number of the primary wing and caudal feathers; the relative +length of wing and tail to each other and to the body; the relative length of +leg and of the feet; the number of scutellæ on the toes, the development of +skin between the toes, are all points of structure which are variable. The +period at which the perfect plumage is acquired varies, as does the state of +the down with which the nestling birds are clothed when hatched. The shape and +size of the eggs vary. The manner of flight differs remarkably; as does in some +breeds the voice and disposition. Lastly, in certain breeds, the males and +females have come to differ to a slight degree from each other. +</p> + +<p> +Altogether at least a score of pigeons might be chosen, which if shown to an +ornithologist, and he were told that they were wild birds, would certainly, I +think, be ranked by him as well-defined species. Moreover, I do not believe +that any ornithologist would place +<a name="Page23"></a> +the English carrier, the short-faced tumbler, the runt, the barb, pouter, and +fantail in the same genus; more especially as in each of these breeds several +truly-inherited sub-breeds, or species as he might have called them, could be +shown him. +</p> + +<p> +Great as the differences are between the breeds of pigeons, I am fully +convinced that the common opinion of naturalists is correct, namely, that all +have descended from the rock-pigeon (Columba livia), including under this term +several geographical races or sub-species, which differ from each other in the +most trifling respects. As several of the reasons which have led me to this +belief are in some degree applicable in other cases, I will here briefly give +them. If the several breeds are not varieties, and have not proceeded from the +rock-pigeon, they must have descended from at least seven or eight aboriginal +stocks; for it is impossible to make the present domestic breeds by the +crossing of any lesser number: how, for instance, could a pouter be produced by +crossing two breeds unless one of the parent-stocks possessed the +characteristic enormous crop? The supposed aboriginal stocks must all have been +rock-pigeons, that is, not breeding or willingly perching on trees. But besides +C. livia, with its geographical sub-species, only two or three other species of +rock-pigeons are known; and these have not any of the characters of the +domestic breeds. Hence the supposed aboriginal stocks must either still exist +in the countries where they were originally domesticated, and yet be unknown to +ornithologists; and this, considering their size, habits, and remarkable +characters, seems very improbable; or they must have become extinct in the wild +state. But birds breeding on precipices, and good fliers, are unlikely to be +exterminated; and the common rock-pigeon, which has the same habits with the +domestic breeds, has not been exterminated +<a name="Page24"></a> +even on several of the smaller British islets, or on the shores of the +Mediterranean. Hence the supposed extermination of so many species having +similar habits with the rock-pigeon seems to me a very rash assumption. +Moreover, the several above-named domesticated breeds have been transported to +all parts of the world, and, therefore, some of them must have been carried +back again into their native country; but not one has ever become wild or +feral, though the dovecot-pigeon, which is the rock-pigeon in a very slightly +altered state, has become feral in several places. Again, all recent experience +shows that it is most difficult to get any wild animal to breed freely under +domestication; yet on the hypothesis of the multiple origin of our pigeons, it +must be assumed that at least seven or eight species were so thoroughly +domesticated in ancient times by half-civilized man, as to be quite prolific +under confinement. +</p> + +<p> +An argument, as it seems to me, of great weight, and applicable in several +other cases, is, that the above-specified breeds, though agreeing generally in +constitution, habits, voice, colouring, and in most parts of their structure, +with the wild rock-pigeon, yet are certainly highly abnormal in other parts of +their structure: we may look in vain throughout the whole great family of +Columbidæ for a beak like that of the English carrier, or that of the +short-faced tumbler, or barb; for reversed feathers like those of the jacobin; +for a crop like that of the pouter; for tail-feathers like those of the +fantail. Hence it must be assumed not only that half-civilized man succeeded in +thoroughly domesticating several species, but that he intentionally or by +chance picked out extraordinarily abnormal species; and further, that these +very species have since all become extinct or unknown. So many strange +contingencies seem to me improbable in the highest degree. +</p> + +<p> +<a name="Page25"></a> +Some facts in regard to the colouring of pigeons well deserve consideration. +The rock-pigeon is of a slaty-blue, and has a white rump (the Indian +sub-species, C. intermedia of Strickland, having it bluish); the tail has a +terminal dark bar, with the bases of the outer feathers externally edged with +white; the wings have two black bars; some semi-domestic breeds and some +apparently truly wild breeds have, besides the two black bars, the wings +chequered with black. These several marks do not occur together in any other +species of the whole family. Now, in every one of the domestic breeds, taking +thoroughly well-bred birds, all the above marks, even to the white edging of +the outer tail-feathers, sometimes concur perfectly developed. Moreover, when +two birds belonging to two distinct breeds are crossed, neither of which is +blue or has any of the above-specified marks, the mongrel offspring are very +apt suddenly to acquire these characters; for instance, I crossed some +uniformly white fantails with some uniformly black barbs, and they produced +mottled brown and black birds; these I again crossed together, and one +grandchild of the pure white fantail and pure black barb was of as beautiful a +blue colour, with the white rump, double black wing-bar, and barred and +white-edged tail-feathers, as any wild rock-pigeon! We can understand these +facts, on the well-known principle of reversion to ancestral characters, if all +the domestic breeds have descended from the rock-pigeon. But if we deny this, +we must make one of the two following highly improbable suppositions. Either, +firstly, that all the several imagined aboriginal stocks were coloured and +marked like the rock-pigeon, although no other existing species is thus +coloured and marked, so that in each separate breed there might be a tendency +to revert to the very same colours and markings. Or, secondly, +<a name="Page26"></a> +that each breed, even the purest, has within a dozen or, at most, within a +score of generations, been crossed by the rock-pigeon: I say within a dozen or +twenty generations, for we know of no fact countenancing the belief that the +child ever reverts to some one ancestor, removed by a greater number of +generations. In a breed which has been crossed only once with some distinct +breed, the tendency to reversion to any character derived from such cross will +naturally become less and less, as in each succeeding generation there will be +less of the foreign blood; but when there has been no cross with a distinct +breed, and there is a tendency in both parents to revert to a character, which +has been lost during some former generation, this tendency, for all that we can +see to the contrary, may be transmitted undiminished for an indefinite number +of generations. These two distinct cases are often confounded in treatises on +inheritance. +</p> + +<p> +Lastly, the hybrids or mongrels from between all the domestic breeds of pigeons +are perfectly fertile. I can state this from my own observations, purposely +made on the most distinct breeds. Now, it is difficult, perhaps impossible, to +bring forward one case of the hybrid offspring of two animals <i>clearly +distinct</i> being themselves perfectly fertile. Some authors believe that +long-continued domestication eliminates this strong tendency to sterility: from +the history of the dog I think there is some probability in this hypothesis, if +applied to species closely related together, though it is unsupported by a +single experiment. But to extend the hypothesis so far as to suppose that +species, aboriginally as distinct as carriers, tumblers, pouters, and fantails +now are, should yield offspring perfectly fertile, <i>inter se</i>, seems to me +rash in the extreme. +</p> + +<p> +From these several reasons, namely, the improbability of man having formerly +got seven or eight supposed +<a name="Page27"></a> +species of pigeons to breed freely under domestication; these supposed species +being quite unknown in a wild state, and their becoming nowhere feral; these +species having very abnormal characters in certain respects, as compared with +all other Columbidæ, though so like in most other respects to the rock-pigeon; +the blue colour and various marks occasionally appearing in all the breeds, +both when kept pure and when crossed; the mongrel offspring being perfectly +fertile;—from these several reasons, taken together, I can feel no doubt +that all our domestic breeds have descended from the Columba livia with its +geographical sub-species. +</p> + +<p> +In favour of this view, I may add, firstly, that C. livia, or the rock-pigeon, +has been found capable of domestication in Europe and in India; and that it +agrees in habits and in a great number of points of structure with all the +domestic breeds. Secondly, although an English carrier or short-faced tumbler +differs immensely in certain characters from the rock-pigeon, yet by comparing +the several sub-breeds of these breeds, more especially those brought from +distant countries, we can make an almost perfect series between the extremes of +structure. Thirdly, those characters which are mainly distinctive of each +breed, for instance the wattle and length of beak of the carrier, the shortness +of that of the tumbler, and the number of tail-feathers in the fantail, are in +each breed eminently variable; and the explanation of this fact will be obvious +when we come to treat of selection. Fourthly, pigeons have been watched, and +tended with the utmost care, and loved by many people. They have been +domesticated for thousands of years in several quarters of the world; the +earliest known record of pigeons is in the fifth Aegyptian dynasty, about 3000 +B.C., as was pointed out to me by Professor Lepsius; but Mr. Birch informs me +that pigeons are given in a bill +<a name="Page28"></a> +of fare in the previous dynasty. In the time of the Romans, as we hear from +Pliny, immense prices were given for pigeons; “nay, they are come to this +pass, that they can reckon up their pedigree and race.” Pigeons were much +valued by Akber Khan in India, about the year 1600; never less than 20,000 +pigeons were taken with the court. “The monarchs of Iran and Turan sent +him some very rare birds;” and, continues the courtly historian, +“His Majesty by crossing the breeds, which method was never practised +before, has improved them astonishingly.” About this same period the +Dutch were as eager about pigeons as were the old Romans. The paramount +importance of these considerations in explaining the immense amount of +variation which pigeons have undergone, will be obvious when we treat of +Selection. We shall then, also, see how it is that the breeds so often have a +somewhat monstrous character. It is also a most favourable circumstance for the +production of distinct breeds, that male and female pigeons can be easily mated +for life; and thus different breeds can be kept together in the same aviary. +</p> + +<p> +I have discussed the probable origin of domestic pigeons at some, yet quite +insufficient, length; because when I first kept pigeons and watched the several +kinds, knowing well how true they bred, I felt fully as much difficulty in +believing that they could ever have descended from a common parent, as any +naturalist could in coming to a similar conclusion in regard to the many +species of finches, or other large groups of birds, in nature. One circumstance +has struck me much; namely, that all the breeders of the various domestic +animals and the cultivators of plants, with whom I have ever conversed, or +whose treatises I have read, are firmly convinced that the several breeds to +which each has attended, are descended from so many aboriginally distinct +species. +<a name="Page29"></a> +Ask, as I have asked, a celebrated raiser of Hereford cattle, whether his +cattle might not have descended from long horns, and he will laugh you to +scorn. I have never met a pigeon, or poultry, or duck, or rabbit fancier, who +was not fully convinced that each main breed was descended from a distinct +species. Van Mons, in his treatise on pears and apples, shows how utterly he +disbelieves that the several sorts, for instance a Ribston-pippin or +Codlin-apple, could ever have proceeded from the seeds of the same tree. +Innumerable other examples could be given. The explanation, I think, is simple: +from long-continued study they are strongly impressed with the differences +between the several races; and though they well know that each race varies +slightly, for they win their prizes by selecting such slight differences, yet +they ignore all general arguments, and refuse to sum up in their minds slight +differences accumulated during many successive generations. May not those +naturalists who, knowing far less of the laws of inheritance than does the +breeder, and knowing no more than he does of the intermediate links in the long +lines of descent, yet admit that many of our domestic races have descended from +the same parents—may they not learn a lesson of caution, when they deride +the idea of species in a state of nature being lineal descendants of other +species? +</p> + +<p> +<i>Selection</i>.—Let us now briefly consider the steps by which +domestic races have been produced, either from one or from several allied +species. Some little effect may, perhaps, be attributed to the direct action of +the external conditions of life, and some little to habit; but he would be a +bold man who would account by such agencies for the differences of a dray and +race horse, a greyhound and bloodhound, a carrier and tumbler pigeon. One of +the most remarkable features in our domesticated races +<a name="Page30"></a> +is that we see in them adaptation, not indeed to the animal’s or +plant’s own good, but to man’s use or fancy. Some variations useful +to him have probably arisen suddenly, or by one step; many botanists, for +instance, believe that the fuller’s teazle, with its hooks, which cannot +be rivalled by any mechanical contrivance, is only a variety of the wild +Dipsacus; and this amount of change may have suddenly arisen in a seedling. So +it has probably been with the turnspit dog; and this is known to have been the +case with the ancon sheep. But when we compare the dray-horse and race-horse, +the dromedary and camel, the various breeds of sheep fitted either for +cultivated land or mountain pasture, with the wool of one breed good for one +purpose, and that of another breed for another purpose; when we compare the +many breeds of dogs, each good for man in very different ways; when we compare +the game-cock, so pertinacious in battle, with other breeds so little +quarrelsome, with “everlasting layers” which never desire to sit, +and with the bantam so small and elegant; when we compare the host of +agricultural, culinary, orchard, and flower-garden races of plants, most useful +to man at different seasons and for different purposes, or so beautiful in his +eyes, we must, I think, look further than to mere variability. We cannot +suppose that all the breeds were suddenly produced as perfect and as useful as +we now see them; indeed, in several cases, we know that this has not been their +history. The key is man’s power of accumulative selection: nature gives +successive variations; man adds them up in certain directions useful to him. In +this sense he may be said to make for himself useful breeds. +</p> + +<p> +The great power of this principle of selection is not hypothetical. It is +certain that several of our eminent breeders have, even within a single +lifetime, modified to +<a name="Page31"></a> +a large extent some breeds of cattle and sheep. In order fully to realise what +they have done, it is almost necessary to read several of the many treatises +devoted to this subject, and to inspect the animals. Breeders habitually speak +of an animal’s organisation as something quite plastic, which they can +model almost as they please. If I had space I could quote numerous passages to +this effect from highly competent authorities. Youatt, who was probably better +acquainted with the works of agriculturalists than almost any other individual, +and who was himself a very good judge of an animal, speaks of the principle of +selection as “that which enables the agriculturist, not only to modify +the character of his flock, but to change it altogether. It is the +magician’s wand, by means of which he may summon into life whatever form +and mould he pleases.” Lord Somerville, speaking of what breeders have +done for sheep, says:—“It would seem as if they had chalked out +upon a wall a form perfect in itself, and then had given it existence.” +That most skilful breeder, Sir John Sebright, used to say, with respect to +pigeons, that “he would produce any given feather in three years, but it +would take him six years to obtain head and beak.” In Saxony the +importance of the principle of selection in regard to merino sheep is so fully +recognised, that men follow it as a trade: the sheep are placed on a table and +are studied, like a picture by a connoisseur; this is done three times at +intervals of months, and the sheep are each time marked and classed, so that +the very best may ultimately be selected for breeding. +</p> + +<p> +What English breeders have actually effected is proved by the enormous prices +given for animals with a good pedigree; and these have now been exported to +almost every quarter of the world. The improvement is by no means generally due +to crossing different breeds; +<a name="Page32"></a> +all the best breeders are strongly opposed to this practice, except sometimes +amongst closely allied sub-breeds. And when a cross has been made, the closest +selection is far more indispensable even than in ordinary cases. If selection +consisted merely in separating some very distinct variety, and breeding from +it, the principle would be so obvious as hardly to be worth notice; but its +importance consists in the great effect produced by the accumulation in one +direction, during successive generations, of differences absolutely +inappreciable by an uneducated eye—differences which I for one have +vainly attempted to appreciate. Not one man in a thousand has accuracy of eye +and judgment sufficient to become an eminent breeder. If gifted with these +qualities, and he studies his subject for years, and devotes his lifetime to it +with indomitable perseverance, he will succeed, and may make great +improvements; if he wants any of these qualities, he will assuredly fail. Few +would readily believe in the natural capacity and years of practice requisite +to become even a skilful pigeon-fancier. +</p> + +<p> +The same principles are followed by horticulturists; but the variations are +here often more abrupt. No one supposes that our choicest productions have been +produced by a single variation from the aboriginal stock. We have proofs that +this is not so in some cases, in which exact records have been kept; thus, to +give a very trifling instance, the steadily-increasing size of the common +gooseberry may be quoted. We see an astonishing improvement in many +florists’ flowers, when the flowers of the present day are compared with +drawings made only twenty or thirty years ago. When a race of plants is once +pretty well established, the seed-raisers do not pick out the best plants, but +merely go over their seed-beds, and pull up the “rogues,” as they +call the plants that deviate from the proper standard. With animals this +<a name="Page33"></a> +kind of selection is, in fact, also followed; for hardly any one is so careless +as to allow his worst animals to breed. +</p> + +<p> +In regard to plants, there is another means of observing the accumulated +effects of selection—namely, by comparing the diversity of flowers in the +different varieties of the same species in the flower-garden; the diversity of +leaves, pods, or tubers, or whatever part is valued, in the kitchen-garden, in +comparison with the flowers of the same varieties; and the diversity of fruit +of the same species in the orchard, in comparison with the leaves and flowers +of the same set of varieties. See how different the leaves of the cabbage are, +and how extremely alike the flowers; how unlike the flowers of the heartsease +are, and how alike the leaves; how much the fruit of the different kinds of +gooseberries differ in size, colour, shape, and hairiness, and yet the flowers +present very slight differences. It is not that the varieties which differ +largely in some one point do not differ at all in other points; this is hardly +ever, perhaps never, the case. The laws of correlation of growth, the +importance of which should never be overlooked, will ensure some differences; +but, as a general rule, I cannot doubt that the continued selection of slight +variations, either in the leaves, the flowers, or the fruit, will produce races +differing from each other chiefly in these characters. +</p> + +<p> +It may be objected that the principle of selection has been reduced to +methodical practice for scarcely more than three-quarters of a century; it has +certainly been more attended to of late years, and many treatises have been +published on the subject; and the result, I may add, has been, in a +corresponding degree, rapid and important. But it is very far from true that +the principle is a modern discovery. I could give several references to the +full acknowledgment of the importance of the principle in works of high +antiquity. In rude and +<a name="Page34"></a> +barbarous periods of English history choice animals were often imported, and +laws were passed to prevent their exportation: the destruction of horses under +a certain size was ordered, and this may be compared to the +“roguing” of plants by nurserymen. The principle of selection I +find distinctly given in an ancient Chinese encyclopædia. Explicit rules are +laid down by some of the Roman classical writers. From passages in Genesis, it +is clear that the colour of domestic animals was at that early period attended +to. Savages now sometimes cross their dogs with wild canine animals, to improve +the breed, and they formerly did so, as is attested by passages in Pliny. The +savages in South Africa match their draught cattle by colour, as do some of the +Esquimaux their teams of dogs. Livingstone shows how much good domestic breeds +are valued by the negroes of the interior of Africa who have not associated +with Europeans. Some of these facts do not show actual selection, but they show +that the breeding of domestic animals was carefully attended to in ancient +times, and is now attended to by the lowest savages. It would, indeed, have +been a strange fact, had attention not been paid to breeding, for the +inheritance of good and bad qualities is so obvious. +</p> + +<p> +At the present time, eminent breeders try by methodical selection, with a +distinct object in view, to make a new strain or sub-breed, superior to +anything existing in the country. But, for our purpose, a kind of Selection, +which may be called Unconscious, and which results from every one trying to +possess and breed from the best individual animals, is more important. Thus, a +man who intends keeping pointers naturally tries to get as good dogs as he can, +and afterwards breeds from his own best dogs, but he has no wish or expectation +of permanently altering the breed. Nevertheless I cannot +<a name="Page35"></a> +doubt that this process, continued during centuries, would improve and modify +any breed, in the same way as Bakewell, Collins, etc., by this very same +process, only carried on more methodically, did greatly modify, even during +their own lifetimes, the forms and qualities of their cattle. Slow and +insensible changes of this kind could never be recognised unless actual +measurements or careful drawings of the breeds in question had been made long +ago, which might serve for comparison. In some cases, however, unchanged or but +little changed individuals of the same breed may be found in less civilised +districts, where the breed has been less improved. There is reason to believe +that King Charles’s spaniel has been unconsciously modified to a large +extent since the time of that monarch. Some highly competent authorities are +convinced that the setter is directly derived from the spaniel, and has +probably been slowly altered from it. It is known that the English pointer has +been greatly changed within the last century, and in this case the change has, +it is believed, been chiefly effected by crosses with the fox-hound; but what +concerns us is, that the change has been effected unconsciously and gradually, +and yet so effectually, that, though the old Spanish pointer certainly came +from Spain, Mr. Borrow has not seen, as I am informed by him, any native dog in +Spain like our pointer. +</p> + +<p> +By a similar process of selection, and by careful training, the whole body of +English racehorses have come to surpass in fleetness and size the parent Arab +stock, so that the latter, by the regulations for the Goodwood Races, are +favoured in the weights they carry. Lord Spencer and others have shown how the +cattle of England have increased in weight and in early maturity, compared with +the stock formerly kept in this country. By comparing the accounts given in old +pigeon treatises of carriers +<a name="Page36"></a> +and tumblers with these breeds as now existing in Britain, India, and Persia, +we can, I think, clearly trace the stages through which they have insensibly +passed, and come to differ so greatly from the rock-pigeon. +</p> + +<p> +Youatt gives an excellent illustration of the effects of a course of selection, +which may be considered as unconsciously followed, in so far that the breeders +could never have expected or even have wished to have produced the result which +ensued—namely, the production of two distinct strains. The two flocks of +Leicester sheep kept by Mr. Buckley and Mr. Burgess, as Mr. Youatt remarks, +“have been purely bred from the original stock of Mr. Bakewell for +upwards of fifty years. There is not a suspicion existing in the mind of any +one at all acquainted with the subject that the owner of either of them has +deviated in any one instance from the pure blood of Mr. Bakewell’s flock, +and yet the difference between the sheep possessed by these two gentlemen is so +great that they have the appearance of being quite different varieties.” +</p> + +<p> +If there exist savages so barbarous as never to think of the inherited +character of the offspring of their domestic animals, yet any one animal +particularly useful to them, for any special purpose, would be carefully +preserved during famines and other accidents, to which savages are so liable, +and such choice animals would thus generally leave more offspring than the +inferior ones; so that in this case there would be a kind of unconscious +selection going on. We see the value set on animals even by the barbarians of +Tierra del Fuego, by their killing and devouring their old women, in times of +dearth, as of less value than their dogs. +</p> + +<p> +In plants the same gradual process of improvement, through the occasional +preservation of the best individuals, whether or not sufficiently distinct to +be ranked +<a name="Page37"></a> +at their first appearance as distinct varieties, and whether or not two or more +species or races have become blended together by crossing, may plainly be +recognised in the increased size and beauty which we now see in the varieties +of the heartsease, rose, pelargonium, dahlia, and other plants, when compared +with the older varieties or with their parent-stocks. No one would ever expect +to get a first-rate heartsease or dahlia from the seed of a wild plant. No one +would expect to raise a first-rate melting pear from the seed of a wild pear, +though he might succeed from a poor seedling growing wild, if it had come from +a garden-stock. The pear, though cultivated in classical times, appears, from +Pliny’s description, to have been a fruit of very inferior quality. I +have seen great surprise expressed in horticultural works at the wonderful +skill of gardeners, in having produced such splendid results from such poor +materials; but the art, I cannot doubt, has been simple, and, as far as the +final result is concerned, has been followed almost unconsciously. It has +consisted in always cultivating the best known variety, sowing its seeds, and, +when a slightly better variety has chanced to appear, selecting it, and so +onwards. But the gardeners of the classical period, who cultivated the best +pear they could procure, never thought what splendid fruit we should eat; +though we owe our excellent fruit, in some small degree, to their having +naturally chosen and preserved the best varieties they could anywhere find. +</p> + +<p> +A large amount of change in our cultivated plants, thus slowly and +unconsciously accumulated, explains, as I believe, the well-known fact, that in +a vast number of cases we cannot recognise, and therefore do not know, the wild +parent-stocks of the plants which have been longest cultivated in our flower +and kitchen gardens. If it has taken centuries or thousands of years to improve +<a name="Page38"></a> +or modify most of our plants up to their present standard of usefulness to man, +we can understand how it is that neither Australia, the Cape of Good Hope, nor +any other region inhabited by quite uncivilised man, has afforded us a single +plant worth culture. It is not that these countries, so rich in species, do not +by a strange chance possess the aboriginal stocks of any useful plants, but +that the native plants have not been improved by continued selection up to a +standard of perfection comparable with that given to the plants in countries +anciently civilised. +</p> + +<p> +In regard to the domestic animals kept by uncivilised man, it should not be +overlooked that they almost always have to struggle for their own food, at +least during certain seasons. And in two countries very differently +circumstanced, individuals of the same species, having slightly different +constitutions or structure, would often succeed better in the one country than +in the other, and thus by a process of “natural selection,” as will +hereafter be more fully explained, two sub-breeds might be formed. This, +perhaps, partly explains what has been remarked by some authors, namely, that +the varieties kept by savages have more of the character of species than the +varieties kept in civilised countries. +</p> + +<p> +On the view here given of the all-important part which selection by man has +played, it becomes at once obvious, how it is that our domestic races show +adaptation in their structure or in their habits to man’s wants or +fancies. We can, I think, further understand the frequently abnormal character +of our domestic races, and likewise their differences being so great in +external characters and relatively so slight in internal parts or organs. Man +can hardly select, or only with much difficulty, any deviation of structure +excepting such as is externally visible; and indeed he rarely cares for what is +internal. He can never act by selection, excepting on variations +<a name="Page39"></a> +which are first given to him in some slight degree by nature. No man would ever +try to make a fantail, till he saw a pigeon with a tail developed in some +slight degree in an unusual manner, or a pouter till he saw a pigeon with a +crop of somewhat unusual size; and the more abnormal or unusual any character +was when it first appeared, the more likely it would be to catch his attention. +But to use such an expression as trying to make a fantail, is, I have no doubt, +in most cases, utterly incorrect. The man who first selected a pigeon with a +slightly larger tail, never dreamed what the descendants of that pigeon would +become through long-continued, partly unconscious and partly methodical +selection. Perhaps the parent bird of all fantails had only fourteen +tail-feathers somewhat expanded, like the present Java fantail, or like +individuals of other and distinct breeds, in which as many as seventeen +tail-feathers have been counted. Perhaps the first pouter-pigeon did not +inflate its crop much more than the turbit now does the upper part of its +oesophagus,—a habit which is disregarded by all fanciers, as it is not +one of the points of the breed. +</p> + +<p> +Nor let it be thought that some great deviation of structure would be necessary +to catch the fancier’s eye: he perceives extremely small differences, and +it is in human nature to value any novelty, however slight, in one’s own +possession. Nor must the value which would formerly be set on any slight +differences in the individuals of the same species, be judged of by the value +which would now be set on them, after several breeds have once fairly been +established. Many slight differences might, and indeed do now, arise amongst +pigeons, which are rejected as faults or deviations from the standard of +perfection of each breed. The common goose has not given rise to any marked +varieties; hence the Thoulouse and the common breed, which differ only in +colour, that +<a name="Page40"></a> +most fleeting of characters, have lately been exhibited as distinct at our +poultry-shows. +</p> + +<p> +I think these views further explain what has sometimes been +noticed—namely that we know nothing about the origin or history of any of +our domestic breeds. But, in fact, a breed, like a dialect of a language, can +hardly be said to have had a definite origin. A man preserves and breeds from +an individual with some slight deviation of structure, or takes more care than +usual in matching his best animals and thus improves them, and the improved +individuals slowly spread in the immediate neighbourhood. But as yet they will +hardly have a distinct name, and from being only slightly valued, their history +will be disregarded. When further improved by the same slow and gradual +process, they will spread more widely, and will get recognised as something +distinct and valuable, and will then probably first receive a provincial name. +In semi-civilised countries, with little free communication, the spreading and +knowledge of any new sub-breed will be a slow process. As soon as the points of +value of the new sub-breed are once fully acknowledged, the principle, as I +have called it, of unconscious selection will always tend,—perhaps more +at one period than at another, as the breed rises or falls in +fashion,—perhaps more in one district than in another, according to the +state of civilisation of the inhabitants—slowly to add to the +characteristic features of the breed, whatever they may be. But the chance will +be infinitely small of any record having been preserved of such slow, varying, +and insensible changes. +</p> + +<p> +I must now say a few words on the circumstances, favourable, or the reverse, to +man’s power of selection. A high degree of variability is obviously +favourable, as freely giving the materials for selection to work on; not that +mere individual differences are not amply +<a name="Page41"></a> +sufficient, with extreme care, to allow of the accumulation of a large amount +of modification in almost any desired direction. But as variations manifestly +useful or pleasing to man appear only occasionally, the chance of their +appearance will be much increased by a large number of individuals being kept; +and hence this comes to be of the highest importance to success. On this +principle Marshall has remarked, with respect to the sheep of parts of +Yorkshire, that “as they generally belong to poor people, and are mostly +<i>in small lots</i>, they never can be improved.” On the other hand, +nurserymen, from raising large stocks of the same plants, are generally far +more successful than amateurs in getting new and valuable varieties. The +keeping of a large number of individuals of a species in any country requires +that the species should be placed under favourable conditions of life, so as to +breed freely in that country. When the individuals of any species are scanty, +all the individuals, whatever their quality may be, will generally be allowed +to breed, and this will effectually prevent selection. But probably the most +important point of all, is, that the animal or plant should be so highly useful +to man, or so much valued by him, that the closest attention should be paid to +even the slightest deviation in the qualities or structure of each individual. +Unless such attention be paid nothing can be effected. I have seen it gravely +remarked, that it was most fortunate that the strawberry began to vary just +when gardeners began to attend closely to this plant. No doubt the strawberry +had always varied since it was cultivated, but the slight varieties had been +neglected. As soon, however, as gardeners picked out individual plants with +slightly larger, earlier, or better fruit, and raised seedlings from them, and +again picked out the best seedlings and bred from them, then, there appeared +(aided by some +<a name="Page42"></a> +crossing with distinct species) those many admirable varieties of the +strawberry which have been raised during the last thirty or forty years. +</p> + +<p> +In the case of animals with separate sexes, facility in preventing crosses is +an important element of success in the formation of new races,—at least, +in a country which is already stocked with other races. In this respect +enclosure of the land plays a part. Wandering savages or the inhabitants of +open plains rarely possess more than one breed of the same species. Pigeons can +be mated for life, and this is a great convenience to the fancier, for thus +many races may be kept true, though mingled in the same aviary; and this +circumstance must have largely favoured the improvement and formation of new +breeds. Pigeons, I may add, can be propagated in great numbers and at a very +quick rate, and inferior birds may be freely rejected, as when killed they +serve for food. On the other hand, cats, from their nocturnal rambling habits, +cannot be matched, and, although so much valued by women and children, we +hardly ever see a distinct breed kept up; such breeds as we do sometimes see +are almost always imported from some other country, often from islands. +Although I do not doubt that some domestic animals vary less than others, yet +the rarity or absence of distinct breeds of the cat, the donkey, peacock, +goose, etc., may be attributed in main part to selection not having been +brought into play: in cats, from the difficulty in pairing them; in donkeys, +from only a few being kept by poor people, and little attention paid to their +breeding; in peacocks, from not being very easily reared and a large stock not +kept; in geese, from being valuable only for two purposes, food and feathers, +and more especially from no pleasure having been felt in the display of +distinct breeds. +</p> + +<p> +<a name="Page43"></a> +To sum up on the origin of our Domestic Races of animals and plants. I believe +that the conditions of life, from their action on the reproductive system, are +so far of the highest importance as causing variability. I do not believe that +variability is an inherent and necessary contingency, under all circumstances, +with all organic beings, as some authors have thought. The effects of +variability are modified by various degrees of inheritance and of reversion. +Variability is governed by many unknown laws, more especially by that of +correlation of growth. Something may be attributed to the direct action of the +conditions of life. Something must be attributed to use and disuse. The final +result is thus rendered infinitely complex. In some cases, I do not doubt that +the intercrossing of species, aboriginally distinct, has played an important +part in the origin of our domestic productions. When in any country several +domestic breeds have once been established, their occasional intercrossing, +with the aid of selection, has, no doubt, largely aided in the formation of new +sub-breeds; but the importance of the crossing of varieties has, I believe, +been greatly exaggerated, both in regard to animals and to those plants which +are propagated by seed. In plants which are temporarily propagated by cuttings, +buds, etc., the importance of the crossing both of distinct species and of +varieties is immense; for the cultivator here quite disregards the extreme +variability both of hybrids and mongrels, and the frequent sterility of +hybrids; but the cases of plants not propagated by seed are of little +importance to us, for their endurance is only temporary. Over all these causes +of Change I am convinced that the accumulative action of Selection, whether +applied methodically and more quickly, or unconsciously and more slowly, but +more efficiently, is by far the predominant Power. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page44"></a><a name="chap02"></a>CHAPTER II.<br /> +VARIATION UNDER NATURE.</h2> + +<p class="letter"> +Variability. Individual differences. Doubtful species. Wide ranging, much +diffused, and common species vary most. Species of the larger genera in any +country vary more than the species of the smaller genera. Many of the species +of the larger genera resemble varieties in being very closely, but unequally, +related to each other, and in having restricted ranges. +</p> + +<p> +Before applying the principles arrived at in the last chapter to organic beings +in a state of nature, we must briefly discuss whether these latter are subject +to any variation. To treat this subject at all properly, a long catalogue of +dry facts should be given; but these I shall reserve for my future work. Nor +shall I here discuss the various definitions which have been given of the term +species. No one definition has as yet satisfied all naturalists; yet every +naturalist knows vaguely what he means when he speaks of a species. Generally +the term includes the unknown element of a distinct act of creation. The term +“variety” is almost equally difficult to define; but here community +of descent is almost universally implied, though it can rarely be proved. We +have also what are called monstrosities; but they graduate into varieties. By a +monstrosity I presume is meant some considerable deviation of structure in one +part, either injurious to or not useful to the species, and not generally +propagated. Some authors use the term “variation” in a technical +sense, as implying a modification directly due to the physical conditions of +life; and “variations” in this sense are supposed not to be +inherited: but who can say that the dwarfed condition of shells in the brackish +waters of the Baltic, or dwarfed +<a name="Page45"></a> +plants on Alpine summits, or the thicker fur of an animal from far northwards, +would not in some cases be inherited for at least some few generations? and in +this case I presume that the form would be called a variety. +</p> + +<p> +Again, we have many slight differences which may be called individual +differences, such as are known frequently to appear in the offspring from the +same parents, or which may be presumed to have thus arisen, from being +frequently observed in the individuals of the same species inhabiting the same +confined locality. No one supposes that all the individuals of the same species +are cast in the very same mould. These individual differences are highly +important for us, as they afford materials for natural selection to accumulate, +in the same manner as man can accumulate in any given direction individual +differences in his domesticated productions. These individual differences +generally affect what naturalists consider unimportant parts; but I could show +by a long catalogue of facts, that parts which must be called important, +whether viewed under a physiological or classificatory point of view, sometimes +vary in the individuals of the same species. I am convinced that the most +experienced naturalist would be surprised at the number of the cases of +variability, even in important parts of structure, which he could collect on +good authority, as I have collected, during a course of years. It should be +remembered that systematists are far from pleased at finding variability in +important characters, and that there are not many men who will laboriously +examine internal and important organs, and compare them in many specimens of +the same species. I should never have expected that the branching of the main +nerves close to the great central ganglion of an insect would have been +variable in the same species; I should have expected that changes of this +nature could have been effected only +<a name="Page46"></a> +by slow degrees: yet quite recently Mr. Lubbock has shown a degree of +variability in these main nerves in Coccus, which may almost be compared to the +irregular branching of the stem of a tree. This philosophical naturalist, I may +add, has also quite recently shown that the muscles in the larvæ of certain +insects are very far from uniform. Authors sometimes argue in a circle when +they state that important organs never vary; for these same authors practically +rank that character as important (as some few naturalists have honestly +confessed) which does not vary; and, under this point of view, no instance of +an important part varying will ever be found: but under any other point of view +many instances assuredly can be given. +</p> + +<p> +There is one point connected with individual differences, which seems to me +extremely perplexing: I refer to those genera which have sometimes been called +“protean” or “polymorphic,” in which the species +present an inordinate amount of variation; and hardly two naturalists can agree +which forms to rank as species and which as varieties. We may instance Rubus, +Rosa, and Hieracium amongst plants, several genera of insects, and several +genera of Brachiopod shells. In most polymorphic genera some of the species +have fixed and definite characters. Genera which are polymorphic in one country +seem to be, with some few exceptions, polymorphic in other countries, and +likewise, judging from Brachiopod shells, at former periods of time. These +facts seem to be very perplexing, for they seem to show that this kind of +variability is independent of the conditions of life. I am inclined to suspect +that we see in these polymorphic genera variations in points of structure which +are of no service or disservice to the species, and which consequently have not +been seized on and rendered definite by natural selection, as hereafter will be +explained. +</p> + +<p> +<a name="Page47"></a> +Those forms which possess in some considerable degree the character of species, +but which are so closely similar to some other forms, or are so closely linked +to them by intermediate gradations, that naturalists do not like to rank them +as distinct species, are in several respects the most important for us. We have +every reason to believe that many of these doubtful and closely-allied forms +have permanently retained their characters in their own country for a long +time; for as long, as far as we know, as have good and true species. +Practically, when a naturalist can unite two forms together by others having +intermediate characters, he treats the one as a variety of the other, ranking +the most common, but sometimes the one first described, as the species, and the +other as the variety. But cases of great difficulty, which I will not here +enumerate, sometimes occur in deciding whether or not to rank one form as a +variety of another, even when they are closely connected by intermediate links; +nor will the commonly-assumed hybrid nature of the intermediate links always +remove the difficulty. In very many cases, however, one form is ranked as a +variety of another, not because the intermediate links have actually been +found, but because analogy leads the observer to suppose either that they do +now somewhere exist, or may formerly have existed; and here a wide door for the +entry of doubt and conjecture is opened. +</p> + +<p> +Hence, in determining whether a form should be ranked as a species or a +variety, the opinion of naturalists having sound judgment and wide experience +seems the only guide to follow. We must, however, in many cases, decide by a +majority of naturalists, for few well-marked and well-known varieties can be +named which have not been ranked as species by at least some competent judges. +</p> + +<p> +<a name="Page48"></a> +That varieties of this doubtful nature are far from uncommon cannot be +disputed. Compare the several floras of Great Britain, of France or of the +United States, drawn up by different botanists, and see what a surprising +number of forms have been ranked by one botanist as good species, and by +another as mere varieties. Mr. H. C. Watson, to whom I lie under deep +obligation for assistance of all kinds, has marked for me 182 British plants, +which are generally considered as varieties, but which have all been ranked by +botanists as species; and in making this list he has omitted many trifling +varieties, but which nevertheless have been ranked by some botanists as +species, and he has entirely omitted several highly polymorphic genera. Under +genera, including the most polymorphic forms, Mr. Babington gives 251 species, +whereas Mr. Bentham gives only 112,—a difference of 139 doubtful forms! +Amongst animals which unite for each birth, and which are highly locomotive, +doubtful forms, ranked by one zoologist as a species and by another as a +variety, can rarely be found within the same country, but are common in +separated areas. How many of those birds and insects in North America and +Europe, which differ very slightly from each other, have been ranked by one +eminent naturalist as undoubted species, and by another as varieties, or, as +they are often called, as geographical races! Many years ago, when comparing, +and seeing others compare, the birds from the separate islands of the Galapagos +Archipelago, both one with another, and with those from the American mainland, +I was much struck how entirely vague and arbitrary is the distinction between +species and varieties. On the islets of the little Madeira group there are many +insects which are characterized as varieties in Mr. Wollaston’s admirable +work, but which it cannot +<a name="Page49"></a> +be doubted would be ranked as distinct species by many entomologists. Even +Ireland has a few animals, now generally regarded as varieties, but which have +been ranked as species by some zoologists. Several most experienced +ornithologists consider our British red grouse as only a strongly-marked race +of a Norwegian species, whereas the greater number rank it as an undoubted +species peculiar to Great Britain. A wide distance between the homes of two +doubtful forms leads many naturalists to rank both as distinct species; but +what distance, it has been well asked, will suffice? if that between America +and Europe is ample, will that between the Continent and the Azores, or +Madeira, or the Canaries, or Ireland, be sufficient? It must be admitted that +many forms, considered by highly-competent judges as varieties, have so +perfectly the character of species that they are ranked by other +highly-competent judges as good and true species. But to discuss whether they +are rightly called species or varieties, before any definition of these terms +has been generally accepted, is vainly to beat the air. +</p> + +<p> +Many of the cases of strongly-marked varieties or doubtful species well deserve +consideration; for several interesting lines of argument, from geographical +distribution, analogical variation, hybridism, etc., have been brought to bear +on the attempt to determine their rank. I will here give only a single +instance,—the well-known one of the primrose and cowslip, or Primula +veris and elatior. These plants differ considerably in appearance; they have a +different flavour and emit a different odour; they flower at slightly different +periods; they grow in somewhat different stations; they ascend mountains to +different heights; they have different geographical ranges; and lastly, +according to very numerous experiments made during several years by +<a name="Page50"></a> +that most careful observer Gärtner, they can be crossed only with much +difficulty. We could hardly wish for better evidence of the two forms being +specifically distinct. On the other hand, they are united by many intermediate +links, and it is very doubtful whether these links are hybrids; and there is, +as it seems to me, an overwhelming amount of experimental evidence, showing +that they descend from common parents, and consequently must be ranked as +varieties. +</p> + +<p> +Close investigation, in most cases, will bring naturalists to an agreement how +to rank doubtful forms. Yet it must be confessed, that it is in the best-known +countries that we find the greatest number of forms of doubtful value. I have +been struck with the fact, that if any animal or plant in a state of nature be +highly useful to man, or from any cause closely attract his attention, +varieties of it will almost universally be found recorded. These varieties, +moreover, will be often ranked by some authors as species. Look at the common +oak, how closely it has been studied; yet a German author makes more than a +dozen species out of forms, which are very generally considered as varieties; +and in this country the highest botanical authorities and practical men can be +quoted to show that the sessile and pedunculated oaks are either good and +distinct species or mere varieties. +</p> + +<p> +When a young naturalist commences the study of a group of organisms quite +unknown to him, he is at first much perplexed to determine what differences to +consider as specific, and what as varieties; for he knows nothing of the amount +and kind of variation to which the group is subject; and this shows, at least, +how very generally there is some variation. But if he confine his attention to +one class within one country, he will soon make up his mind how to rank most of +the doubtful forms. His +<a name="Page51"></a> +general tendency will be to make many species, for he will become impressed, +just like the pigeon or poultry-fancier before alluded to, with the amount of +difference in the forms which he is continually studying; and he has little +general knowledge of analogical variation in other groups and in other +countries, by which to correct his first impressions. As he extends the range +of his observations, he will meet with more cases of difficulty; for he will +encounter a greater number of closely-allied forms. But if his observations be +widely extended, he will in the end generally be enabled to make up his own +mind which to call varieties and which species; but he will succeed in this at +the expense of admitting much variation,—and the truth of this admission +will often be disputed by other naturalists. When, moreover, he comes to study +allied forms brought from countries not now continuous, in which case he can +hardly hope to find the intermediate links between his doubtful forms, he will +have to trust almost entirely to analogy, and his difficulties will rise to a +climax. +</p> + +<p> +Certainly no clear line of demarcation has as yet been drawn between species +and sub-species—that is, the forms which in the opinion of some +naturalists come very near to, but do not quite arrive at the rank of species; +or, again, between sub-species and well-marked varieties, or between lesser +varieties and individual differences. These differences blend into each other +in an insensible series; and a series impresses the mind with the idea of an +actual passage. +</p> + +<p> +Hence I look at individual differences, though of small interest to the +systematist, as of high importance for us, as being the first step towards such +slight varieties as are barely thought worth recording in works on natural +history. And I look at varieties which are in any degree more distinct and +permanent, as steps leading to more +<a name="Page52"></a> +strongly marked and more permanent varieties; and at these latter, as leading +to sub-species, and to species. The passage from one stage of difference to +another and higher stage may be, in some cases, due merely to the +long-continued action of different physical conditions in two different +regions; but I have not much faith in this view; and I attribute the passage of +a variety, from a state in which it differs very slightly from its parent to +one in which it differs more, to the action of natural selection in +accumulating (as will hereafter be more fully explained) differences of +structure in certain definite directions. Hence I believe a well-marked variety +may be justly called an incipient species; but whether this belief be +justifiable must be judged of by the general weight of the several facts and +views given throughout this work. +</p> + +<p> +It need not be supposed that all varieties or incipient species necessarily +attain the rank of species. They may whilst in this incipient state become +extinct, or they may endure as varieties for very long periods, as has been +shown to be the case by Mr. Wollaston with the varieties of certain fossil +land-shells in Madeira. If a variety were to flourish so as to exceed in +numbers the parent species, it would then rank as the species, and the species +as the variety; or it might come to supplant and exterminate the parent +species; or both might co-exist, and both rank as independent species. But we +shall hereafter have to return to this subject. +</p> + +<p> +From these remarks it will be seen that I look at the term species, as one +arbitrarily given for the sake of convenience to a set of individuals closely +resembling each other, and that it does not essentially differ from the term +variety, which is given to less distinct and more fluctuating forms. The term +variety, again, in comparison with mere individual differences, is also applied +arbitrarily, and for mere convenience sake. +</p> + +<p> +<a name="Page53"></a> +Guided by theoretical considerations, I thought that some interesting results +might be obtained in regard to the nature and relations of the species which +vary most, by tabulating all the varieties in several well-worked floras. At +first this seemed a simple task; but Mr. H. C. Watson, to whom I am much +indebted for valuable advice and assistance on this subject, soon convinced me +that there were many difficulties, as did subsequently Dr. Hooker, even in +stronger terms. I shall reserve for my future work the discussion of these +difficulties, and the tables themselves of the proportional numbers of the +varying species. Dr. Hooker permits me to add, that after having carefully read +my manuscript, and examined the tables, he thinks that the following statements +are fairly well established. The whole subject, however, treated as it +necessarily here is with much brevity, is rather perplexing, and allusions +cannot be avoided to the “struggle for existence,” +“divergence of character,” and other questions, hereafter to be +discussed. +</p> + +<p> +Alph. De Candolle and others have shown that plants which have very wide ranges +generally present varieties; and this might have been expected, as they become +exposed to diverse physical conditions, and as they come into competition +(which, as we shall hereafter see, is a far more important circumstance) with +different sets of organic beings. But my tables further show that, in any +limited country, the species which are most common, that is abound most in +individuals, and the species which are most widely diffused within their own +country (and this is a different consideration from wide range, and to a +certain extent from commonness), often give rise to varieties sufficiently +well-marked to have been recorded in botanical works. Hence it is the most +flourishing, or, as they may be called, the dominant species,—those +<a name="Page54"></a> +which range widely over the world, are the most diffused in their own country, +and are the most numerous in individuals,—which oftenest produce +well-marked varieties, or, as I consider them, incipient species. And this, +perhaps, might have been anticipated; for, as varieties, in order to become in +any degree permanent, necessarily have to struggle with the other inhabitants +of the country, the species which are already dominant will be the most likely +to yield offspring which, though in some slight degree modified, will still +inherit those advantages that enabled their parents to become dominant over +their compatriots. +</p> + +<p> +If the plants inhabiting a country and described in any Flora be divided into +two equal masses, all those in the larger genera being placed on one side, and +all those in the smaller genera on the other side, a somewhat larger number of +the very common and much diffused or dominant species will be found on the side +of the larger genera. This, again, might have been anticipated; for the mere +fact of many species of the same genus inhabiting any country, shows that there +is something in the organic or inorganic conditions of that country favourable +to the genus; and, consequently, we might have expected to have found in the +larger genera, or those including many species, a large proportional number of +dominant species. But so many causes tend to obscure this result, that I am +surprised that my tables show even a small majority on the side of the larger +genera. I will here allude to only two causes of obscurity. Fresh-water and +salt-loving plants have generally very wide ranges and are much diffused, but +this seems to be connected with the nature of the stations inhabited by them, +and has little or no relation to the size of the genera to which the species +belong. Again, plants low in the scale of organisation are +<a name="Page55"></a> +generally much more widely diffused than plants higher in the scale; and here +again there is no close relation to the size of the genera. The cause of +lowly-organised plants ranging widely will be discussed in our chapter on +geographical distribution. +</p> + +<p> +From looking at species as only strongly-marked and well-defined varieties, I +was led to anticipate that the species of the larger genera in each country +would oftener present varieties, than the species of the smaller genera; for +wherever many closely related species (<i>i.e.</i> species of the same genus) +have been formed, many varieties or incipient species ought, as a general rule, +to be now forming. Where many large trees grow, we expect to find saplings. +Where many species of a genus have been formed through variation, circumstances +have been favourable for variation; and hence we might expect that the +circumstances would generally be still favourable to variation. On the other +hand, if we look at each species as a special act of creation, there is no +apparent reason why more varieties should occur in a group having many species, +than in one having few. +</p> + +<p> +To test the truth of this anticipation I have arranged the plants of twelve +countries, and the coleopterous insects of two districts, into two nearly equal +masses, the species of the larger genera on one side, and those of the smaller +genera on the other side, and it has invariably proved to be the case that a +larger proportion of the species on the side of the larger genera present +varieties, than on the side of the smaller genera. Moreover, the species of the +large genera which present any varieties, invariably present a larger average +number of varieties than do the species of the small genera. Both these results +follow when another division is made, and when all the smallest genera, with +from only one to four species, are absolutely excluded from the tables. These +<a name="Page56"></a> +facts are of plain signification on the view that species are only strongly +marked and permanent varieties; for wherever many species of the same genus +have been formed, or where, if we may use the expression, the manufactory of +species has been active, we ought generally to find the manufactory still in +action, more especially as we have every reason to believe the process of +manufacturing new species to be a slow one. And this certainly is the case, if +varieties be looked at as incipient species; for my tables clearly show as a +general rule that, wherever many species of a genus have been formed, the +species of that genus present a number of varieties, that is of incipient +species, beyond the average. It is not that all large genera are now varying +much, and are thus increasing in the number of their species, or that no small +genera are now varying and increasing; for if this had been so, it would have +been fatal to my theory; inasmuch as geology plainly tells us that small genera +have in the lapse of time often increased greatly in size; and that large +genera have often come to their maxima, declined, and disappeared. All that we +want to show is, that where many species of a genus have been formed, on an +average many are still forming; and this holds good. +</p> + +<p> +There are other relations between the species of large genera and their +recorded varieties which deserve notice. We have seen that there is no +infallible criterion by which to distinguish species and well-marked varieties; +and in those cases in which intermediate links have not been found between +doubtful forms, naturalists are compelled to come to a determination by the +amount of difference between them, judging by analogy whether or not the amount +suffices to raise one or both to the rank of species. Hence the amount of +difference is one very important criterion in settling whether two forms should +<a name="Page57"></a> +be ranked as species or varieties. Now Fries has remarked in regard to plants, +and Westwood in regard to insects, that in large genera the amount of +difference between the species is often exceedingly small. I have endeavoured +to test this numerically by averages, and, as far as my imperfect results go, +they always confirm the view. I have also consulted some sagacious and most +experienced observers, and, after deliberation, they concur in this view. In +this respect, therefore, the species of the larger genera resemble varieties, +more than do the species of the smaller genera. Or the case may be put in +another way, and it may be said, that in the larger genera, in which a number +of varieties or incipient species greater than the average are now +manufacturing, many of the species already manufactured still to a certain +extent resemble varieties, for they differ from each other by a less than usual +amount of difference. +</p> + +<p> +Moreover, the species of the large genera are related to each other, in the +same manner as the varieties of any one species are related to each other. No +naturalist pretends that all the species of a genus are equally distinct from +each other; they may generally be divided into sub-genera, or sections, or +lesser groups. As Fries has well remarked, little groups of species are +generally clustered like satellites around certain other species. And what are +varieties but groups of forms, unequally related to each other, and clustered +round certain forms—that is, round their parent-species? Undoubtedly +there is one most important point of difference between varieties and species; +namely, that the amount of difference between varieties, when compared with +each other or with their parent-species, is much less than that between the +species of the same genus. But when we come to discuss the principle, as I call +it, of Divergence of Character, +<a name="Page58"></a> +we shall see how this may be explained, and how the lesser differences between +varieties will tend to increase into the greater differences between species. +</p> + +<p> +There is one other point which seems to me worth notice. Varieties generally +have much restricted ranges: this statement is indeed scarcely more than a +truism, for if a variety were found to have a wider range than that of its +supposed parent-species, their denominations ought to be reversed. But there is +also reason to believe, that those species which are very closely allied to +other species, and in so far resemble varieties, often have much restricted +ranges. For instance, Mr. H. C. Watson has marked for me in the well-sifted +London Catalogue of plants (4th edition) 63 plants which are therein ranked as +species, but which he considers as so closely allied to other species as to be +of doubtful value: these 63 reputed species range on an average over 6.9 of the +provinces into which Mr. Watson has divided Great Britain. Now, in this same +catalogue, 53 acknowledged varieties are recorded, and these range over 7.7 +provinces; whereas, the species to which these varieties belong range over 14.3 +provinces. So that the acknowledged varieties have very nearly the same +restricted average range, as have those very closely allied forms, marked for +me by Mr. Watson as doubtful species, but which are almost universally ranked +by British botanists as good and true species. +</p> + +<p class="p2"> +Finally, then, varieties have the same general characters as species, for they +cannot be distinguished from species,—except, firstly, by the discovery +of intermediate linking forms, and the occurrence of such links cannot affect +the actual characters of the forms which they connect; and except, secondly, by +a certain amount of +<a name="Page59"></a> +difference, for two forms, if differing very little, are generally ranked as +varieties, notwithstanding that intermediate linking forms have not been +discovered; but the amount of difference considered necessary to give to two +forms the rank of species is quite indefinite. In genera having more than the +average number of species in any country, the species of these genera have more +than the average number of varieties. In large genera the species are apt to be +closely, but unequally, allied together, forming little clusters round certain +species. Species very closely allied to other species apparently have +restricted ranges. In all these several respects the species of large genera +present a strong analogy with varieties. And we can clearly understand these +analogies, if species have once existed as varieties, and have thus originated: +whereas, these analogies are utterly inexplicable if each species has been +independently created. +</p> + +<p> +We have, also, seen that it is the most flourishing and dominant species of the +larger genera which on an average vary most; and varieties, as we shall +hereafter see, tend to become converted into new and distinct species. The +larger genera thus tend to become larger; and throughout nature the forms of +life which are now dominant tend to become still more dominant by leaving many +modified and dominant descendants. But by steps hereafter to be explained, the +larger genera also tend to break up into smaller genera. And thus, the forms of +life throughout the universe become divided into groups subordinate to groups. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page60"></a><a name="chap03"></a>CHAPTER III.<br /> +STRUGGLE FOR EXISTENCE.</h2> + +<p class="letter"> +Bears on natural selection. The term used in a wide sense. Geometrical powers +of increase. Rapid increase of naturalised animals and plants. Nature of the +checks to increase. Competition universal. Effects of climate. Protection from +the number of individuals. Complex relations of all animals and plants +throughout nature. Struggle for life most severe between individuals and +varieties of the same species; often severe between species of the same genus. +The relation of organism to organism the most important of all relations. +</p> + +<p> +Before entering on the subject of this chapter, I must make a few preliminary +remarks, to show how the struggle for existence bears on Natural Selection. It +has been seen in the last chapter that amongst organic beings in a state of +nature there is some individual variability; indeed I am not aware that this +has ever been disputed. It is immaterial for us whether a multitude of doubtful +forms be called species or sub-species or varieties; what rank, for instance, +the two or three hundred doubtful forms of British plants are entitled to hold, +if the existence of any well-marked varieties be admitted. But the mere +existence of individual variability and of some few well-marked varieties, +though necessary as the foundation for the work, helps us but little in +understanding how species arise in nature. How have all those exquisite +adaptations of one part of the organisation to another part, and to the +conditions of life, and of one distinct organic being to another being, been +perfected? We see these beautiful co-adaptations most plainly in the woodpecker +and missletoe; and only a little less plainly in the humblest parasite which +clings +<a name="Page61"></a> +to the hairs of a quadruped or feathers of a bird; in the structure of the +beetle which dives through the water; in the plumed seed which is wafted by the +gentlest breeze; in short, we see beautiful adaptations everywhere and in every +part of the organic world. +</p> + +<p> +Again, it may be asked, how is it that varieties, which I have called incipient +species, become ultimately converted into good and distinct species, which in +most cases obviously differ from each other far more than do the varieties of +the same species? How do those groups of species, which constitute what are +called distinct genera, and which differ from each other more than do the +species of the same genus, arise? All these results, as we shall more fully see +in the next chapter, follow inevitably from the struggle for life. Owing to +this struggle for life, any variation, however slight and from whatever cause +proceeding, if it be in any degree profitable to an individual of any species, +in its infinitely complex relations to other organic beings and to external +nature, will tend to the preservation of that individual, and will generally be +inherited by its offspring. The offspring, also, will thus have a better chance +of surviving, for, of the many individuals of any species which are +periodically born, but a small number can survive. I have called this +principle, by which each slight variation, if useful, is preserved, by the term +of Natural Selection, in order to mark its relation to man’s power of +selection. We have seen that man by selection can certainly produce great +results, and can adapt organic beings to his own uses, through the accumulation +of slight but useful variations, given to him by the hand of Nature. But +Natural Selection, as we shall hereafter see, is a power incessantly ready for +action, and is as immeasurably superior to man’s feeble efforts, as the +works of Nature are to those of Art. +</p> + +<p> +<a name="Page62"></a> +We will now discuss in a little more detail the struggle for existence. In my +future work this subject shall be treated, as it well deserves, at much greater +length. The elder De Candolle and Lyell have largely and philosophically shown +that all organic beings are exposed to severe competition. In regard to plants, +no one has treated this subject with more spirit and ability than W. Herbert, +Dean of Manchester, evidently the result of his great horticultural knowledge. +Nothing is easier than to admit in words the truth of the universal struggle +for life, or more difficult—at least I have found it so—than +constantly to bear this conclusion in mind. Yet unless it be thoroughly +engrained in the mind, I am convinced that the whole economy of nature, with +every fact on distribution, rarity, abundance, extinction, and variation, will +be dimly seen or quite misunderstood. We behold the face of nature bright with +gladness, we often see superabundance of food; we do not see, or we forget, +that the birds which are idly singing round us mostly live on insects or seeds, +and are thus constantly destroying life; or we forget how largely these +songsters, or their eggs, or their nestlings, are destroyed by birds and beasts +of prey; we do not always bear in mind, that though food may be now +superabundant, it is not so at all seasons of each recurring year. +</p> + +<p> +I should premise that I use the term Struggle for Existence in a large and +metaphorical sense, including dependence of one being on another, and including +(which is more important) not only the life of the individual, but success in +leaving progeny. Two canine animals in a time of dearth, may be truly said to +struggle with each other which shall get food and live. But a plant on the edge +of a desert is said to struggle for life against the drought, though more +properly it should be said to be dependent on the moisture. A +<a name="Page63"></a> +plant which annually produces a thousand seeds, of which on an average only one +comes to maturity, may be more truly said to struggle with the plants of the +same and other kinds which already clothe the ground. The missletoe is +dependent on the apple and a few other trees, but can only in a far-fetched +sense be said to struggle with these trees, for if too many of these parasites +grow on the same tree, it will languish and die. But several seedling +missletoes, growing close together on the same branch, may more truly be said +to struggle with each other. As the missletoe is disseminated by birds, its +existence depends on birds; and it may metaphorically be said to struggle with +other fruit-bearing plants, in order to tempt birds to devour and thus +disseminate its seeds rather than those of other plants. In these several +senses, which pass into each other, I use for convenience sake the general term +of struggle for existence. +</p> + +<p> +A struggle for existence inevitably follows from the high rate at which all +organic beings tend to increase. Every being, which during its natural lifetime +produces several eggs or seeds, must suffer destruction during some period of +its life, and during some season or occasional year, otherwise, on the +principle of geometrical increase, its numbers would quickly become so +inordinately great that no country could support the product. Hence, as more +individuals are produced than can possibly survive, there must in every case be +a struggle for existence, either one individual with another of the same +species, or with the individuals of distinct species, or with the physical +conditions of life. It is the doctrine of Malthus applied with manifold force +to the whole animal and vegetable kingdoms; for in this case there can be no +artificial increase of food, and no prudential restraint from marriage. +Although some species may be +<a name="Page64"></a> +now increasing, more or less rapidly, in numbers, all cannot do so, for the +world would not hold them. +</p> + +<p> +There is no exception to the rule that every organic being naturally increases +at so high a rate, that if not destroyed, the earth would soon be covered by +the progeny of a single pair. Even slow-breeding man has doubled in twenty-five +years, and at this rate, in a few thousand years, there would literally not be +standing room for his progeny. Linnæus has calculated that if an annual plant +produced only two seeds—and there is no plant so unproductive as +this—and their seedlings next year produced two, and so on, then in +twenty years there would be a million plants. The elephant is reckoned to be +the slowest breeder of all known animals, and I have taken some pains to +estimate its probable minimum rate of natural increase: it will be under the +mark to assume that it breeds when thirty years old, and goes on breeding till +ninety years old, bringing forth three pair of young in this interval; if this +be so, at the end of the fifth century there would be alive fifteen million +elephants, descended from the first pair. +</p> + +<p> +But we have better evidence on this subject than mere theoretical calculations, +namely, the numerous recorded cases of the astonishingly rapid increase of +various animals in a state of nature, when circumstances have been favourable +to them during two or three following seasons. Still more striking is the +evidence from our domestic animals of many kinds which have run wild in several +parts of the world: if the statements of the rate of increase of slow-breeding +cattle and horses in South America, and latterly in Australia, had not been +well authenticated, they would have been quite incredible. So it is with +plants: cases could be given of introduced plants which have become common +throughout whole islands in a period of less than ten years. Several +<a name="Page65"></a> +of the plants now most numerous over the wide plains of La Plata, clothing +square leagues of surface almost to the exclusion of all other plants, have +been introduced from Europe; and there are plants which now range in India, as +I hear from Dr. Falconer, from Cape Comorin to the Himalaya, which have been +imported from America since its discovery. In such cases, and endless instances +could be given, no one supposes that the fertility of these animals or plants +has been suddenly and temporarily increased in any sensible degree. The obvious +explanation is that the conditions of life have been very favourable, and that +there has consequently been less destruction of the old and young, and that +nearly all the young have been enabled to breed. In such cases the geometrical +ratio of increase, the result of which never fails to be surprising, simply +explains the extraordinarily rapid increase and wide diffusion of naturalised +productions in their new homes. +</p> + +<p> +In a state of nature almost every plant produces seed, and amongst animals +there are very few which do not annually pair. Hence we may confidently assert, +that all plants and animals are tending to increase at a geometrical ratio, +that all would most rapidly stock every station in which they could any how +exist, and that the geometrical tendency to increase must be checked by +destruction at some period of life. Our familiarity with the larger domestic +animals tends, I think, to mislead us: we see no great destruction falling on +them, and we forget that thousands are annually slaughtered for food, and that +in a state of nature an equal number would have somehow to be disposed of. +</p> + +<p> +The only difference between organisms which annually produce eggs or seeds by +the thousand, and those which produce extremely few, is, that the slow-breeders +would require a few more years to people, under favourable +<a name="Page66"></a> +conditions, a whole district, let it be ever so large. The condor lays a couple +of eggs and the ostrich a score, and yet in the same country the condor may be +the more numerous of the two: the Fulmar petrel lays but one egg, yet it is +believed to be the most numerous bird in the world. One fly deposits hundreds +of eggs, and another, like the hippobosca, a single one; but this difference +does not determine how many individuals of the two species can be supported in +a district. A large number of eggs is of some importance to those species, +which depend on a rapidly fluctuating amount of food, for it allows them +rapidly to increase in number. But the real importance of a large number of +eggs or seeds is to make up for much destruction at some period of life; and +this period in the great majority of cases is an early one. If an animal can in +any way protect its own eggs or young, a small number may be produced, and yet +the average stock be fully kept up; but if many eggs or young are destroyed, +many must be produced, or the species will become extinct. It would suffice to +keep up the full number of a tree, which lived on an average for a thousand +years, if a single seed were produced once in a thousand years, supposing that +this seed were never destroyed, and could be ensured to germinate in a fitting +place. So that in all cases, the average number of any animal or plant depends +only indirectly on the number of its eggs or seeds. +</p> + +<p> +In looking at Nature, it is most necessary to keep the foregoing considerations +always in mind—never to forget that every single organic being around us +may be said to be striving to the utmost to increase in numbers; that each +lives by a struggle at some period of its life; that heavy destruction +inevitably falls either on the young or old, during each generation or at +recurrent intervals. Lighten any check, mitigate the +<a name="Page67"></a> +destruction ever so little, and the number of the species will almost +instantaneously increase to any amount. The face of Nature may be compared to a +yielding surface, with ten thousand sharp wedges packed close together and +driven inwards by incessant blows, sometimes one wedge being struck, and then +another with greater force. +</p> + +<p> +What checks the natural tendency of each species to increase in number is most +obscure. Look at the most vigorous species; by as much as it swarms in numbers, +by so much will its tendency to increase be still further increased. We know +not exactly what the checks are in even one single instance. Nor will this +surprise any one who reflects how ignorant we are on this head, even in regard +to mankind, so incomparably better known than any other animal. This subject +has been ably treated by several authors, and I shall, in my future work, +discuss some of the checks at considerable length, more especially in regard to +the feral animals of South America. Here I will make only a few remarks, just +to recall to the reader’s mind some of the chief points. Eggs or very +young animals seem generally to suffer most, but this is not invariably the +case. With plants there is a vast destruction of seeds, but, from some +observations which I have made, I believe that it is the seedlings which suffer +most from germinating in ground already thickly stocked with other plants. +Seedlings, also, are destroyed in vast numbers by various enemies; for +instance, on a piece of ground three feet long and two wide, dug and cleared, +and where there could be no choking from other plants, I marked all the +seedlings of our native weeds as they came up, and out of the 357 no less than +295 were destroyed, chiefly by slugs and insects. If turf which has long been +mown, and the case would be the same with turf closely browsed by quadrupeds, +be let to grow, +<a name="Page68"></a> +the more vigorous plants gradually kill the less vigorous, though fully grown, +plants: thus out of twenty species growing on a little plot of turf (three feet +by four) nine species perished from the other species being allowed to grow up +freely. +</p> + +<p> +The amount of food for each species of course gives the extreme limit to which +each can increase; but very frequently it is not the obtaining food, but the +serving as prey to other animals, which determines the average numbers of a +species. Thus, there seems to be little doubt that the stock of partridges, +grouse, and hares on any large estate depends chiefly on the destruction of +vermin. If not one head of game were shot during the next twenty years in +England, and, at the same time, if no vermin were destroyed, there would, in +all probability, be less game than at present, although hundreds of thousands +of game animals are now annually killed. On the other hand, in some cases, as +with the elephant and rhinoceros, none are destroyed by beasts of prey: even +the tiger in India most rarely dares to attack a young elephant protected by +its dam. +</p> + +<p> +Climate plays an important part in determining the average numbers of a +species, and periodical seasons of extreme cold or drought, I believe to be the +most effective of all checks. I estimated that the winter of 1854-55 destroyed +four-fifths of the birds in my own grounds; and this is a tremendous +destruction, when we remember that ten per cent. is an extraordinarily severe +mortality from epidemics with man. The action of climate seems at first sight +to be quite independent of the struggle for existence; but in so far as climate +chiefly acts in reducing food, it brings on the most severe struggle between +the individuals, whether of the same or of distinct species, which subsist on +the same kind of food. Even when climate, for instance extreme +<a name="Page69"></a> +cold, acts directly, it will be the least vigorous, or those which have got +least food through the advancing winter, which will suffer most. When we travel +from south to north, or from a damp region to a dry, we invariably see some +species gradually getting rarer and rarer, and finally disappearing; and the +change of climate being conspicuous, we are tempted to attribute the whole +effect to its direct action. But this is a very false view: we forget that each +species, even where it most abounds, is constantly suffering enormous +destruction at some period of its life, from enemies or from competitors for +the same place and food; and if these enemies or competitors be in the least +degree favoured by any slight change of climate, they will increase in numbers, +and, as each area is already fully stocked with inhabitants, the other species +will decrease. When we travel southward and see a species decreasing in +numbers, we may feel sure that the cause lies quite as much in other species +being favoured, as in this one being hurt. So it is when we travel northward, +but in a somewhat lesser degree, for the number of species of all kinds, and +therefore of competitors, decreases northwards; hence in going northward, or in +ascending a mountain, we far oftener meet with stunted forms, due to the +<i>directly</i> injurious action of climate, than we do in proceeding +southwards or in descending a mountain. When we reach the Arctic regions, or +snow-capped summits, or absolute deserts, the struggle for life is almost +exclusively with the elements. +</p> + +<p> +That climate acts in main part indirectly by favouring other species, we may +clearly see in the prodigious number of plants in our gardens which can +perfectly well endure our climate, but which never become naturalised, for they +cannot compete with our native plants, nor resist destruction by our native +animals. +</p> + +<p> +<a name="Page70"></a> +When a species, owing to highly favourable circumstances, increases +inordinately in numbers in a small tract, epidemics—at least, this seems +generally to occur with our game animals—often ensue: and here we have a +limiting check independent of the struggle for life. But even some of these +so-called epidemics appear to be due to parasitic worms, which have from some +cause, possibly in part through facility of diffusion amongst the crowded +animals, been disproportionably favoured: and here comes in a sort of struggle +between the parasite and its prey. +</p> + +<p> +On the other hand, in many cases, a large stock of individuals of the same +species, relatively to the numbers of its enemies, is absolutely necessary for +its preservation. Thus we can easily raise plenty of corn and rape-seed, etc., +in our fields, because the seeds are in great excess compared with the number +of birds which feed on them; nor can the birds, though having a superabundance +of food at this one season, increase in number proportionally to the supply of +seed, as their numbers are checked during winter: but any one who has tried, +knows how troublesome it is to get seed from a few wheat or other such plants +in a garden; I have in this case lost every single seed. This view of the +necessity of a large stock of the same species for its preservation, explains, +I believe, some singular facts in nature, such as that of very rare plants +being sometimes extremely abundant in the few spots where they do occur; and +that of some social plants being social, that is, abounding in individuals, +even on the extreme confines of their range. For in such cases, we may believe, +that a plant could exist only where the conditions of its life were so +favourable that many could exist together, and thus save each other from utter +destruction. I should add that the good effects of frequent intercrossing, and +the ill effects +<a name="Page71"></a> +of close interbreeding, probably come into play in some of these cases; but on +this intricate subject I will not here enlarge. +</p> + +<p> +Many cases are on record showing how complex and unexpected are the checks and +relations between organic beings, which have to struggle together in the same +country. I will give only a single instance, which, though a simple one, has +interested me. In Staffordshire, on the estate of a relation where I had ample +means of investigation, there was a large and extremely barren heath, which had +never been touched by the hand of man; but several hundred acres of exactly the +same nature had been enclosed twenty-five years previously and planted with +Scotch fir. The change in the native vegetation of the planted part of the +heath was most remarkable, more than is generally seen in passing from one +quite different soil to another: not only the proportional numbers of the +heath-plants were wholly changed, but twelve species of plants (not counting +grasses and carices) flourished in the plantations, which could not be found on +the heath. The effect on the insects must have been still greater, for six +insectivorous birds were very common in the plantations, which were not to be +seen on the heath; and the heath was frequented by two or three distinct +insectivorous birds. Here we see how potent has been the effect of the +introduction of a single tree, nothing whatever else having been done, with the +exception that the land had been enclosed, so that cattle could not enter. But +how important an element enclosure is, I plainly saw near Farnham, in Surrey. +Here there are extensive heaths, with a few clumps of old Scotch firs on the +distant hill-tops: within the last ten years large spaces have been enclosed, +and self-sown firs are now springing up in multitudes, so close together that +all cannot live. +<a name="Page72"></a> +When I ascertained that these young trees had not been sown or planted, I was +so much surprised at their numbers that I went to several points of view, +whence I could examine hundreds of acres of the unenclosed heath, and literally +I could not see a single Scotch fir, except the old planted clumps. But on +looking closely between the stems of the heath, I found a multitude of +seedlings and little trees, which had been perpetually browsed down by the +cattle. In one square yard, at a point some hundred yards distant from one of +the old clumps, I counted thirty-two little trees; and one of them, judging +from the rings of growth, had during twenty-six years tried to raise its head +above the stems of the heath, and had failed. No wonder that, as soon as the +land was enclosed, it became thickly clothed with vigorously growing young +firs. Yet the heath was so extremely barren and so extensive that no one would +ever have imagined that cattle would have so closely and effectually searched +it for food. +</p> + +<p> +Here we see that cattle absolutely determine the existence of the Scotch fir; +but in several parts of the world insects determine the existence of cattle. +Perhaps Paraguay offers the most curious instance of this; for here neither +cattle nor horses nor dogs have ever run wild, though they swarm southward and +northward in a feral state; and Azara and Rengger have shown that this is +caused by the greater number in Paraguay of a certain fly, which lays its eggs +in the navels of these animals when first born. The increase of these flies, +numerous as they are, must be habitually checked by some means, probably by +birds. Hence, if certain insectivorous birds (whose numbers are probably +regulated by hawks or beasts of prey) were to increase in Paraguay, the flies +would decrease—then cattle and horses would become feral, and this would +certainly greatly alter (as +<a name="Page73"></a> +indeed I have observed in parts of South America) the vegetation: this again +would largely affect the insects; and this, as we just have seen in +Staffordshire, the insectivorous birds, and so onwards in ever-increasing +circles of complexity. We began this series by insectivorous birds, and we have +ended with them. Not that in nature the relations can ever be as simple as +this. Battle within battle must ever be recurring with varying success; and yet +in the long-run the forces are so nicely balanced, that the face of nature +remains uniform for long periods of time, though assuredly the merest trifle +would often give the victory to one organic being over another. Nevertheless so +profound is our ignorance, and so high our presumption, that we marvel when we +hear of the extinction of an organic being; and as we do not see the cause, we +invoke cataclysms to desolate the world, or invent laws on the duration of the +forms of life! +</p> + +<p> +I am tempted to give one more instance showing how plants and animals, most +remote in the scale of nature, are bound together by a web of complex +relations. I shall hereafter have occasion to show that the exotic Lobelia +fulgens, in this part of England, is never visited by insects, and +consequently, from its peculiar structure, never can set a seed. Many of our +orchidaceous plants absolutely require the visits of moths to remove their +pollen-masses and thus to fertilise them. I have, also, reason to believe that +humble-bees are indispensable to the fertilisation of the heartsease (Viola +tricolor), for other bees do not visit this flower. From experiments which I +have tried, I have found that the visits of bees, if not indispensable, are at +least highly beneficial to the fertilisation of our clovers; but humble-bees +alone visit the common red clover (Trifolium pratense), as other bees cannot +reach the nectar. Hence I have very little doubt, that if the whole genus of +humble-bees became +<a name="Page74"></a> +extinct or very rare in England, the heartsease and red clover would become +very rare, or wholly disappear. The number of humble-bees in any district +depends in a great degree on the number of field-mice, which destroy their +combs and nests; and Mr. H. Newman, who has long attended to the habits of +humble-bees, believes that “more than two thirds of them are thus +destroyed all over England.” Now the number of mice is largely dependent, +as every one knows, on the number of cats; and Mr. Newman says, “Near +villages and small towns I have found the nests of humble-bees more numerous +than elsewhere, which I attribute to the number of cats that destroy the +mice.” Hence it is quite credible that the presence of a feline animal in +large numbers in a district might determine, through the intervention first of +mice and then of bees, the frequency of certain flowers in that district! +</p> + +<p> +In the case of every species, many different checks, acting at different +periods of life, and during different seasons or years, probably come into +play; some one check or some few being generally the most potent, but all +concurring in determining the average number or even the existence of the +species. In some cases it can be shown that widely-different checks act on the +same species in different districts. When we look at the plants and bushes +clothing an entangled bank, we are tempted to attribute their proportional +numbers and kinds to what we call chance. But how false a view is this! Every +one has heard that when an American forest is cut down, a very different +vegetation springs up; but it has been observed that the trees now growing on +the ancient Indian mounds, in the Southern United States, display the same +beautiful diversity and proportion of kinds as in the surrounding virgin +forests. What a struggle between the several kinds of trees +<a name="Page75"></a> +must here have gone on during long centuries, each annually scattering its +seeds by the thousand; what war between insect and insect—between +insects, snails, and other animals with birds and beasts of prey—all +striving to increase, and all feeding on each other or on the trees or their +seeds and seedlings, or on the other plants which first clothed the ground and +thus checked the growth of the trees! Throw up a handful of feathers, and all +must fall to the ground according to definite laws; but how simple is this +problem compared to the action and reaction of the innumerable plants and +animals which have determined, in the course of centuries, the proportional +numbers and kinds of trees now growing on the old Indian ruins! +</p> + +<p> +The dependency of one organic being on another, as of a parasite on its prey, +lies generally between beings remote in the scale of nature. This is often the +case with those which may strictly be said to struggle with each other for +existence, as in the case of locusts and grass-feeding quadrupeds. But the +struggle almost invariably will be most severe between the individuals of the +same species, for they frequent the same districts, require the same food, and +are exposed to the same dangers. In the case of varieties of the same species, +the struggle will generally be almost equally severe, and we sometimes see the +contest soon decided: for instance, if several varieties of wheat be sown +together, and the mixed seed be resown, some of the varieties which best suit +the soil or climate, or are naturally the most fertile, will beat the others +and so yield more seed, and will consequently in a few years quite supplant the +other varieties. To keep up a mixed stock of even such extremely close +varieties as the variously coloured sweet-peas, they must be each year +harvested separately, and the seed then mixed in due proportion, +<a name="Page76"></a> +otherwise the weaker kinds will steadily decrease in numbers and disappear. So +again with the varieties of sheep: it has been asserted that certain +mountain-varieties will starve out other mountain-varieties, so that they +cannot be kept together. The same result has followed from keeping together +different varieties of the medicinal leech. It may even be doubted whether the +varieties of any one of our domestic plants or animals have so exactly the same +strength, habits, and constitution, that the original proportions of a mixed +stock could be kept up for half a dozen generations, if they were allowed to +struggle together, like beings in a state of nature, and if the seed or young +were not annually sorted. +</p> + +<p> +As species of the same genus have usually, though by no means invariably, some +similarity in habits and constitution, and always in structure, the struggle +will generally be more severe between species of the same genus, when they come +into competition with each other, than between species of distinct genera. We +see this in the recent extension over parts of the United States of one species +of swallow having caused the decrease of another species. The recent increase +of the missel-thrush in parts of Scotland has caused the decrease of the +song-thrush. How frequently we hear of one species of rat taking the place of +another species under the most different climates! In Russia the small Asiatic +cockroach has everywhere driven before it its great congener. One species of +charlock will supplant another, and so in other cases. We can dimly see why the +competition should be most severe between allied forms, which fill nearly the +same place in the economy of nature; but probably in no one case could we +precisely say why one species has been victorious over another in the great +battle of life. +</p> + +<p> +<a name="Page77"></a> +A corollary of the highest importance may be deduced from the foregoing +remarks, namely, that the structure of every organic being is related, in the +most essential yet often hidden manner, to that of all other organic beings, +with which it comes into competition for food or residence, or from which it +has to escape, or on which it preys. This is obvious in the structure of the +teeth and talons of the tiger; and in that of the legs and claws of the +parasite which clings to the hair on the tiger’s body. But in the +beautifully plumed seed of the dandelion, and in the flattened and fringed legs +of the water-beetle, the relation seems at first confined to the elements of +air and water. Yet the advantage of plumed seeds no doubt stands in the closest +relation to the land being already thickly clothed by other plants; so that the +seeds may be widely distributed and fall on unoccupied ground. In the +water-beetle, the structure of its legs, so well adapted for diving, allows it +to compete with other aquatic insects, to hunt for its own prey, and to escape +serving as prey to other animals. +</p> + +<p> +The store of nutriment laid up within the seeds of many plants seems at first +sight to have no sort of relation to other plants. But from the strong growth +of young plants produced from such seeds (as peas and beans), when sown in the +midst of long grass, I suspect that the chief use of the nutriment in the seed +is to favour the growth of the young seedling, whilst struggling with other +plants growing vigorously all around. +</p> + +<p> +Look at a plant in the midst of its range, why does it not double or quadruple +its numbers? We know that it can perfectly well withstand a little more heat or +cold, dampness or dryness, for elsewhere it ranges +<a name="Page78"></a> +into slightly hotter or colder, damper or drier districts. In this case we can +clearly see that if we wished in imagination to give the plant the power of +increasing in number, we should have to give it some advantage over its +competitors, or over the animals which preyed on it. On the confines of its +geographical range, a change of constitution with respect to climate would +clearly be an advantage to our plant; but we have reason to believe that only a +few plants or animals range so far, that they are destroyed by the rigour of +the climate alone. Not until we reach the extreme confines of life, in the +arctic regions or on the borders of an utter desert, will competition cease. +The land may be extremely cold or dry, yet there will be competition between +some few species, or between the individuals of the same species, for the +warmest or dampest spots. +</p> + +<p> +Hence, also, we can see that when a plant or animal is placed in a new country +amongst new competitors, though the climate may be exactly the same as in its +former home, yet the conditions of its life will generally be changed in an +essential manner. If we wished to increase its average numbers in its new home, +we should have to modify it in a different way to what we should have done in +its native country; for we should have to give it some advantage over a +different set of competitors or enemies. +</p> + +<p> +It is good thus to try in our imagination to give any form some advantage over +another. Probably in no single instance should we know what to do, so as to +succeed. It will convince us of our ignorance on the mutual relations of all +organic beings; a conviction as necessary, as it seems to be difficult to +acquire. All that we can do, is to keep steadily in mind that each organic +being is striving to increase at a geometrical +<a name="Page79"></a> +ratio; that each at some period of its life, during some season of the year, +during each generation or at intervals, has to struggle for life, and to suffer +great destruction. When we reflect on this struggle, we may console ourselves +with the full belief, that the war of nature is not incessant, that no fear is +felt, that death is generally prompt, and that the vigorous, the healthy, and +the happy survive and multiply. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page80"></a><a name="chap04"></a>CHAPTER IV.<br /> +NATURAL SELECTION.</h2> + +<p class="letter"> +Natural Selection: its power compared with man’s selection, its power on +characters of trifling importance, its power at all ages and on both sexes. +Sexual Selection. On the generality of intercrosses between individuals of the +same species. Circumstances favourable and unfavourable to Natural Selection, +namely, intercrossing, isolation, number of individuals. Slow action. +Extinction caused by Natural Selection. Divergence of Character, related to the +diversity of inhabitants of any small area, and to naturalisation. Action of +Natural Selection, through Divergence of Character and Extinction, on the +descendants from a common parent. Explains the Grouping of all organic beings. +</p> + +<p> +How will the struggle for existence, discussed too briefly in the last chapter, +act in regard to variation? Can the principle of selection, which we have seen +is so potent in the hands of man, apply in nature? I think we shall see that it +can act most effectually. Let it be borne in mind in what an endless number of +strange peculiarities our domestic productions, and, in a lesser degree, those +under nature, vary; and how strong the hereditary tendency is. Under +domestication, it may be truly said that the whole organisation becomes in some +degree plastic. Let it be borne in mind how infinitely complex and +close-fitting are the mutual relations of all organic beings to each other and +to their physical conditions of life. Can it, then, be thought improbable, +seeing that variations useful to man have undoubtedly occurred, that other +variations useful in some way to each being in the great and complex battle of +life, should sometimes occur in the course of thousands of generations? If such +do occur, can we doubt (remembering +<a name="Page81"></a> +that many more individuals are born than can possibly survive) that individuals +having any advantage, however slight, over others, would have the best chance +of surviving and of procreating their kind? On the other hand, we may feel sure +that any variation in the least degree injurious would be rigidly destroyed. +This preservation of favourable variations and the rejection of injurious +variations, I call Natural Selection. Variations neither useful nor injurious +would not be affected by natural selection, and would be left a fluctuating +element, as perhaps we see in the species called polymorphic. +</p> + +<p> +We shall best understand the probable course of natural selection by taking the +case of a country undergoing some physical change, for instance, of climate. +The proportional numbers of its inhabitants would almost immediately undergo a +change, and some species might become extinct. We may conclude, from what we +have seen of the intimate and complex manner in which the inhabitants of each +country are bound together, that any change in the numerical proportions of +some of the inhabitants, independently of the change of climate itself, would +most seriously affect many of the others. If the country were open on its +borders, new forms would certainly immigrate, and this also would seriously +disturb the relations of some of the former inhabitants. Let it be remembered +how powerful the influence of a single introduced tree or mammal has been shown +to be. But in the case of an island, or of a country partly surrounded by +barriers, into which new and better adapted forms could not freely enter, we +should then have places in the economy of nature which would assuredly be +better filled up, if some of the original inhabitants were in some manner +modified; for, had the area been open to immigration, these same +<a name="Page82"></a> +places would have been seized on by intruders. In such case, every slight +modification, which in the course of ages chanced to arise, and which in any +way favoured the individuals of any of the species, by better adapting them to +their altered conditions, would tend to be preserved; and natural selection +would thus have free scope for the work of improvement. +</p> + +<p> +We have reason to believe, as stated in the first chapter, that a change in the +conditions of life, by specially acting on the reproductive system, causes or +increases variability; and in the foregoing case the conditions of life are +supposed to have undergone a change, and this would manifestly be favourable to +natural selection, by giving a better chance of profitable variations +occurring; and unless profitable variations do occur, natural selection can do +nothing. Not that, as I believe, any extreme amount of variability is +necessary; as man can certainly produce great results by adding up in any given +direction mere individual differences, so could Nature, but far more easily, +from having incomparably longer time at her disposal. Nor do I believe that any +great physical change, as of climate, or any unusual degree of isolation to +check immigration, is actually necessary to produce new and unoccupied places +for natural selection to fill up by modifying and improving some of the varying +inhabitants. For as all the inhabitants of each country are struggling together +with nicely balanced forces, extremely slight modifications in the structure or +habits of one inhabitant would often give it an advantage over others; and +still further modifications of the same kind would often still further increase +the advantage. No country can be named in which all the native inhabitants are +now so perfectly adapted to each other and to the physical conditions under +which they live, that none of +<a name="Page83"></a> +them could anyhow be improved; for in all countries, the natives have been so +far conquered by naturalised productions, that they have allowed foreigners to +take firm possession of the land. And as foreigners have thus everywhere beaten +some of the natives, we may safely conclude that the natives might have been +modified with advantage, so as to have better resisted such intruders. +</p> + +<p> +As man can produce and certainly has produced a great result by his methodical +and unconscious means of selection, what may not nature effect? Man can act +only on external and visible characters: nature cares nothing for appearances, +except in so far as they may be useful to any being. She can act on every +internal organ, on every shade of constitutional difference, on the whole +machinery of life. Man selects only for his own good; Nature only for that of +the being which she tends. Every selected character is fully exercised by her; +and the being is placed under well-suited conditions of life. Man keeps the +natives of many climates in the same country; he seldom exercises each selected +character in some peculiar and fitting manner; he feeds a long and a short +beaked pigeon on the same food; he does not exercise a long-backed or +long-legged quadruped in any peculiar manner; he exposes sheep with long and +short wool to the same climate. He does not allow the most vigorous males to +struggle for the females. He does not rigidly destroy all inferior animals, but +protects during each varying season, as far as lies in his power, all his +productions. He often begins his selection by some half-monstrous form; or at +least by some modification prominent enough to catch his eye, or to be plainly +useful to him. Under nature, the slightest difference of structure or +constitution may well turn the nicely-balanced scale in the +<a name="Page84"></a> +struggle for life, and so be preserved. How fleeting are the wishes and efforts +of man! how short his time! and consequently how poor will his products be, +compared with those accumulated by nature during whole geological periods. Can +we wonder, then, that nature’s productions should be far +“truer” in character than man’s productions; that they should +be infinitely better adapted to the most complex conditions of life, and should +plainly bear the stamp of far higher workmanship? +</p> + +<p> +It may be said that natural selection is daily and hourly scrutinising, +throughout the world, every variation, even the slightest; rejecting that which +is bad, preserving and adding up all that is good; silently and insensibly +working, whenever and wherever opportunity offers, at the improvement of each +organic being in relation to its organic and inorganic conditions of life. We +see nothing of these slow changes in progress, until the hand of time has +marked the long lapse of ages, and then so imperfect is our view into long past +geological ages, that we only see that the forms of life are now different from +what they formerly were. +</p> + +<p> +Although natural selection can act only through and for the good of each being, +yet characters and structures, which we are apt to consider as of very trifling +importance, may thus be acted on. When we see leaf-eating insects green, and +bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse +the colour of heather, and the black-grouse that of peaty earth, we must +believe that these tints are of service to these birds and insects in +preserving them from danger. Grouse, if not destroyed at some period of their +lives, would increase in countless numbers; they are known to suffer largely +from birds of prey; and hawks are guided by eyesight to their prey,—so +much so, that on +<a name="Page85"></a> +parts of the Continent persons are warned not to keep white pigeons, as being +the most liable to destruction. Hence I can see no reason to doubt that natural +selection might be most effective in giving the proper colour to each kind of +grouse, and in keeping that colour, when once acquired, true and constant. Nor +ought we to think that the occasional destruction of an animal of any +particular colour would produce little effect: we should remember how essential +it is in a flock of white sheep to destroy every lamb with the faintest trace +of black. In plants the down on the fruit and the colour of the flesh are +considered by botanists as characters of the most trifling importance: yet we +hear from an excellent horticulturist, Downing, that in the United States +smooth-skinned fruits suffer far more from a beetle, a curculio, than those +with down; that purple plums suffer far more from a certain disease than yellow +plums; whereas another disease attacks yellow-fleshed peaches far more than +those with other coloured flesh. If, with all the aids of art, these slight +differences make a great difference in cultivating the several varieties, +assuredly, in a state of nature, where the trees would have to struggle with +other trees and with a host of enemies, such differences would effectually +settle which variety, whether a smooth or downy, a yellow or purple fleshed +fruit, should succeed. +</p> + +<p> +In looking at many small points of difference between species, which, as far as +our ignorance permits us to judge, seem to be quite unimportant, we must not +forget that climate, food, etc., probably produce some slight and direct +effect. It is, however, far more necessary to bear in mind that there are many +unknown laws of correlation of growth, which, when one part of the organisation +is modified through variation, and the modifications are accumulated by natural +selection for +<a name="Page86"></a> +the good of the being, will cause other modifications, often of the most +unexpected nature. +</p> + +<p> +As we see that those variations which under domestication appear at any +particular period of life, tend to reappear in the offspring at the same +period;—for instance, in the seeds of the many varieties of our culinary +and agricultural plants; in the caterpillar and cocoon stages of the varieties +of the silkworm; in the eggs of poultry, and in the colour of the down of their +chickens; in the horns of our sheep and cattle when nearly adult;—so in a +state of nature, natural selection will be enabled to act on and modify organic +beings at any age, by the accumulation of profitable variations at that age, +and by their inheritance at a corresponding age. If it profit a plant to have +its seeds more and more widely disseminated by the wind, I can see no greater +difficulty in this being effected through natural selection, than in the +cotton-planter increasing and improving by selection the down in the pods on +his cotton-trees. Natural selection may modify and adapt the larva of an insect +to a score of contingencies, wholly different from those which concern the +mature insect. These modifications will no doubt affect, through the laws of +correlation, the structure of the adult; and probably in the case of those +insects which live only for a few hours, and which never feed, a large part of +their structure is merely the correlated result of successive changes in the +structure of their larvæ. So, conversely, modifications in the adult will +probably often affect the structure of the larva; but in all cases natural +selection will ensure that modifications consequent on other modifications at a +different period of life, shall not be in the least degree injurious: for if +they became so, they would cause the extinction of the species. +</p> + +<p> +Natural selection will modify the structure of the +<a name="Page87"></a> +young in relation to the parent, and of the parent in relation to the young. In +social animals it will adapt the structure of each individual for the benefit +of the community; if each in consequence profits by the selected change. What +natural selection cannot do, is to modify the structure of one species, without +giving it any advantage, for the good of another species; and though statements +to this effect may be found in works of natural history, I cannot find one case +which will bear investigation. A structure used only once in an animal’s +whole life, if of high importance to it, might be modified to any extent by +natural selection; for instance, the great jaws possessed by certain insects, +and used exclusively for opening the cocoon—or the hard tip to the beak +of nestling birds, used for breaking the egg. It has been asserted, that of the +best short-beaked tumbler-pigeons more perish in the egg than are able to get +out of it; so that fanciers assist in the act of hatching. Now, if nature had +to make the beak of a full-grown pigeon very short for the bird’s own +advantage, the process of modification would be very slow, and there would be +simultaneously the most rigorous selection of the young birds within the egg, +which had the most powerful and hardest beaks, for all with weak beaks would +inevitably perish: or, more delicate and more easily broken shells might be +selected, the thickness of the shell being known to vary like every other +structure. +</p> + +<p class="p2"> +<i>Sexual Selection</i>.—Inasmuch as peculiarities often appear under +domestication in one sex and become hereditarily attached to that sex, the same +fact probably occurs under nature, and if so, natural selection will be able to +modify one sex in its functional relations to the other sex, or in relation to +wholly different habits of life in the two sexes, as is sometimes the case +<a name="Page88"></a> +with insects. And this leads me to say a few words on what I call Sexual +Selection. This depends, not on a struggle for existence, but on a struggle +between the males for possession of the females; the result is not death to the +unsuccessful competitor, but few or no offspring. Sexual selection is, +therefore, less rigorous than natural selection. Generally, the most vigorous +males, those which are best fitted for their places in nature, will leave most +progeny. But in many cases, victory will depend not on general vigour, but on +having special weapons, confined to the male sex. A hornless stag or spurless +cock would have a poor chance of leaving offspring. Sexual selection by always +allowing the victor to breed might surely give indomitable courage, length to +the spur, and strength to the wing to strike in the spurred leg, as well as the +brutal cock-fighter, who knows well that he can improve his breed by careful +selection of the best cocks. How low in the scale of nature this law of battle +descends, I know not; male alligators have been described as fighting, +bellowing, and whirling round, like Indians in a war-dance, for the possession +of the females; male salmons have been seen fighting all day long; male +stag-beetles often bear wounds from the huge mandibles of other males. The war +is, perhaps, severest between the males of polygamous animals, and these seem +oftenest provided with special weapons. The males of carnivorous animals are +already well armed; though to them and to others, special means of defence may +be given through means of sexual selection, as the mane to the lion, the +shoulder-pad to the boar, and the hooked jaw to the male salmon; for the shield +may be as important for victory, as the sword or spear. +</p> + +<p> +Amongst birds, the contest is often of a more peaceful character. All those who +have attended to the subject, +<a name="Page89"></a> +believe that there is the severest rivalry between the males of many species to +attract by singing the females. The rock-thrush of Guiana, birds of Paradise, +and some others, congregate; and successive males display their gorgeous +plumage and perform strange antics before the females, which standing by as +spectators, at last choose the most attractive partner. Those who have closely +attended to birds in confinement well know that they often take individual +preferences and dislikes: thus Sir R. Heron has described how one pied peacock +was eminently attractive to all his hen birds. It may appear childish to +attribute any effect to such apparently weak means: I cannot here enter on the +details necessary to support this view; but if man can in a short time give +elegant carriage and beauty to his bantams, according to his standard of +beauty, I can see no good reason to doubt that female birds, by selecting, +during thousands of generations, the most melodious or beautiful males, +according to their standard of beauty, might produce a marked effect. I +strongly suspect that some well-known laws with respect to the plumage of male +and female birds, in comparison with the plumage of the young, can be explained +on the view of plumage having been chiefly modified by sexual selection, acting +when the birds have come to the breeding age or during the breeding season; the +modifications thus produced being inherited at corresponding ages or seasons, +either by the males alone, or by the males and females; but I have not space +here to enter on this subject. +</p> + +<p> +Thus it is, as I believe, that when the males and females of any animal have +the same general habits of life, but differ in structure, colour, or ornament, +such differences have been mainly caused by sexual selection; that is, +individual males have had, in successive generations, some slight advantage +over other +<a name="Page90"></a> +males, in their weapons, means of defence, or charms; and have transmitted +these advantages to their male offspring. Yet, I would not wish to attribute +all such sexual differences to this agency: for we see peculiarities arising +and becoming attached to the male sex in our domestic animals (as the wattle in +male carriers, horn-like protuberances in the cocks of certain fowls, etc.), +which we cannot believe to be either useful to the males in battle, or +attractive to the females. We see analogous cases under nature, for instance, +the tuft of hair on the breast of the turkey-cock, which can hardly be either +useful or ornamental to this bird;—indeed, had the tuft appeared under +domestication, it would have been called a monstrosity. +</p> + +<p class="p2"> +<i>Illustrations of the action of Natural Selection</i>.—In order to make +it clear how, as I believe, natural selection acts, I must beg permission to +give one or two imaginary illustrations. Let us take the case of a wolf, which +preys on various animals, securing some by craft, some by strength, and some by +fleetness; and let us suppose that the fleetest prey, a deer for instance, had +from any change in the country increased in numbers, or that other prey had +decreased in numbers, during that season of the year when the wolf is hardest +pressed for food. I can under such circumstances see no reason to doubt that +the swiftest and slimmest wolves would have the best chance of surviving, and +so be preserved or selected,—provided always that they retained strength +to master their prey at this or at some other period of the year, when they +might be compelled to prey on other animals. I can see no more reason to doubt +this, than that man can improve the fleetness of his greyhounds by careful and +methodical selection, or by that unconscious selection which results from each +man trying +<a name="Page91"></a> +to keep the best dogs without any thought of modifying the breed. +</p> + +<p> +Even without any change in the proportional numbers of the animals on which our +wolf preyed, a cub might be born with an innate tendency to pursue certain +kinds of prey. Nor can this be thought very improbable; for we often observe +great differences in the natural tendencies of our domestic animals; one cat, +for instance, taking to catch rats, another mice; one cat, according to Mr. St. +John, bringing home winged game, another hares or rabbits, and another hunting +on marshy ground and almost nightly catching woodcocks or snipes. The tendency +to catch rats rather than mice is known to be inherited. Now, if any slight +innate change of habit or of structure benefited an individual wolf, it would +have the best chance of surviving and of leaving offspring. Some of its young +would probably inherit the same habits or structure, and by the repetition of +this process, a new variety might be formed which would either supplant or +coexist with the parent-form of wolf. Or, again, the wolves inhabiting a +mountainous district, and those frequenting the lowlands, would naturally be +forced to hunt different prey; and from the continued preservation of the +individuals best fitted for the two sites, two varieties might slowly be +formed. These varieties would cross and blend where they met; but to this +subject of intercrossing we shall soon have to return. I may add, that, +according to Mr. Pierce, there are two varieties of the wolf inhabiting the +Catskill Mountains in the United States, one with a light greyhound-like form, +which pursues deer, and the other more bulky, with shorter legs, which more +frequently attacks the shepherd’s flocks. +</p> + +<p> +Let us now take a more complex case. Certain plants excrete a sweet juice, +apparently for the sake of eliminating something injurious from their sap: this +is +<a name="Page92"></a> +effected by glands at the base of the stipules in some Leguminosæ, and at the +back of the leaf of the common laurel. This juice, though small in quantity, is +greedily sought by insects. Let us now suppose a little sweet juice or nectar +to be excreted by the inner bases of the petals of a flower. In this case +insects in seeking the nectar would get dusted with pollen, and would certainly +often transport the pollen from one flower to the stigma of another flower. The +flowers of two distinct individuals of the same species would thus get crossed; +and the act of crossing, we have good reason to believe (as will hereafter be +more fully alluded to), would produce very vigorous seedlings, which +consequently would have the best chance of flourishing and surviving. Some of +these seedlings would probably inherit the nectar-excreting power. Those +individual flowers which had the largest glands or nectaries, and which +excreted most nectar, would be oftenest visited by insects, and would be +oftenest crossed; and so in the long-run would gain the upper hand. Those +flowers, also, which had their stamens and pistils placed, in relation to the +size and habits of the particular insects which visited them, so as to favour +in any degree the transportal of their pollen from flower to flower, would +likewise be favoured or selected. We might have taken the case of insects +visiting flowers for the sake of collecting pollen instead of nectar; and as +pollen is formed for the sole object of fertilisation, its destruction appears +a simple loss to the plant; yet if a little pollen were carried, at first +occasionally and then habitually, by the pollen-devouring insects from flower +to flower, and a cross thus effected, although nine-tenths of the pollen were +destroyed, it might still be a great gain to the plant; and those individuals +which produced more and more pollen, and had larger and larger anthers, would +be selected. +</p> + +<p> +<a name="Page93"></a> +When our plant, by this process of the continued preservation or natural +selection of more and more attractive flowers, had been rendered highly +attractive to insects, they would, unintentionally on their part, regularly +carry pollen from flower to flower; and that they can most effectually do this, +I could easily show by many striking instances. I will give only one—not +as a very striking case, but as likewise illustrating one step in the +separation of the sexes of plants, presently to be alluded to. Some holly-trees +bear only male flowers, which have four stamens producing rather a small +quantity of pollen, and a rudimentary pistil; other holly-trees bear only +female flowers; these have a full-sized pistil, and four stamens with +shrivelled anthers, in which not a grain of pollen can be detected. Having +found a female tree exactly sixty yards from a male tree, I put the stigmas of +twenty flowers, taken from different branches, under the microscope, and on +all, without exception, there were pollen-grains, and on some a profusion of +pollen. As the wind had set for several days from the female to the male tree, +the pollen could not thus have been carried. The weather had been cold and +boisterous, and therefore not favourable to bees, nevertheless every female +flower which I examined had been effectually fertilised by the bees, +accidentally dusted with pollen, having flown from tree to tree in search of +nectar. But to return to our imaginary case: as soon as the plant had been +rendered so highly attractive to insects that pollen was regularly carried from +flower to flower, another process might commence. No naturalist doubts the +advantage of what has been called the “physiological division of +labour;” hence we may believe that it would be advantageous to a plant to +produce stamens alone in one flower or on one whole plant, and pistils alone in +<a name="Page94"></a> +another flower or on another plant. In plants under culture and placed under +new conditions of life, sometimes the male organs and sometimes the female +organs become more or less impotent; now if we suppose this to occur in ever so +slight a degree under nature, then as pollen is already carried regularly from +flower to flower, and as a more complete separation of the sexes of our plant +would be advantageous on the principle of the division of labour, individuals +with this tendency more and more increased, would be continually favoured or +selected, until at last a complete separation of the sexes would be effected. +</p> + +<p> +Let us now turn to the nectar-feeding insects in our imaginary case: we may +suppose the plant of which we have been slowly increasing the nectar by +continued selection, to be a common plant; and that certain insects depended in +main part on its nectar for food. I could give many facts, showing how anxious +bees are to save time; for instance, their habit of cutting holes and sucking +the nectar at the bases of certain flowers, which they can, with a very little +more trouble, enter by the mouth. Bearing such facts in mind, I can see no +reason to doubt that an accidental deviation in the size and form of the body, +or in the curvature and length of the proboscis, etc., far too slight to be +appreciated by us, might profit a bee or other insect, so that an individual so +characterised would be able to obtain its food more quickly, and so have a +better chance of living and leaving descendants. Its descendants would probably +inherit a tendency to a similar slight deviation of structure. The tubes of the +corollas of the common red and incarnate clovers (Trifolium pratense and +incarnatum) do not on a hasty glance appear to differ in length; yet the +hive-bee can easily suck the nectar out of the incarnate clover, but not out of +the common red +<a name="Page95"></a> +clover, which is visited by humble-bees alone; so that whole fields of the red +clover offer in vain an abundant supply of precious nectar to the hive-bee. +Thus it might be a great advantage to the hive-bee to have a slightly longer or +differently constructed proboscis. On the other hand, I have found by +experiment that the fertility of clover greatly depends on bees visiting and +moving parts of the corolla, so as to push the pollen on to the stigmatic +surface. Hence, again, if humble-bees were to become rare in any country, it +might be a great advantage to the red clover to have a shorter or more deeply +divided tube to its corolla, so that the hive-bee could visit its flowers. Thus +I can understand how a flower and a bee might slowly become, either +simultaneously or one after the other, modified and adapted in the most perfect +manner to each other, by the continued preservation of individuals presenting +mutual and slightly favourable deviations of structure. +</p> + +<p> +I am well aware that this doctrine of natural selection, exemplified in the +above imaginary instances, is open to the same objections which were at first +urged against Sir Charles Lyell’s noble views on “the modern +changes of the earth, as illustrative of geology;” but we now very seldom +hear the action, for instance, of the coast-waves, called a trifling and +insignificant cause, when applied to the excavation of gigantic valleys or to +the formation of the longest lines of inland cliffs. Natural selection can act +only by the preservation and accumulation of infinitesimally small inherited +modifications, each profitable to the preserved being; and as modern geology +has almost banished such views as the excavation of a great valley by a single +diluvial wave, so will natural selection, if it be a true principle, banish the +belief of the continued creation of new organic +<a name="Page96"></a> +beings, or of any great and sudden modification in their structure. +</p> + +<p class="p2"> +<i>On the Intercrossing of Individuals</i>.—I must here introduce a short +digression. In the case of animals and plants with separated sexes, it is of +course obvious that two individuals must always unite for each birth; but in +the case of hermaphrodites this is far from obvious. Nevertheless I am strongly +inclined to believe that with all hermaphrodites two individuals, either +occasionally or habitually, concur for the reproduction of their kind. This +view, I may add, was first suggested by Andrew Knight. We shall presently see +its importance; but I must here treat the subject with extreme brevity, though +I have the materials prepared for an ample discussion. All vertebrate animals, +all insects, and some other large groups of animals, pair for each birth. +Modern research has much diminished the number of supposed hermaphrodites, and +of real hermaphrodites a large number pair; that is, two individuals regularly +unite for reproduction, which is all that concerns us. But still there are many +hermaphrodite animals which certainly do not habitually pair, and a vast +majority of plants are hermaphrodites. What reason, it may be asked, is there +for supposing in these cases that two individuals ever concur in reproduction? +As it is impossible here to enter on details, I must trust to some general +considerations alone. +</p> + +<p> +In the first place, I have collected so large a body of facts, showing, in +accordance with the almost universal belief of breeders, that with animals and +plants a cross between different varieties, or between individuals of the same +variety but of another strain, gives vigour and fertility to the offspring; and +on the other hand, that <i>close</i> interbreeding diminishes vigour and +fertility; that +<a name="Page97"></a> +these facts alone incline me to believe that it is a general law of nature +(utterly ignorant though we be of the meaning of the law) that no organic being +self-fertilises itself for an eternity of generations; but that a cross with +another individual is occasionally—perhaps at very long +intervals—indispensable. +</p> + +<p> +On the belief that this is a law of nature, we can, I think, understand several +large classes of facts, such as the following, which on any other view are +inexplicable. Every hybridizer knows how unfavourable exposure to wet is to the +fertilisation of a flower, yet what a multitude of flowers have their anthers +and stigmas fully exposed to the weather! but if an occasional cross be +indispensable, the fullest freedom for the entrance of pollen from another +individual will explain this state of exposure, more especially as the +plant’s own anthers and pistil generally stand so close together that +self-fertilisation seems almost inevitable. Many flowers, on the other hand, +have their organs of fructification closely enclosed, as in the great +papilionaceous or pea-family; but in several, perhaps in all, such flowers, +there is a very curious adaptation between the structure of the flower and the +manner in which bees suck the nectar; for, in doing this, they either push the +flower’s own pollen on the stigma, or bring pollen from another flower. +So necessary are the visits of bees to papilionaceous flowers, that I have +found, by experiments published elsewhere, that their fertility is greatly +diminished if these visits be prevented. Now, it is scarcely possible that bees +should fly from flower to flower, and not carry pollen from one to the other, +to the great good, as I believe, of the plant. Bees will act like a camel-hair +pencil, and it is quite sufficient just to touch the anthers of one flower and +then the stigma of another with the same brush to ensure fertilisation; but it +must not be +<a name="Page98"></a> +supposed that bees would thus produce a multitude of hybrids between distinct +species; for if you bring on the same brush a plant’s own pollen and +pollen from another species, the former will have such a prepotent effect, that +it will invariably and completely destroy, as has been shown by Gärtner, any +influence from the foreign pollen. +</p> + +<p> +When the stamens of a flower suddenly spring towards the pistil, or slowly move +one after the other towards it, the contrivance seems adapted solely to ensure +self-fertilisation; and no doubt it is useful for this end: but, the agency of +insects is often required to cause the stamens to spring forward, as Kölreuter +has shown to be the case with the barberry; and curiously in this very genus, +which seems to have a special contrivance for self-fertilisation, it is well +known that if very closely-allied forms or varieties are planted near each +other, it is hardly possible to raise pure seedlings, so largely do they +naturally cross. In many other cases, far from there being any aids for +self-fertilisation, there are special contrivances, as I could show from the +writings of C. C. Sprengel and from my own observations, which effectually +prevent the stigma receiving pollen from its own flower: for instance, in +Lobelia fulgens, there is a really beautiful and elaborate contrivance by which +every one of the infinitely numerous pollen-granules are swept out of the +conjoined anthers of each flower, before the stigma of that individual flower +is ready to receive them; and as this flower is never visited, at least in my +garden, by insects, it never sets a seed, though by placing pollen from one +flower on the stigma of another, I raised plenty of seedlings; and whilst +another species of Lobelia growing close by, which is visited by bees, seeds +freely. In very many other cases, though there be no special mechanical +contrivance to prevent the stigma of a flower receiving its own pollen, yet, as +<a name="Page99"></a> +C. C. Sprengel has shown, and as I can confirm, either the anthers burst before +the stigma is ready for fertilisation, or the stigma is ready before the pollen +of that flower is ready, so that these plants have in fact separated sexes, and +must habitually be crossed. How strange are these facts! How strange that the +pollen and stigmatic surface of the same flower, though placed so close +together, as if for the very purpose of self-fertilisation, should in so many +cases be mutually useless to each other! How simply are these facts explained +on the view of an occasional cross with a distinct individual being +advantageous or indispensable! +</p> + +<p> +If several varieties of the cabbage, radish, onion, and of some other plants, +be allowed to seed near each other, a large majority, as I have found, of the +seedlings thus raised will turn out mongrels: for instance, I raised 233 +seedling cabbages from some plants of different varieties growing near each +other, and of these only 78 were true to their kind, and some even of these +were not perfectly true. Yet the pistil of each cabbage-flower is surrounded +not only by its own six stamens, but by those of the many other flowers on the +same plant. How, then, comes it that such a vast number of the seedlings are +mongrelized? I suspect that it must arise from the pollen of a distinct +<i>variety</i> having a prepotent effect over a flower’s own pollen; and +that this is part of the general law of good being derived from the +intercrossing of distinct individuals of the same species. When distinct +<i>species</i> are crossed the case is directly the reverse, for a +plant’s own pollen is always prepotent over foreign pollen; but to this +subject we shall return in a future chapter. +</p> + +<p> +In the case of a gigantic tree covered with innumerable flowers, it may be +objected that pollen could seldom be carried from tree to tree, and at most +only from flower +<a name="Page100"></a> +to flower on the same tree, and that flowers on the same tree can be considered +as distinct individuals only in a limited sense. I believe this objection to be +valid, but that nature has largely provided against it by giving to trees a +strong tendency to bear flowers with separated sexes. When the sexes are +separated, although the male and female flowers may be produced on the same +tree, we can see that pollen must be regularly carried from flower to flower; +and this will give a better chance of pollen being occasionally carried from +tree to tree. That trees belonging to all Orders have their sexes more often +separated than other plants, I find to be the case in this country; and at my +request Dr. Hooker tabulated the trees of New Zealand, and Dr. Asa Gray those +of the United States, and the result was as I anticipated. On the other hand, +Dr. Hooker has recently informed me that he finds that the rule does not hold +in Australia; and I have made these few remarks on the sexes of trees simply to +call attention to the subject. +</p> + +<p> +Turning for a very brief space to animals: on the land there are some +hermaphrodites, as land-mollusca and earth-worms; but these all pair. As yet I +have not found a single case of a terrestrial animal which fertilises itself. +We can understand this remarkable fact, which offers so strong a contrast with +terrestrial plants, on the view of an occasional cross being indispensable, by +considering the medium in which terrestrial animals live, and the nature of the +fertilising element; for we know of no means, analogous to the action of +insects and of the wind in the case of plants, by which an occasional cross +could be effected with terrestrial animals without the concurrence of two +individuals. Of aquatic animals, there are many self-fertilising +hermaphrodites; but here currents in the water offer an obvious means for an +occasional cross. And, as in the case of flowers, I have as yet +<a name="Page101"></a> +failed, after consultation with one of the highest authorities, namely, +Professor Huxley, to discover a single case of an hermaphrodite animal with the +organs of reproduction so perfectly enclosed within the body, that access from +without and the occasional influence of a distinct individual can be shown to +be physically impossible. Cirripedes long appeared to me to present a case of +very great difficulty under this point of view; but I have been enabled, by a +fortunate chance, elsewhere to prove that two individuals, though both are +self-fertilising hermaphrodites, do sometimes cross. +</p> + +<p> +It must have struck most naturalists as a strange anomaly that, in the case of +both animals and plants, species of the same family and even of the same genus, +though agreeing closely with each other in almost their whole organisation, yet +are not rarely, some of them hermaphrodites, and some of them unisexual. But +if, in fact, all hermaphrodites do occasionally intercross with other +individuals, the difference between hermaphrodites and unisexual species, as +far as function is concerned, becomes very small. +</p> + +<p> +From these several considerations and from the many special facts which I have +collected, but which I am not here able to give, I am strongly inclined to +suspect that, both in the vegetable and animal kingdoms, an occasional +intercross with a distinct individual is a law of nature. I am well aware that +there are, on this view, many cases of difficulty, some of which I am trying to +investigate. Finally then, we may conclude that in many organic beings, a cross +between two individuals is an obvious necessity for each birth; in many others +it occurs perhaps only at long intervals; but in none, as I suspect, can +self-fertilisation go on for perpetuity. +</p> + +<p class="p2"> +<i>Circumstances favourable to Natural Selection</i>.—This +<a name="Page102"></a> +is an extremely intricate subject. A large amount of inheritable and +diversified variability is favourable, but I believe mere individual +differences suffice for the work. A large number of individuals, by giving a +better chance for the appearance within any given period of profitable +variations, will compensate for a lesser amount of variability in each +individual, and is, I believe, an extremely important element of success. +Though nature grants vast periods of time for the work of natural selection, +she does not grant an indefinite period; for as all organic beings are +striving, it may be said, to seize on each place in the economy of nature, if +any one species does not become modified and improved in a corresponding degree +with its competitors, it will soon be exterminated. +</p> + +<p> +In man’s methodical selection, a breeder selects for some definite +object, and free intercrossing will wholly stop his work. But when many men, +without intending to alter the breed, have a nearly common standard of +perfection, and all try to get and breed from the best animals, much +improvement and modification surely but slowly follow from this unconscious +process of selection, notwithstanding a large amount of crossing with inferior +animals. Thus it will be in nature; for within a confined area, with some place +in its polity not so perfectly occupied as might be, natural selection will +always tend to preserve all the individuals varying in the right direction, +though in different degrees, so as better to fill up the unoccupied place. But +if the area be large, its several districts will almost certainly present +different conditions of life; and then if natural selection be modifying and +improving a species in the several districts, there will be intercrossing with +the other individuals of the same species on the confines of each. And in this +case the effects of intercrossing can hardly be counterbalanced +<a name="Page103"></a> +by natural selection always tending to modify all the individuals in each +district in exactly the same manner to the conditions of each; for in a +continuous area, the conditions will generally graduate away insensibly from +one district to another. The intercrossing will most affect those animals which +unite for each birth, which wander much, and which do not breed at a very quick +rate. Hence in animals of this nature, for instance in birds, varieties will +generally be confined to separated countries; and this I believe to be the +case. In hermaphrodite organisms which cross only occasionally, and likewise in +animals which unite for each birth, but which wander little and which can +increase at a very rapid rate, a new and improved variety might be quickly +formed on any one spot, and might there maintain itself in a body, so that +whatever intercrossing took place would be chiefly between the individuals of +the same new variety. A local variety when once thus formed might subsequently +slowly spread to other districts. On the above principle, nurserymen always +prefer getting seed from a large body of plants of the same variety, as the +chance of intercrossing with other varieties is thus lessened. +</p> + +<p> +Even in the case of slow-breeding animals, which unite for each birth, we must +not overrate the effects of intercrosses in retarding natural selection; for I +can bring a considerable catalogue of facts, showing that within the same area, +varieties of the same animal can long remain distinct, from haunting different +stations, from breeding at slightly different seasons, or from varieties of the +same kind preferring to pair together. +</p> + +<p> +Intercrossing plays a very important part in nature in keeping the individuals +of the same species, or of the same variety, true and uniform in character. It +will obviously thus act far more efficiently with those animals +<a name="Page104"></a> +which unite for each birth; but I have already attempted to show that we have +reason to believe that occasional intercrosses take place with all animals and +with all plants. Even if these take place only at long intervals, I am +convinced that the young thus produced will gain so much in vigour and +fertility over the offspring from long-continued self-fertilisation, that they +will have a better chance of surviving and propagating their kind; and thus, in +the long run, the influence of intercrosses, even at rare intervals, will be +great. If there exist organic beings which never intercross, uniformity of +character can be retained amongst them, as long as their conditions of life +remain the same, only through the principle of inheritance, and through natural +selection destroying any which depart from the proper type; but if their +conditions of life change and they undergo modification, uniformity of +character can be given to their modified offspring, solely by natural selection +preserving the same favourable variations. +</p> + +<p> +Isolation, also, is an important element in the process of natural selection. +In a confined or isolated area, if not very large, the organic and inorganic +conditions of life will generally be in a great degree uniform; so that natural +selection will tend to modify all the individuals of a varying species +throughout the area in the same manner in relation to the same conditions. +Intercrosses, also, with the individuals of the same species, which otherwise +would have inhabited the surrounding and differently circumstanced districts, +will be prevented. But isolation probably acts more efficiently in checking the +immigration of better adapted organisms, after any physical change, such as of +climate or elevation of the land, etc.; and thus new places in the natural +economy of the country are left open for the old inhabitants to struggle for, +and become adapted to, through modifications +<a name="Page105"></a> +in their structure and constitution. Lastly, isolation, by checking immigration +and consequently competition, will give time for any new variety to be slowly +improved; and this may sometimes be of importance in the production of new +species. If, however, an isolated area be very small, either from being +surrounded by barriers, or from having very peculiar physical conditions, the +total number of the individuals supported on it will necessarily be very small; +and fewness of individuals will greatly retard the production of new species +through natural selection, by decreasing the chance of the appearance of +favourable variations. +</p> + +<p> +If we turn to nature to test the truth of these remarks, and look at any small +isolated area, such as an oceanic island, although the total number of the +species inhabiting it, will be found to be small, as we shall see in our +chapter on geographical distribution; yet of these species a very large +proportion are endemic,—that is, have been produced there, and nowhere +else. Hence an oceanic island at first sight seems to have been highly +favourable for the production of new species. But we may thus greatly deceive +ourselves, for to ascertain whether a small isolated area, or a large open area +like a continent, has been most favourable for the production of new organic +forms, we ought to make the comparison within equal times; and this we are +incapable of doing. +</p> + +<p> +Although I do not doubt that isolation is of considerable importance in the +production of new species, on the whole I am inclined to believe that largeness +of area is of more importance, more especially in the production of species, +which will prove capable of enduring for a long period, and of spreading +widely. Throughout a great and open area, not only will there be a better +chance of favourable variations arising from the large number of individuals of +the same species +<a name="Page106"></a> +there supported, but the conditions of life are infinitely complex from the +large number of already existing species; and if some of these many species +become modified and improved, others will have to be improved in a +corresponding degree or they will be exterminated. Each new form, also, as soon +as it has been much improved, will be able to spread over the open and +continuous area, and will thus come into competition with many others. Hence +more new places will be formed, and the competition to fill them will be more +severe, on a large than on a small and isolated area. Moreover, great areas, +though now continuous, owing to oscillations of level, will often have recently +existed in a broken condition, so that the good effects of isolation will +generally, to a certain extent, have concurred. Finally, I conclude that, +although small isolated areas probably have been in some respects highly +favourable for the production of new species, yet that the course of +modification will generally have been more rapid on large areas; and what is +more important, that the new forms produced on large areas, which already have +been victorious over many competitors, will be those that will spread most +widely, will give rise to most new varieties and species, and will thus play an +important part in the changing history of the organic world. +</p> + +<p> +We can, perhaps, on these views, understand some facts which will be again +alluded to in our chapter on geographical distribution; for instance, that the +productions of the smaller continent of Australia have formerly yielded, and +apparently are now yielding, before those of the larger Europæo-Asiatic area. +Thus, also, it is that continental productions have everywhere become so +largely naturalised on islands. On a small island, the race for life will have +been less severe, and there will have been less modification and less +extermination. +<a name="Page107"></a> +Hence, perhaps, it comes that the flora of Madeira, according to Oswald Heer, +resembles the extinct tertiary flora of Europe. All fresh-water basins, taken +together, make a small area compared with that of the sea or of the land; and, +consequently, the competition between fresh-water productions will have been +less severe than elsewhere; new forms will have been more slowly formed, and +old forms more slowly exterminated. And it is in fresh water that we find seven +genera of Ganoid fishes, remnants of a once preponderant order: and in fresh +water we find some of the most anomalous forms now known in the world, as the +Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain +extent orders now widely separated in the natural scale. These anomalous forms +may almost be called living fossils; they have endured to the present day, from +having inhabited a confined area, and from having thus been exposed to less +severe competition. +</p> + +<p> +To sum up the circumstances favourable and unfavourable to natural selection, +as far as the extreme intricacy of the subject permits. I conclude, looking to +the future, that for terrestrial productions a large continental area, which +will probably undergo many oscillations of level, and which consequently will +exist for long periods in a broken condition, will be the most favourable for +the production of many new forms of life, likely to endure long and to spread +widely. For the area will first have existed as a continent, and the +inhabitants, at this period numerous in individuals and kinds, will have been +subjected to very severe competition. When converted by subsidence into large +separate islands, there will still exist many individuals of the same species +on each island: intercrossing on the confines of the range of each species will +thus be checked: after physical changes of any kind, immigration will be +prevented, +<a name="Page108"></a> +so that new places in the polity of each island will have to be filled up by +modifications of the old inhabitants; and time will be allowed for the +varieties in each to become well modified and perfected. When, by renewed +elevation, the islands shall be re-converted into a continental area, there +will again be severe competition: the most favoured or improved varieties will +be enabled to spread: there will be much extinction of the less improved forms, +and the relative proportional numbers of the various inhabitants of the renewed +continent will again be changed; and again there will be a fair field for +natural selection to improve still further the inhabitants, and thus produce +new species. +</p> + +<p> +That natural selection will always act with extreme slowness, I fully admit. +Its action depends on there being places in the polity of nature, which can be +better occupied by some of the inhabitants of the country undergoing +modification of some kind. The existence of such places will often depend on +physical changes, which are generally very slow, and on the immigration of +better adapted forms having been checked. But the action of natural selection +will probably still oftener depend on some of the inhabitants becoming slowly +modified; the mutual relations of many of the other inhabitants being thus +disturbed. Nothing can be effected, unless favourable variations occur, and +variation itself is apparently always a very slow process. The process will +often be greatly retarded by free intercrossing. Many will exclaim that these +several causes are amply sufficient wholly to stop the action of natural +selection. I do not believe so. On the other hand, I do believe that natural +selection will always act very slowly, often only at long intervals of time, +and generally on only a very few of the inhabitants of the same region at the +same time. I further believe, that this very slow, intermittent +<a name="Page109"></a> +action of natural selection accords perfectly well with what geology tells us +of the rate and manner at which the inhabitants of this world have changed. +</p> + +<p> +Slow though the process of selection may be, if feeble man can do much by his +powers of artificial selection, I can see no limit to the amount of change, to +the beauty and infinite complexity of the coadaptations between all organic +beings, one with another and with their physical conditions of life, which may +be effected in the long course of time by nature’s power of selection. +</p> + +<p class="p2"> +<i>Extinction</i>.—This subject will be more fully discussed in our +chapter on Geology; but it must be here alluded to from being intimately +connected with natural selection. Natural selection acts solely through the +preservation of variations in some way advantageous, which consequently endure. +But as from the high geometrical powers of increase of all organic beings, each +area is already fully stocked with inhabitants, it follows that as each +selected and favoured form increases in number, so will the less favoured forms +decrease and become rare. Rarity, as geology tells us, is the precursor to +extinction. We can, also, see that any form represented by few individuals +will, during fluctuations in the seasons or in the number of its enemies, run a +good chance of utter extinction. But we may go further than this; for as new +forms are continually and slowly being produced, unless we believe that the +number of specific forms goes on perpetually and almost indefinitely +increasing, numbers inevitably must become extinct. That the number of specific +forms has not indefinitely increased, geology shows us plainly; and indeed we +can see reason why they should not have thus increased, for the number of +places in the polity of nature is not indefinitely great,—not that we +<a name="Page110"></a> +have any means of knowing that any one region has as yet got its maximum of +species. Probably no region is as yet fully stocked, for at the Cape of Good +Hope, where more species of plants are crowded together than in any other +quarter of the world, some foreign plants have become naturalised, without +causing, as far as we know, the extinction of any natives. +</p> + +<p> +Furthermore, the species which are most numerous in individuals will have the +best chance of producing within any given period favourable variations. We have +evidence of this, in the facts given in the second chapter, showing that it is +the common species which afford the greatest number of recorded varieties, or +incipient species. Hence, rare species will be less quickly modified or +improved within any given period, and they will consequently be beaten in the +race for life by the modified descendants of the commoner species. +</p> + +<p> +From these several considerations I think it inevitably follows, that as new +species in the course of time are formed through natural selection, others will +become rarer and rarer, and finally extinct. The forms which stand in closest +competition with those undergoing modification and improvement, will naturally +suffer most. And we have seen in the chapter on the Struggle for Existence that +it is the most closely-allied forms,—varieties of the same species, and +species of the same genus or of related genera,—which, from having nearly +the same structure, constitution, and habits, generally come into the severest +competition with each other. Consequently, each new variety or species, during +the progress of its formation, will generally press hardest on its nearest +kindred, and tend to exterminate them. We see the same process of extermination +amongst our domesticated productions, through the selection of improved forms +by man. Many curious +<a name="Page111"></a> +instances could be given showing how quickly new breeds of cattle, sheep, and +other animals, and varieties of flowers, take the place of older and inferior +kinds. In Yorkshire, it is historically known that the ancient black cattle +were displaced by the long-horns, and that these “were swept away by the +short-horns” (I quote the words of an agricultural writer) “as if +by some murderous pestilence.” +</p> + +<p class="p2"> +<i>Divergence of Character</i>.—The principle, which I have designated by +this term, is of high importance on my theory, and explains, as I believe, +several important facts. In the first place, varieties, even strongly-marked +ones, though having somewhat of the character of species—as is shown by +the hopeless doubts in many cases how to rank them—yet certainly differ +from each other far less than do good and distinct species. Nevertheless, +according to my view, varieties are species in the process of formation, or +are, as I have called them, incipient species. How, then, does the lesser +difference between varieties become augmented into the greater difference +between species? That this does habitually happen, we must infer from most of +the innumerable species throughout nature presenting well-marked differences; +whereas varieties, the supposed prototypes and parents of future well-marked +species, present slight and ill-defined differences. Mere chance, as we may +call it, might cause one variety to differ in some character from its parents, +and the offspring of this variety again to differ from its parent in the very +same character and in a greater degree; but this alone would never account for +so habitual and large an amount of difference as that between varieties of the +same species and species of the same genus. +</p> + +<p> +As has always been my practice, let us seek light on +<a name="Page112"></a> +this head from our domestic productions. We shall here find something +analogous. A fancier is struck by a pigeon having a slightly shorter beak; +another fancier is struck by a pigeon having a rather longer beak; and on the +acknowledged principle that “fanciers do not and will not admire a medium +standard, but like extremes,” they both go on (as has actually occurred +with tumbler-pigeons) choosing and breeding from birds with longer and longer +beaks, or with shorter and shorter beaks. Again, we may suppose that at an +early period one man preferred swifter horses; another stronger and more bulky +horses. The early differences would be very slight; in the course of time, from +the continued selection of swifter horses by some breeders, and of stronger +ones by others, the differences would become greater, and would be noted as +forming two sub-breeds; finally, after the lapse of centuries, the sub-breeds +would become converted into two well-established and distinct breeds. As the +differences slowly become greater, the inferior animals with intermediate +characters, being neither very swift nor very strong, will have been neglected, +and will have tended to disappear. Here, then, we see in man’s +productions the action of what may be called the principle of divergence, +causing differences, at first barely appreciable, steadily to increase, and the +breeds to diverge in character both from each other and from their common +parent. +</p> + +<p> +But how, it may be asked, can any analogous principle apply in nature? I +believe it can and does apply most efficiently, from the simple circumstance +that the more diversified the descendants from any one species become in +structure, constitution, and habits, by so much will they be better enabled to +seize on many and widely diversified places in the polity of nature, and so be +enabled to increase in numbers. +</p> + +<p> +<a name="Page113"></a> +We can clearly see this in the case of animals with simple habits. Take the +case of a carnivorous quadruped, of which the number that can be supported in +any country has long ago arrived at its full average. If its natural powers of +increase be allowed to act, it can succeed in increasing (the country not +undergoing any change in its conditions) only by its varying descendants +seizing on places at present occupied by other animals: some of them, for +instance, being enabled to feed on new kinds of prey, either dead or alive; +some inhabiting new stations, climbing trees, frequenting water, and some +perhaps becoming less carnivorous. The more diversified in habits and structure +the descendants of our carnivorous animal became, the more places they would be +enabled to occupy. What applies to one animal will apply throughout all time to +all animals—that is, if they vary—for otherwise natural selection +can do nothing. So it will be with plants. It has been experimentally proved, +that if a plot of ground be sown with one species of grass, and a similar plot +be sown with several distinct genera of grasses, a greater number of plants and +a greater weight of dry herbage can thus be raised. The same has been found to +hold good when first one variety and then several mixed varieties of wheat have +been sown on equal spaces of ground. Hence, if any one species of grass were to +go on varying, and those varieties were continually selected which differed +from each other in at all the same manner as distinct species and genera of +grasses differ from each other, a greater number of individual plants of this +species of grass, including its modified descendants, would succeed in living +on the same piece of ground. And we well know that each species and each +variety of grass is annually sowing almost countless seeds; and thus, as it may +be said, is striving its utmost to increase its numbers. Consequently, +<a name="Page114"></a> +I cannot doubt that in the course of many thousands of generations, the most +distinct varieties of any one species of grass would always have the best +chance of succeeding and of increasing in numbers, and thus of supplanting the +less distinct varieties; and varieties, when rendered very distinct from each +other, take the rank of species. +</p> + +<p> +The truth of the principle, that the greatest amount of life can be supported +by great diversification of structure, is seen under many natural +circumstances. In an extremely small area, especially if freely open to +immigration, and where the contest between individual and individual must be +severe, we always find great diversity in its inhabitants. For instance, I +found that a piece of turf, three feet by four in size, which had been exposed +for many years to exactly the same conditions, supported twenty species of +plants, and these belonged to eighteen genera and to eight orders, which shows +how much these plants differed from each other. So it is with the plants and +insects on small and uniform islets; and so in small ponds of fresh water. +Farmers find that they can raise most food by a rotation of plants belonging to +the most different orders: nature follows what may be called a simultaneous +rotation. Most of the animals and plants which live close round any small piece +of ground, could live on it (supposing it not to be in any way peculiar in its +nature), and may be said to be striving to the utmost to live there; but, it is +seen, that where they come into the closest competition with each other, the +advantages of diversification of structure, with the accompanying differences +of habit and constitution, determine that the inhabitants, which thus jostle +each other most closely, shall, as a general rule, belong to what we call +different genera and orders. +</p> + +<p> +The same principle is seen in the naturalisation of +<a name="Page115"></a> +plants through man’s agency in foreign lands. It might have been expected +that the plants which have succeeded in becoming naturalised in any land would +generally have been closely allied to the indigenes; for these are commonly +looked at as specially created and adapted for their own country. It might, +also, perhaps have been expected that naturalised plants would have belonged to +a few groups more especially adapted to certain stations in their new homes. +But the case is very different; and Alph. De Candolle has well remarked in his +great and admirable work, that floras gain by naturalisation, proportionally +with the number of the native genera and species, far more in new genera than +in new species. To give a single instance: in the last edition of Dr. Asa +Gray’s ‘Manual of the Flora of the Northern United States,’ +260 naturalised plants are enumerated, and these belong to 162 genera. We thus +see that these naturalised plants are of a highly diversified nature. They +differ, moreover, to a large extent from the indigenes, for out of the 162 +genera, no less than 100 genera are not there indigenous, and thus a large +proportional addition is made to the genera of these States. +</p> + +<p> +By considering the nature of the plants or animals which have struggled +successfully with the indigenes of any country, and have there become +naturalised, we can gain some crude idea in what manner some of the natives +would have had to be modified, in order to have gained an advantage over the +other natives; and we may, I think, at least safely infer that diversification +of structure, amounting to new generic differences, would have been profitable +to them. +</p> + +<p> +The advantage of diversification in the inhabitants of the same region is, in +fact, the same as that of the physiological division of labour in the organs of +the same individual body—a subject so well elucidated by +<a name="Page116"></a> +Milne Edwards. No physiologist doubts that a stomach by being adapted to digest +vegetable matter alone, or flesh alone, draws most nutriment from these +substances. So in the general economy of any land, the more widely and +perfectly the animals and plants are diversified for different habits of life, +so will a greater number of individuals be capable of there supporting +themselves. A set of animals, with their organisation but little diversified, +could hardly compete with a set more perfectly diversified in structure. It may +be doubted, for instance, whether the Australian marsupials, which are divided +into groups differing but little from each other, and feebly representing, as +Mr. Waterhouse and others have remarked, our carnivorous, ruminant, and rodent +mammals, could successfully compete with these well-pronounced orders. In the +Australian mammals, we see the process of diversification in an early and +incomplete stage of development. After the foregoing discussion, which ought to +have been much amplified, we may, I think, assume that the modified descendants +of any one species will succeed by so much the better as they become more +diversified in structure, and are thus enabled to encroach on places occupied +by other beings. Now let us see how this principle of great benefit being +derived from divergence of character, combined with the principles of natural +selection and of extinction, will tend to act. +</p> + +<p> +The accompanying diagram will aid us in understanding this rather perplexing +subject. Let A to L represent the species of a genus large in its own country; +these species are supposed to resemble each other in unequal degrees, as is so +generally the case in nature, and as is represented in the diagram by the +letters standing at unequal distances. I have said a large genus, because we +have seen in the second chapter, +<a name="Page117"></a> +that on an average more of the species of large genera vary than of small +genera; and the varying species of the large genera present a greater number of +varieties. We have, also, seen that the species, which are the commonest and +the most widely-diffused, vary more than rare species with restricted ranges. +Let (A) be a common, widely-diffused, and varying species, belonging to a genus +large in its own country. The little fan of diverging dotted lines of unequal +lengths proceeding from (A), may represent its varying offspring. The +variations are supposed to be extremely slight, but of the most diversified +nature; they are not supposed all to appear simultaneously, but often after +long intervals of time; nor are they all supposed to endure for equal periods. +Only those variations which are in some way profitable will be preserved or +naturally selected. And here the importance of the principle of benefit being +derived from divergence of character comes in; for this will generally lead to +the most different or divergent variations (represented by the outer dotted +lines) being preserved and accumulated by natural selection. When a dotted line +reaches one of the horizontal lines, and is there marked by a small numbered +letter, a sufficient amount of variation is supposed to have been accumulated +to have formed a fairly well-marked variety, such as would be thought worthy of +record in a systematic work. +</p> + +<p> +The intervals between the horizontal lines in the diagram, may represent each a +thousand generations; but it would have been better if each had represented ten +thousand generations. After a thousand generations, species (A) is supposed to +have produced two fairly well-marked varieties, namely <i>a</i><sup>1</sup> and +<i>m</i><sup>1</sup>. These two varieties will generally continue to be exposed +to the same conditions which made their parents variable, +<a name="Page118"></a> +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 advantages 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 own country. And these +circumstances we know to be favourable to the production of new varieties. +</p> + +<p> +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 <i>a</i><sup>1</sup> is supposed in the +diagram to have produced variety <i>a</i><sup>2</sup>, which will, owing to the +principle of divergence, differ more from (A) than did variety +<i>a</i><sup>1</sup>. Variety <i>m</i><sup>1</sup> is supposed to have produced +two varieties, namely <i>m</i><sup>2</sup> and <i>s</i><sup>2</sup>, 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 +increasing in number and diverging in character. In the diagram the process is +represented up to the ten-thousandth generation, and under a condensed and +simplified form up to the fourteen-thousandth generation. +</p> + +<p> +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. +<a name="Page119"></a> +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 selection 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 infinitely 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 +numbered 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. +</p> + +<p> +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 improved branches in the lines of descent, will, it +is probable, 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 reaching to the upper horizontal lines. In some cases I do not +doubt that the process of modification will be confined to a single line of +descent, and the number of the descendants will not be increased; although the +amount +<a name="Page120"></a> +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 <i>a</i><sup>1</sup> to +<i>a</i><sup>10</sup>. 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, without either having given off any fresh branches +or races. +</p> + +<p> +After ten thousand generations, species (A) is supposed to have produced three +forms, <i>a</i><sup>10</sup>, <i>f</i><sup>10</sup>, and <i>m</i><sup>10</sup>, +which, 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 well-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 +<i>a</i><sup>14</sup> and <i>m</i><sup>14</sup>, all descended from (A). Thus, +as I believe, species are multiplied and genera are formed. +</p> + +<p> +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 well-marked varieties +(<i>w</i><sup>10</sup> and <i>z</i><sup>10</sup>) or two species, according to +the amount of change supposed to be represented between +<a name="Page121"></a> +the horizontal lines. +After fourteen thousand generations, six new species, marked by the letters +<i>n</i><sup>14</sup> to <i>z</i><sup>14</sup>, are supposed to have been +produced. In each genus, the species, which are already extremely different in +character, will generally tend to produce the greatest number of modified +descendants; for these will 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 transmitting unaltered descendants; and this is shown in +the diagram by the dotted lines not prolonged far upwards from want of space. +</p> + +<p> +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 fully stocked country natural selection necessarily acts by the +selected form having some advantage in the struggle for life over other forms, +there will be a constant 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, constitution, 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 well as the original +parent-species itself, will generally tend to become extinct. So it probably +will be with many whole collateral lines of descent, which will be conquered by +later and improved lines of descent. If, however, the +<a name="Page122"></a> +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. +</p> + +<p> +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 (<i>a</i><sup>14</sup> to +<i>m</i><sup>14</sup>); and (I) will have been replaced by six +(<i>n</i><sup>14</sup> to <i>z</i><sup>14</sup>) new species. +</p> + +<p> +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 +diffused 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. +</p> + +<p> +<a name="Page123"></a> +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 +<i>a</i><sup>14</sup> and <i>z</i><sup>14</sup> 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 <i>a</i><sup>14</sup>, +<i>q</i><sup>14</sup>, <i>p</i><sup>14</sup>, will be nearly related from +having recently branched off from <i>a</i><sup>10</sup>; <i>b</i><sup>14</sup> +and <i>f</i><sup>14</sup>, from having diverged at an earlier period from +<i>a</i><sup>5</sup>, will be in some degree distinct from the three +first-named species; and lastly, <i>o</i><sup>14</sup>, <i>e</i><sup>14</sup>, +and <i>m</i><sup>14</sup>, will be nearly related one to the other, but from +having diverged 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. +</p> + +<p> +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, moreover, are supposed to have gone on diverging in different +directions. The intermediate species, also (and this is a very important +consideration), which connected the original species (A) and (I), have all +become, excepting (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. +</p> + +<p> +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 more +<a name="Page124"></a> +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 downwards towards a single +point; this point representing a single species, the supposed single parent of +our several new sub-genera and genera. +</p> + +<p> +It is worth while to reflect for a moment on the character of the new species +<small>F</small><sup>14</sup>, 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 supposed to be +extinct and unknown, it will 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 (<small>F</small><sup>14</sup>) 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. +</p> + +<p> +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 light 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 existing groups; and we can understand this fact, for +<a name="Page125"></a> +the extinct species lived at very ancient epochs when the branching lines of +descent had diverged less. +</p> + +<p> +I see no reason to limit the process of modification, as now explained, to the +formation of genera alone. If, in our diagram, we suppose the amount of change +represented by each successive group of diverging dotted lines to be very +great, the forms marked <i>a</i><sup>14</sup> to <i>p</i><sup>14</sup>, those +marked <i>b</i><sup>14</sup> and <i>f</i><sup>14</sup>, and those marked +<i>o</i><sup>14</sup> to <i>m</i><sup>14</sup>, 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 form 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 two 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. +</p> + +<p> +We have seen that in each country it is the species of the larger genera which +oftenest present varieties or incipient species. This, indeed, might have been +expected; 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 shows that +its species have inherited from a common ancestor some advantage in common. +Hence, the struggle for the production of new and modified descendants, will +mainly lie between the larger groups, which are all trying to increase in +number. One large group will slowly conquer another large group, reduce its +numbers, and thus lessen its chance of further variation and improvement. +Within the same large +<a name="Page126"></a> +group, the later and more highly perfected sub-groups, from branching out and +seizing on many new places in the polity of Nature, will constantly tend to +supplant and destroy the earlier and less improved sub-groups. Small and broken +groups and sub-groups will finally tend to disappear. Looking to the future, we +can predict that the groups of organic beings which are now large and +triumphant, and which are least broken up, that is, which as yet have suffered +least extinction, will for a long period continue to increase. But which groups +will ultimately prevail, no man can predict; for we well know that many groups, +formerly most extensively developed, have now become extinct. Looking still +more remotely to the future, we may predict that, owing to the continued and +steady increase of the larger groups, a multitude of smaller groups will become +utterly extinct, and leave no modified descendants; and consequently that of +the species living at any one period, extremely few will transmit descendants +to a remote futurity. I shall have to return to this subject in the chapter on +Classification, but I may add that on this view of extremely few of the more +ancient species having transmitted descendants, and on the view of all the +descendants of the same species making a class, we can understand how it is +that there exist but very few classes in each main division of the animal and +vegetable kingdoms. Although extremely few of the most ancient species may now +have living and modified descendants, yet at the most remote geological period, +the earth may have been as well peopled with many species of many genera, +families, orders, and classes, as at the present day. +</p> + +<p class="p2"> +<i>Summary of the Chapter</i>.—If during the long course of ages and +under varying conditions of life, organic beings +<a name="Page127"></a> +vary at all in the several parts of their organisation, and I think this cannot +be disputed; if there be, owing to the high geometrical powers of increase of +each species, at some age, season, or year, a severe struggle for life, and +this certainly cannot be disputed; then, considering the infinite complexity of +the relations of all organic beings to each other and to their conditions of +existence, causing an infinite diversity in structure, constitution, and +habits, to be advantageous to them, I think it would be a most extraordinary +fact if no variation ever had occurred useful to each being’s own +welfare, in the same way as so many variations have occurred useful to man. But +if variations useful to any organic being do occur, assuredly individuals thus +characterised will have the best chance of being preserved in the struggle for +life; and from the strong principle of inheritance they will tend to produce +offspring similarly characterised. This principle of preservation, I have +called, for the sake of brevity, Natural Selection. Natural selection, on the +principle of qualities being inherited at corresponding ages, can modify the +egg, seed, or young, as easily as the adult. Amongst many animals, sexual +selection will give its aid to ordinary selection, by assuring to the most +vigorous and best adapted males the greatest number of offspring. Sexual +selection will also give characters useful to the males alone, in their +struggles with other males. +</p> + +<p> +Whether natural selection has really thus acted in nature, in modifying and +adapting the various forms of life to their several conditions and stations, +must be judged of by the general tenour and balance of evidence given in the +following chapters. But we already see how it entails extinction; and how +largely extinction has acted in the world’s history, geology plainly +declares. Natural selection, also, leads to divergence of +<a name="Page128"></a> +character; for more living beings can be supported on the same area the more +they diverge in structure, habits, and constitution, of which we see proof by +looking at the inhabitants of any small spot or at naturalised productions. +Therefore during the modification of the descendants of any one species, and +during the incessant struggle of all species to increase in numbers, the more +diversified these descendants become, the better will be their chance of +succeeding in the battle of life. Thus the small differences distinguishing +varieties of the same species, will steadily tend to increase till they come to +equal the greater differences between species of the same genus, or even of +distinct genera. +</p> + +<p> +We have seen that it is the common, the widely-diffused, and widely-ranging +species, belonging to the larger genera, which vary most; and these will tend +to transmit to their modified offspring that superiority which now makes them +dominant in their own countries. Natural selection, as has just been remarked, +leads to divergence of character and to much extinction of the less improved +and intermediate forms of life. On these principles, I believe, the nature of +the affinities of all organic beings may be explained. It is a truly wonderful +fact—the wonder of which we are apt to overlook from +familiarity—that all animals and all plants throughout all time and space +should be related to each other in group subordinate to group, in the manner +which we everywhere behold—namely, varieties of the same species most +closely related together, species of the same genus less closely and unequally +related together, forming sections and sub-genera, species of distinct genera +much less closely related, and genera related in different degrees, forming +sub-families, families, orders, sub-classes, and classes. The several +subordinate groups in any class cannot be +<a name="Page129"></a> +ranked in a single file, but seem rather to be clustered round points, and +these round other points, and so on in almost endless cycles. On the view that +each species has been independently created, I can see no explanation of this +great fact in the classification of all organic beings; but, to the best of my +judgment, it is explained through inheritance and the complex action of natural +selection, entailing extinction and divergence of character, as we have seen +illustrated in the diagram. +</p> + +<p> +The affinities of all the beings of the same class have sometimes been +represented by a great tree. I believe this simile largely speaks the truth. +The green and budding twigs may represent existing species; and those produced +during each former year may represent the long succession of extinct species. +At each period of growth all the growing twigs have tried to branch out on all +sides, and to overtop and kill the surrounding twigs and branches, in the same +manner as species and groups of species have tried to overmaster other species +in the great battle for life. The limbs divided into great branches, and these +into lesser and lesser branches, were themselves once, when the tree was small, +budding twigs; and this connexion of the former and present buds by ramifying +branches may well represent the classification of all extinct and living +species in groups subordinate to groups. Of the many twigs which flourished +when the tree was a mere bush, only two or three, now grown into great +branches, yet survive and bear all the other branches; so with the species +which lived during long-past geological periods, very few now have living and +modified descendants. From the first growth of the tree, many a limb and branch +has decayed and dropped off; and these lost branches of various sizes may +represent those whole orders, families, and genera which have now no living +representatives, and +<a name="Page130"></a> +which are known to us only from having been found in a fossil state. As we here +and there see a thin straggling branch springing from a fork low down in a +tree, and which by some chance has been favoured and is still alive on its +summit, so we occasionally see an animal like the Ornithorhynchus or +Lepidosiren, which in some small degree connects by its affinities two large +branches of life, and which has apparently been saved from fatal competition by +having inhabited a protected station. As buds give rise by growth to fresh +buds, and these, if vigorous, branch out and overtop on all sides many a +feebler branch, so by generation I believe it has been with the great Tree of +Life, which fills with its dead and broken branches the crust of the earth, and +covers the surface with its ever branching and beautiful ramifications. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page131"></a><a name="chap05"></a>CHAPTER V.<br /> +LAWS OF VARIATION.</h2> + +<p class="letter"> +Effects of external conditions. Use and disuse, combined with natural +selection; organs of flight and of vision. Acclimatisation. Correlation of +growth. Compensation and economy of growth. False correlations. Multiple, +rudimentary, and lowly organised structures variable. Parts developed in an +unusual manner are highly variable: specific characters more variable than +generic: secondary sexual characters variable. Species of the same genus vary +in an analogous manner. Reversions to long lost characters. Summary. +</p> + +<p> +I have hitherto sometimes spoken as if the variations—so common and +multiform in organic beings under domestication, and in a lesser degree in +those in a state of nature—had been due to chance. This, of course, is a +wholly incorrect expression, but it serves to acknowledge plainly our ignorance +of the cause of each particular variation. Some authors believe it to be as +much the function of the reproductive system to produce individual differences, +or very slight deviations of structure, as to make the child like its parents. +But the much greater variability, as well as the greater frequency of +monstrosities, under domestication or cultivation, than under nature, leads me +to believe that deviations of structure are in some way due to the nature of +the conditions of life, to which the parents and their more remote ancestors +have been exposed during several generations. I have remarked in the first +chapter—but a long catalogue of facts which cannot be here given would be +necessary to show the truth of the remark—that the reproductive system is +eminently susceptible to changes in the conditions of life; and to +<a name="Page132"></a> +this system being functionally disturbed in the parents, I chiefly attribute +the varying or plastic condition of the offspring. The male and female sexual +elements seem to be affected before that union takes place which is to form a +new being. In the case of “sporting” plants, the bud, which in its +earliest condition does not apparently differ essentially from an ovule, is +alone affected. But why, because the reproductive system is disturbed, this or +that part should vary more or less, we are profoundly ignorant. Nevertheless, +we can here and there dimly catch a faint ray of light, and we may feel sure +that there must be some cause for each deviation of structure, however slight. +</p> + +<p> +How much direct effect difference of climate, food, etc., produces on any being +is extremely doubtful. My impression is, that the effect is extremely small in +the case of animals, but perhaps rather more in that of plants. We may, at +least, safely conclude that such influences cannot have produced the many +striking and complex co-adaptations of structure between one organic being and +another, which we see everywhere throughout nature. Some little influence may +be attributed to climate, food, etc.: thus, E. Forbes speaks confidently that +shells at their southern limit, and when living in shallow water, are more +brightly coloured than those of the same species further north or from greater +depths. Gould believes that birds of the same species are more brightly +coloured under a clear atmosphere, than when living on islands or near the +coast. So with insects, Wollaston is convinced that residence near the sea +affects their colours. Moquin-Tandon gives a list of plants which when growing +near the sea-shore have their leaves in some degree fleshy, though not +elsewhere fleshy. Several other such cases could be given. +</p> + +<p> +The fact of varieties of one species, when they range +<a name="Page133"></a> +into the zone of habitation of other species, often acquiring in a very slight +degree some of the characters of such species, accords with our view that +species of all kinds are only well-marked and permanent varieties. Thus the +species of shells which are confined to tropical and shallow seas are generally +brighter-coloured than those confined to cold and deeper seas. The birds which +are confined to continents are, according to Mr. Gould, brighter-coloured than +those of islands. The insect-species confined to sea-coasts, as every collector +knows, are often brassy or lurid. Plants which live exclusively on the sea-side +are very apt to have fleshy leaves. He who believes in the creation of each +species, will have to say that this shell, for instance, was created with +bright colours for a warm sea; but that this other shell became bright-coloured +by variation when it ranged into warmer or shallower waters. +</p> + +<p> +When a variation is of the slightest use to a being, we cannot tell how much of +it to attribute to the accumulative action of natural selection, and how much +to the conditions of life. Thus, it is well known to furriers that animals of +the same species have thicker and better fur the more severe the climate is +under which they have lived; but who can tell how much of this difference may +be due to the warmest-clad individuals having been favoured and preserved +during many generations, and how much to the direct action of the severe +climate? for it would appear that climate has some direct action on the hair of +our domestic quadrupeds. +</p> + +<p> +Instances could be given of the same variety being produced under conditions of +life as different as can well be conceived; and, on the other hand, of +different varieties being produced from the same species under the same +conditions. Such facts show how indirectly +<a name="Page134"></a> +the conditions of life must act. Again, innumerable instances are known to +every naturalist of species keeping true, or not varying at all, although +living under the most opposite climates. Such considerations as these incline +me to lay very little weight on the direct action of the conditions of life. +Indirectly, as already remarked, they seem to play an important part in +affecting the reproductive system, and in thus inducing variability; and +natural selection will then accumulate all profitable variations, however +slight, until they become plainly developed and appreciable by us. +</p> + +<p class="p2"> +<i>Effects of Use and Disuse</i>.—From the facts alluded to in the first +chapter, I think there can be little doubt that use in our domestic animals +strengthens and enlarges certain parts, and disuse diminishes them; and that +such modifications are inherited. Under free nature, we can have no standard of +comparison, by which to judge of the effects of long-continued use or disuse, +for we know not the parent-forms; but many animals have structures which can be +explained by the effects of disuse. As Professor Owen has remarked, there is no +greater anomaly in nature than a bird that cannot fly; yet there are several in +this state. The logger-headed duck of South America can only flap along the +surface of the water, and has its wings in nearly the same condition as the +domestic Aylesbury duck. As the larger ground-feeding birds seldom take flight +except to escape danger, I believe that the nearly wingless condition of +several birds, which now inhabit or have lately inhabited several oceanic +islands, tenanted by no beast of prey, has been caused by disuse. The ostrich +indeed inhabits continents and is exposed to danger from which it cannot escape +by flight, but by kicking it can defend itself from enemies, as well as any of +the smaller +<a name="Page135"></a> +quadrupeds. We may imagine that the early progenitor of the ostrich had habits +like those of a bustard, and that as natural selection increased in successive +generations the size and weight of its body, its legs were used more, and its +wings less, until they became incapable of flight. +</p> + +<p> +Kirby has remarked (and I have observed the same fact) that the anterior tarsi, +or feet, of many male dung-feeding beetles are very often broken off; he +examined seventeen specimens in his own collection, and not one had even a +relic left. In the Onites apelles the tarsi are so habitually lost, that the +insect has been described as not having them. In some other genera they are +present, but in a rudimentary condition. In the Ateuchus or sacred beetle of +the Egyptians, they are totally deficient. There is not sufficient evidence to +induce us to believe that mutilations are ever inherited; and I should prefer +explaining the entire absence of the anterior tarsi in Ateuchus, and their +rudimentary condition in some other genera, by the long-continued effects of +disuse in their progenitors; for as the tarsi are almost always lost in many +dung-feeding beetles, they must be lost early in life, and therefore cannot be +much used by these insects. +</p> + +<p> +In some cases we might easily put down to disuse modifications of structure +which are wholly, or mainly, due to natural selection. Mr. Wollaston has +discovered the remarkable fact that 200 beetles, out of the 550 species +inhabiting Madeira, are so far deficient in wings that they cannot fly; and +that of the twenty-nine endemic genera, no less than twenty-three genera have +all their species in this condition! Several facts, namely, that beetles in +many parts of the world are very frequently blown to sea and perish; that the +beetles in Madeira, as observed by Mr. Wollaston, lie much concealed, +<a name="Page136"></a> +until the wind lulls and the sun shines; that the proportion of wingless +beetles is larger on the exposed Dezertas than in Madeira itself; and +especially the extraordinary fact, so strongly insisted on by Mr. Wollaston, of +the almost entire absence of certain large groups of beetles, elsewhere +excessively numerous, and which groups have habits of life almost necessitating +frequent flight;—these several considerations have made me believe that +the wingless condition of so many Madeira beetles is mainly due to the action +of natural selection, but combined probably with disuse. For during thousands +of successive generations each individual beetle which flew least, either from +its wings having been ever so little less perfectly developed or from indolent +habit, will have had the best chance of surviving from not being blown out to +sea; and, on the other hand, those beetles which most readily took to flight +will oftenest have been blown to sea and thus have been destroyed. +</p> + +<p> +The insects in Madeira which are not ground-feeders, and which, as the +flower-feeding coleoptera and lepidoptera, must habitually use their wings to +gain their subsistence, have, as Mr. Wollaston suspects, their wings not at all +reduced, but even enlarged. This is quite compatible with the action of natural +selection. For when a new insect first arrived on the island, the tendency of +natural selection to enlarge or to reduce the wings, would depend on whether a +greater number of individuals were saved by successfully battling with the +winds, or by giving up the attempt and rarely or never flying. As with mariners +shipwrecked near a coast, it would have been better for the good swimmers if +they had been able to swim still further, whereas it would have been better for +the bad swimmers if they had not been able to swim at all and had stuck to the +wreck. +</p> + +<p> +<a name="Page137"></a> +The eyes of moles and of some burrowing rodents are rudimentary in size, and in +some cases are quite covered up by skin and fur. This state of the eyes is +probably due to gradual reduction from disuse, but aided perhaps by natural +selection. In South America, a burrowing rodent, the tuco-tuco, or Ctenomys, is +even more subterranean in its habits than the mole; and I was assured by a +Spaniard, who had often caught them, that they were frequently blind; one which +I kept alive was certainly in this condition, the cause, as appeared on +dissection, having been inflammation of the nictitating membrane. As frequent +inflammation of the eyes must be injurious to any animal, and as eyes are +certainly not indispensable to animals with subterranean habits, a reduction in +their size with the adhesion of the eyelids and growth of fur over them, might +in such case be an advantage; and if so, natural selection would constantly aid +the effects of disuse. +</p> + +<p> +It is well known that several animals, belonging to the most different classes, +which inhabit the caves of Styria and of Kentucky, are blind. In some of the +crabs the foot-stalk for the eye remains, though the eye is gone; the stand for +the telescope is there, though the telescope with its glasses has been lost. As +it is difficult to imagine that eyes, though useless, could be in any way +injurious to animals living in darkness, I attribute their loss wholly to +disuse. In one of the blind animals, namely, the cave-rat, the eyes are of +immense size; and Professor Silliman thought that it regained, after living +some days in the light, some slight power of vision. In the same manner as in +Madeira the wings of some of the insects have been enlarged, and the wings of +others have been reduced by natural selection aided by use and disuse, so in +the case of the cave-rat natural selection seems to have struggled with the +loss of light and +<a name="Page138"></a> +to have increased the size of the eyes; whereas with all the other inhabitants +of the caves, disuse by itself seems to have done its work. +</p> + +<p> +It is difficult to imagine conditions of life more similar than deep limestone +caverns under a nearly similar climate; so that on the common view of the blind +animals having been separately created for the American and European caverns, +close similarity in their organisation and affinities might have been expected; +but, as Schiödte and others have remarked, this is not the case, and the +cave-insects of the two continents are not more closely allied than might have +been anticipated from the general resemblance of the other inhabitants of North +America and Europe. On my view we must suppose that American animals, having +ordinary powers of vision, slowly migrated by successive generations from the +outer world into the deeper and deeper recesses of the Kentucky caves, as did +European animals into the caves of Europe. We have some evidence of this +gradation of habit; for, as Schiödte remarks, “animals not far remote +from ordinary forms, prepare the transition from light to darkness. Next follow +those that are constructed for twilight; and, last of all, those destined for +total darkness.” By the time that an animal had reached, after numberless +generations, the deepest recesses, disuse will on this view have more or less +perfectly obliterated its eyes, and natural selection will often have effected +other changes, such as an increase in the length of the antennæ or palpi, as a +compensation for blindness. Notwithstanding such modifications, we might expect +still to see in the cave-animals of America, affinities to the other +inhabitants of that continent, and in those of Europe, to the inhabitants of +the European continent. And this is the case with some of the American +cave-animals, as I hear from +<a name="Page139"></a> +Professor Dana; and some of the European cave-insects are very closely allied +to those of the surrounding country. It would be most difficult to give any +rational explanation of the affinities of the blind cave-animals to the other +inhabitants of the two continents on the ordinary view of their independent +creation. That several of the inhabitants of the caves of the Old and New +Worlds should be closely related, we might expect from the well-known +relationship of most of their other productions. Far from feeling any surprise +that some of the cave-animals should be very anomalous, as Agassiz has remarked +in regard to the blind fish, the Amblyopsis, and as is the case with the blind +Proteus with reference to the reptiles of Europe, I am only surprised that more +wrecks of ancient life have not been preserved, owing to the less severe +competition to which the inhabitants of these dark abodes will probably have +been exposed. +</p> + +<p class="p2"> +<i>Acclimatisation</i>.—Habit is hereditary with plants, as in the period +of flowering, in the amount of rain requisite for seeds to germinate, in the +time of sleep, etc., and this leads me to say a few words on acclimatisation. +As it is extremely common for species of the same genus to inhabit very hot and +very cold countries, and as I believe that all the species of the same genus +have descended from a single parent, if this view be correct, acclimatisation +must be readily effected during long-continued descent. It is notorious that +each species is adapted to the climate of its own home: species from an arctic +or even from a temperate region cannot endure a tropical climate, or +conversely. So again, many succulent plants cannot endure a damp climate. But +the degree of adaptation of species to the climates under which they live is +often overrated. +<a name="Page140"></a> +We may infer this from our frequent inability to predict whether or not an +imported plant will endure our climate, and from the number of plants and +animals brought from warmer countries which here enjoy good health. We have +reason to believe that species in a state of nature are limited in their ranges +by the competition of other organic beings quite as much as, or more than, by +adaptation to particular climates. But whether or not the adaptation be +generally very close, we have evidence, in the case of some few plants, of +their becoming, to a certain extent, naturally habituated to different +temperatures, or becoming acclimatised: thus the pines and rhododendrons, +raised from seed collected by Dr. Hooker from trees growing at different +heights on the Himalaya, were found in this country to possess different +constitutional powers of resisting cold. Mr. Thwaites informs me that he has +observed similar facts in Ceylon, and analogous observations have been made by +Mr. H. C. Watson on European species of plants brought from the Azores to +England. In regard to animals, several authentic cases could be given of +species within historical times having largely extended their range from warmer +to cooler latitudes, and conversely; but we do not positively know that these +animals were strictly adapted to their native climate, but in all ordinary +cases we assume such to be the case; nor do we know that they have subsequently +become acclimatised to their new homes. +</p> + +<p> +As I believe that our domestic animals were originally chosen by uncivilised +man because they were useful and bred readily under confinement, and not +because they were subsequently found capable of far-extended transportation, I +think the common and extraordinary capacity in our domestic animals of not only +withstanding the most different climates but of being perfectly +<a name="Page141"></a> +fertile (a far severer test) under them, may be used as an argument that a +large proportion of other animals, now in a state of nature, could easily be +brought to bear widely different climates. We must not, however, push the +foregoing argument too far, on account of the probable origin of some of our +domestic animals from several wild stocks: the blood, for instance, of a +tropical and arctic wolf or wild dog may perhaps be mingled in our domestic +breeds. The rat and mouse cannot be considered as domestic animals, but they +have been transported by man to many parts of the world, and now have a far +wider range than any other rodent, living free under the cold climate of Faroe +in the north and of the Falklands in the south, and on many islands in the +torrid zones. Hence I am inclined to look at adaptation to any special climate +as a quality readily grafted on an innate wide flexibility of constitution, +which is common to most animals. On this view, the capacity of enduring the +most different climates by man himself and by his domestic animals, and such +facts as that former species of the elephant and rhinoceros were capable of +enduring a glacial climate, whereas the living species are now all tropical or +sub-tropical in their habits, ought not to be looked at as anomalies, but +merely as examples of a very common flexibility of constitution, brought, under +peculiar circumstances, into play. +</p> + +<p> +How much of the acclimatisation of species to any peculiar climate is due to +mere habit, and how much to the natural selection of varieties having different +innate constitutions, and how much to both means combined, is a very obscure +question. That habit or custom has some influence I must believe, both from +analogy, and from the incessant advice given in agricultural works, even in the +ancient Encyclopædias of China, to be very cautious +<a name="Page142"></a> +in transposing animals from one district to another; for it is not likely that +man should have succeeded in selecting so many breeds and sub-breeds with +constitutions specially fitted for their own districts: the result must, I +think, be due to habit. On the other hand, I can see no reason to doubt that +natural selection will continually tend to preserve those individuals which are +born with constitutions best adapted to their native countries. In treatises on +many kinds of cultivated plants, certain varieties are said to withstand +certain climates better than others: this is very strikingly shown in works on +fruit trees published in the United States, in which certain varieties are +habitually recommended for the northern, and others for the southern States; +and as most of these varieties are of recent origin, they cannot owe their +constitutional differences to habit. The case of the Jerusalem artichoke, which +is never propagated by seed, and of which consequently new varieties have not +been produced, has even been advanced—for it is now as tender as ever it +was—as proving that acclimatisation cannot be effected! The case, also, +of the kidney-bean has been often cited for a similar purpose, and with much +greater weight; but until some one will sow, during a score of generations, his +kidney-beans so early that a very large proportion are destroyed by frost, and +then collect seed from the few survivors, with care to prevent accidental +crosses, and then again get seed from these seedlings, with the same +precautions, the experiment cannot be said to have been even tried. Nor let it +be supposed that no differences in the constitution of seedling kidney-beans +ever appear, for an account has been published how much more hardy some +seedlings appeared to be than others. +</p> + +<p> +On the whole, I think we may conclude that habit, +<a name="Page143"></a> +use, and disuse, have, in some cases, played a considerable part in the +modification of the constitution, and of the structure of various organs; but +that the effects of use and disuse have often been largely combined with, and +sometimes overmastered by, the natural selection of innate differences. +</p> + +<p class="p2"> +<i>Correlation of Growth</i>.—I mean by this expression that the whole +organisation is so tied together during its growth and development, that when +slight variations in any one part occur, and are accumulated through natural +selection, other parts become modified. This is a very important subject, most +imperfectly understood. The most obvious case is, that modifications +accumulated solely for the good of the young or larva, will, it may safely be +concluded, affect the structure of the adult; in the same manner as any +malconformation affecting the early embryo, seriously affects the whole +organisation of the adult. The several parts of the body which are homologous, +and which, at an early embryonic period, are alike, seem liable to vary in an +allied manner: we see this in the right and left sides of the body varying in +the same manner; in the front and hind legs, and even in the jaws and limbs, +varying together, for the lower jaw is believed to be homologous with the +limbs. These tendencies, I do not doubt, may be mastered more or less +completely by natural selection: thus a family of stags once existed with an +antler only on one side; and if this had been of any great use to the breed it +might probably have been rendered permanent by natural selection. +</p> + +<p> +Homologous parts, as has been remarked by some authors, tend to cohere; this is +often seen in monstrous plants; and nothing is more common than the union of +homologous parts in normal structures, as the union of +<a name="Page144"></a> +the petals of the corolla into a tube. Hard parts seem to affect the form of +adjoining soft parts; it is believed by some authors that the diversity in the +shape of the pelvis in birds causes the remarkable diversity in the shape of +their kidneys. Others believe that the shape of the pelvis in the human mother +influences by pressure the shape of the head of the child. In snakes, according +to Schlegel, the shape of the body and the manner of swallowing determine the +position of several of the most important viscera. +</p> + +<p> +The nature of the bond of correlation is very frequently quite obscure. M. Is. +Geoffroy St. Hilaire has forcibly remarked, that certain malconformations very +frequently, and that others rarely coexist, without our being able to assign +any reason. What can be more singular than the relation between blue eyes and +deafness in cats, and the tortoise-shell colour with the female sex; the +feathered feet and skin between the outer toes in pigeons, and the presence of +more or less down on the young birds when first hatched, with the future colour +of their plumage; or, again, the relation between the hair and teeth in the +naked Turkish dog, though here probably homology comes into play? With respect +to this latter case of correlation, I think it can hardly be accidental, that +if we pick out the two orders of mammalia which are most abnormal in their +dermal coverings, viz. Cetacea (whales) and Edentata (armadilloes, scaly +ant-eaters, etc.), that these are likewise the most abnormal in their teeth. +</p> + +<p> +I know of no case better adapted to show the importance of the laws of +correlation in modifying important structures, independently of utility and, +therefore, of natural selection, than that of the difference between the outer +and inner flowers in some Compositous and Umbelliferous plants. Every one knows +the +<a name="Page145"></a> +difference in the ray and central florets of, for instance, the daisy, and this +difference is often accompanied with the abortion of parts of the flower. But, +in some Compositous plants, the seeds also differ in shape and sculpture; and +even the ovary itself, with its accessory parts, differs, as has been described +by Cassini. These differences have been attributed by some authors to pressure, +and the shape of the seeds in the ray-florets in some Compositæ countenances +this idea; but, in the case of the corolla of the Umbelliferæ, it is by no +means, as Dr. Hooker informs me, in species with the densest heads that the +inner and outer flowers most frequently differ. It might have been thought that +the development of the ray-petals by drawing nourishment from certain other +parts of the flower had caused their abortion; but in some Compositæ there is +a difference in the seeds of the outer and inner florets without any difference +in the corolla. Possibly, these several differences may be connected with some +difference in the flow of nutriment towards the central and external flowers: +we know, at least, that in irregular flowers, those nearest to the axis are +oftenest subject to peloria, and become regular. I may add, as an instance of +this, and of a striking case of correlation, that I have recently observed in +some garden pelargoniums, that the central flower of the truss often loses the +patches of darker colour in the two upper petals; and that when this occurs, +the adherent nectary is quite aborted; when the colour is absent from only one +of the two upper petals, the nectary is only much shortened. +</p> + +<p> +With respect to the difference in the corolla of the central and exterior +flowers of a head or umbel, I do not feel at all sure that C. C. +Sprengel’s idea that the ray-florets serve to attract insects, whose +agency is highly advantageous in the fertilisation of plants of +<a name="Page146"></a> +these two orders, is so far-fetched, as it may at first appear: and if it be +advantageous, natural selection may have come into play. But in regard to the +differences both in the internal and external structure of the seeds, which are +not always correlated with any differences in the flowers, it seems impossible +that they can be in any way advantageous to the plant: yet in the Umbelliferæ +these differences are of such apparent importance—the seeds being in some +cases, according to Tausch, orthospermous in the exterior flowers and +coelospermous in the central flowers,—that the elder De Candolle founded +his main divisions of the order on analogous differences. Hence we see that +modifications of structure, viewed by systematists as of high value, may be +wholly due to unknown laws of correlated growth, and without being, as far as +we can see, of the slightest service to the species. +</p> + +<p> +We may often falsely attribute to correlation of growth, structures which are +common to whole groups of species, and which in truth are simply due to +inheritance; for an ancient progenitor may have acquired through natural +selection some one modification in structure, and, after thousands of +generations, some other and independent modification; and these two +modifications, having been transmitted to a whole group of descendants with +diverse habits, would naturally be thought to be correlated in some necessary +manner. So, again, I do not doubt that some apparent correlations, occurring +throughout whole orders, are entirely due to the manner alone in which natural +selection can act. For instance, Alph. De Candolle has remarked that winged +seeds are never found in fruits which do not open: I should explain the rule by +the fact that seeds could not gradually become winged through natural +selection, except in fruits which opened; so that the individual plants +producing +<a name="Page147"></a> +seeds which were a little better fitted to be wafted further, might get an +advantage over those producing seed less fitted for dispersal; and this process +could not possibly go on in fruit which did not open. +</p> + +<p> +The elder Geoffroy and Goethe propounded, at about the same period, their law +of compensation or balancement of growth; or, as Goethe expressed it, “in +order to spend on one side, nature is forced to economise on the other +side.” I think this holds true to a certain extent with our domestic +productions: if nourishment flows to one part or organ in excess, it rarely +flows, at least in excess, to another part; thus it is difficult to get a cow +to give much milk and to fatten readily. The same varieties of the cabbage do +not yield abundant and nutritious foliage and a copious supply of oil-bearing +seeds. When the seeds in our fruits become atrophied, the fruit itself gains +largely in size and quality. In our poultry, a large tuft of feathers on the +head is generally accompanied by a diminished comb, and a large beard by +diminished wattles. With species in a state of nature it can hardly be +maintained that the law is of universal application; but many good observers, +more especially botanists, believe in its truth. I will not, however, here give +any instances, for I see hardly any way of distinguishing between the effects, +on the one hand, of a part being largely developed through natural selection +and another and adjoining part being reduced by this same process or by disuse, +and, on the other hand, the actual withdrawal of nutriment from one part owing +to the excess of growth in another and adjoining part. +</p> + +<p> +I suspect, also, that some of the cases of compensation which have been +advanced, and likewise some other facts, may be merged under a more general +principle, namely, that natural selection is continually trying to economise in +every part of the organisation. If under +<a name="Page148"></a> +changed conditions of life a structure before useful becomes less useful, any +diminution, however slight, in its development, will be seized on by natural +selection, for it will profit the individual not to have its nutriment wasted +in building up an useless structure. I can thus only understand a fact with +which I was much struck when examining cirripedes, and of which many other +instances could be given: namely, that when a cirripede is parasitic within +another and is thus protected, it loses more or less completely its own shell +or carapace. This is the case with the male Ibla, and in a truly extraordinary +manner with the Proteolepas: for the carapace in all other cirripedes consists +of the three highly-important anterior segments of the head enormously +developed, and furnished with great nerves and muscles; but in the parasitic +and protected Proteolepas, the whole anterior part of the head is reduced to +the merest rudiment attached to the bases of the prehensile antennæ. Now the +saving of a large and complex structure, when rendered superfluous by the +parasitic habits of the Proteolepas, though effected by slow steps, would be a +decided advantage to each successive individual of the species; for in the +struggle for life to which every animal is exposed, each individual Proteolepas +would have a better chance of supporting itself, by less nutriment being wasted +in developing a structure now become useless. +</p> + +<p> +Thus, as I believe, natural selection will always succeed in the long run in +reducing and saving every part of the organisation, as soon as it is rendered +superfluous, without by any means causing some other part to be largely +developed in a corresponding degree. And, conversely, that natural selection +may perfectly well succeed in largely developing any organ, without requiring +as a necessary compensation the reduction of some adjoining part. +</p> + +<p> +<a name="Page149"></a> +It seems to be a rule, as remarked by Is. Geoffroy St. Hilaire, both in +varieties and in species, that when any part or organ is repeated many times in +the structure of the same individual (as the vertebræ in snakes, and the +stamens in polyandrous flowers) the number is variable; whereas the number of +the same part or organ, when it occurs in lesser numbers, is constant. The same +author and some botanists have further remarked that multiple parts are also +very liable to variation in structure. Inasmuch as this “vegetative +repetition,” to use Professor Owen’s expression, seems to be a sign +of low organisation; the foregoing remark seems connected with the very general +opinion of naturalists, that beings low in the scale of nature are more +variable than those which are higher. I presume that lowness in this case means +that the several parts of the organisation have been but little specialised for +particular functions; and as long as the same part has to perform diversified +work, we can perhaps see why it should remain variable, that is, why natural +selection should have preserved or rejected each little deviation of form less +carefully than when the part has to serve for one special purpose alone. In the +same way that a knife which has to cut all sorts of things may be of almost any +shape; whilst a tool for some particular object had better be of some +particular shape. Natural selection, it should never be forgotten, can act on +each part of each being, solely through and for its advantage. +</p> + +<p> +Rudimentary parts, it has been stated by some authors, and I believe with +truth, are apt to be highly variable. We shall have to recur to the general +subject of rudimentary and aborted organs; and I will here only add that their +variability seems to be owing to their uselessness, and therefore to natural +selection having no power to check deviations in their structure. Thus +<a name="Page150"></a> +rudimentary parts are left to the free play of the various laws of growth, to +the effects of long-continued disuse, and to the tendency to reversion. +</p> + +<p class="p2"> +<i>A part developed in any species in an extraordinary degree or manner, in +comparison with the same part in allied species, tends to be highly +variable</i>.—Several years ago I was much struck with a remark, nearly +to the above effect, published by Mr. Waterhouse. I infer also from an +observation made by Professor Owen, with respect to the length of the arms of +the ourang-outang, that he has come to a nearly similar conclusion. It is +hopeless to attempt to convince any one of the truth of this proposition +without giving the long array of facts which I have collected, and which cannot +possibly be here introduced. I can only state my conviction that it is a rule +of high generality. I am aware of several causes of error, but I hope that I +have made due allowance for them. It should be understood that the rule by no +means applies to any part, however unusually developed, unless it be unusually +developed in comparison with the same part in closely allied species. Thus, the +bat’s wing is a most abnormal structure in the class mammalia; but the +rule would not here apply, because there is a whole group of bats having wings; +it would apply only if some one species of bat had its wings developed in some +remarkable manner in comparison with the other species of the same genus. The +rule applies very strongly in the case of secondary sexual characters, when +displayed in any unusual manner. The term, secondary sexual characters, used by +Hunter, applies to characters which are attached to one sex, but are not +directly connected with the act of reproduction. The rule applies to males and +females; but as females more rarely offer remarkable secondary sexual +characters, it applies +<a name="Page151"></a> +more rarely to them. The rule being so plainly applicable in the case of +secondary sexual characters, may be due to the great variability of these +characters, whether or not displayed in any unusual manner—of which fact +I think there can be little doubt. But that our rule is not confined to +secondary sexual characters is clearly shown in the case of hermaphrodite +cirripedes; and I may here add, that I particularly attended to Mr. +Waterhouse’s remark, whilst investigating this Order, and I am fully +convinced that the rule almost invariably holds good with cirripedes. I shall, +in my future work, give a list of the more remarkable cases; I will here only +briefly give one, as it illustrates the rule in its largest application. The +opercular valves of sessile cirripedes (rock barnacles) are, in every sense of +the word, very important structures, and they differ extremely little even in +different genera; but in the several species of one genus, Pyrgoma, these +valves present a marvellous amount of diversification: the homologous valves in +the different species being sometimes wholly unlike in shape; and the amount of +variation in the individuals of several of the species is so great, that it is +no exaggeration to state that the varieties differ more from each other in the +characters of these important valves than do other species of distinct genera. +</p> + +<p> +As birds within the same country vary in a remarkably small degree, I have +particularly attended to them, and the rule seems to me certainly to hold good +in this class. I cannot make out that it applies to plants, and this would +seriously have shaken my belief in its truth, had not the great variability in +plants made it particularly difficult to compare their relative degrees of +variability. +</p> + +<p> +When we see any part or organ developed in a remarkable degree or manner in any +species, the fair +<a name="Page152"></a> +presumption is that it is of high importance to that species; nevertheless the +part in this case is eminently liable to variation. Why should this be so? On +the view that each species has been independently created, with all its parts +as we now see them, I can see no explanation. But on the view that groups of +species have descended from other species, and have been modified through +natural selection, I think we can obtain some light. In our domestic animals, +if any part, or the whole animal, be neglected and no selection be applied, +that part (for instance, the comb in the Dorking fowl) or the whole breed will +cease to have a nearly uniform character. The breed will then be said to have +degenerated. In rudimentary organs, and in those which have been but little +specialised for any particular purpose, and perhaps in polymorphic groups, we +see a nearly parallel natural case; for in such cases natural selection either +has not or cannot come into full play, and thus the organisation is left in a +fluctuating condition. But what here more especially concerns us is, that in +our domestic animals those points, which at the present time are undergoing +rapid change by continued selection, are also eminently liable to variation. +Look at the breeds of the pigeon; see what a prodigious amount of difference +there is in the beak of the different tumblers, in the beak and wattle of the +different carriers, in the carriage and tail of our fantails, etc., these being +the points now mainly attended to by English fanciers. Even in the sub-breeds, +as in the short-faced tumbler, it is notoriously difficult to breed them nearly +to perfection, and frequently individuals are born which depart widely from the +standard. There may be truly said to be a constant struggle going on between, +on the one hand, the tendency to reversion to a less modified state, as well as +an innate tendency to further +<a name="Page153"></a> +variability of all kinds, and, on the other hand, the power of steady selection +to keep the breed true. In the long run selection gains the day, and we do not +expect to fail so far as to breed a bird as coarse as a common tumbler from a +good short-faced strain. But as long as selection is rapidly going on, there +may always be expected to be much variability in the structure undergoing +modification. It further deserves notice that these variable characters, +produced by man’s selection, sometimes become attached, from causes quite +unknown to us, more to one sex than to the other, generally to the male sex, as +with the wattle of carriers and the enlarged crop of pouters. +</p> + +<p> +Now let us turn to nature. When a part has been developed in an extraordinary +manner in any one species, compared with the other species of the same genus, +we may conclude that this part has undergone an extraordinary amount of +modification, since the period when the species branched off from the common +progenitor of the genus. This period will seldom be remote in any extreme +degree, as species very rarely endure for more than one geological period. An +extraordinary amount of modification implies an unusually large and +long-continued amount of variability, which has continually been accumulated by +natural selection for the benefit of the species. But as the variability of the +extraordinarily-developed part or organ has been so great and long-continued +within a period not excessively remote, we might, as a general rule, expect +still to find more variability in such parts than in other parts of the +organisation, which have remained for a much longer period nearly constant. And +this, I am convinced, is the case. That the struggle between natural selection +on the one hand, and the tendency to reversion and variability on the other +hand, will in the +<a name="Page154"></a> +course of time cease; and that the most abnormally developed organs may be made +constant, I can see no reason to doubt. Hence when an organ, however abnormal +it may be, has been transmitted in approximately the same condition to many +modified descendants, as in the case of the wing of the bat, it must have +existed, according to my theory, for an immense period in nearly the same +state; and thus it comes to be no more variable than any other structure. It is +only in those cases in which the modification has been comparatively recent and +extraordinarily great that we ought to find the <i>generative variability</i>, +as it may be called, still present in a high degree. For in this case the +variability will seldom as yet have been fixed by the continued selection of +the individuals varying in the required manner and degree, and by the continued +rejection of those tending to revert to a former and less modified condition. +</p> + +<p> +The principle included in these remarks may be extended. It is notorious that +specific characters are more variable than generic. To explain by a simple +example what is meant. If some species in a large genus of plants had blue +flowers and some had red, the colour would be only a specific character, and no +one would be surprised at one of the blue species varying into red, or +conversely; but if all the species had blue flowers, the colour would become a +generic character, and its variation would be a more unusual circumstance. I +have chosen this example because an explanation is not in this case applicable, +which most naturalists would advance, namely, that specific characters are more +variable than generic, because they are taken from parts of less physiological +importance than those commonly used for classing genera. I believe this +explanation is partly, yet only indirectly, true; I shall, however, have to +return +<a name="Page155"></a> +to this subject in our chapter on Classification. It would be almost +superfluous to adduce evidence in support of the above statement, that specific +characters are more variable than generic; but I have repeatedly noticed in +works on natural history, that when an author has remarked with surprise that +some <i>important</i> organ or part, which is generally very constant +throughout large groups of species, has <i>differed</i> considerably in +closely-allied species, that it has, also, been <i>variable</i> in the +individuals of some of the species. And this fact shows that a character, which +is generally of generic value, when it sinks in value and becomes only of +specific value, often becomes variable, though its physiological importance may +remain the same. Something of the same kind applies to monstrosities: at least +Is. Geoffroy St. Hilaire seems to entertain no doubt, that the more an organ +normally differs in the different species of the same group, the more subject +it is to individual anomalies. +</p> + +<p> +On the ordinary view of each species having been independently created, why +should that part of the structure, which differs from the same part in other +independently-created species of the same genus, be more variable than those +parts which are closely alike in the several species? I do not see that any +explanation can be given. But on the view of species being only strongly marked +and fixed varieties, we might surely expect to find them still often continuing +to vary in those parts of their structure which have varied within a moderately +recent period, and which have thus come to differ. Or to state the case in +another manner:—the points in which all the species of a genus resemble +each other, and in which they differ from the species of some other genus, are +called generic characters; and these characters in common I attribute to +inheritance from a common +<a name="Page156"></a> +progenitor, for it can rarely have happened that natural selection will have +modified several species, fitted to more or less widely-different habits, in +exactly the same manner: and as these so-called generic characters have been +inherited from a remote period, since that period when the species first +branched off from their common progenitor, and subsequently have not varied or +come to differ in any degree, or only in a slight degree, it is not probable +that they should vary at the present day. On the other hand, the points in +which species differ from other species of the same genus, are called specific +characters; and as these specific characters have varied and come to differ +within the period of the branching off of the species from a common progenitor, +it is probable that they should still often be in some degree +variable,—at least more variable than those parts of the organisation +which have for a very long period remained constant. +</p> + +<p> +In connexion with the present subject, I will make only two other remarks. I +think it will be admitted, without my entering on details, that secondary +sexual characters are very variable; I think it also will be admitted that +species of the same group differ from each other more widely in their secondary +sexual characters, than in other parts of their organisation; compare, for +instance, the amount of difference between the males of gallinaceous birds, in +which secondary sexual characters are strongly displayed, with the amount of +difference between their females; and the truth of this proposition will be +granted. The cause of the original variability of secondary sexual characters +is not manifest; but we can see why these characters should not have been +rendered as constant and uniform as other parts of the organisation; for +secondary sexual characters have been accumulated by sexual selection, which +<a name="Page157"></a> +is less rigid in its action than ordinary selection, as it does not entail +death, but only gives fewer offspring to the less favoured males. Whatever the +cause may be of the variability of secondary sexual characters, as they are +highly variable, sexual selection will have had a wide scope for action, and +may thus readily have succeeded in giving to the species of the same group a +greater amount of difference in their sexual characters, than in other parts of +their structure. +</p> + +<p> +It is a remarkable fact, that the secondary sexual differences between the two +sexes of the same species are generally displayed in the very same parts of the +organisation in which the different species of the same genus differ from each +other. Of this fact I will give in illustration two instances, the first which +happen to stand on my list; and as the differences in these cases are of a very +unusual nature, the relation can hardly be accidental. The same number of +joints in the tarsi is a character generally common to very large groups of +beetles, but in the Engidæ, as Westwood has remarked, the number varies +greatly; and the number likewise differs in the two sexes of the same species: +again in fossorial hymenoptera, the manner of neuration of the wings is a +character of the highest importance, because common to large groups; but in +certain genera the neuration differs in the different species, and likewise in +the two sexes of the same species. This relation has a clear meaning on my view +of the subject: I look at all the species of the same genus as having as +certainly descended from the same progenitor, as have the two sexes of any one +of the species. Consequently, whatever part of the structure of the common +progenitor, or of its early descendants, became variable; variations of this +part would it is highly probable, be taken advantage of by natural and sexual +selection, in +<a name="Page158"></a> +order to fit the several species to their several places in the economy of +nature, and likewise to fit the two sexes of the same species to each other, or +to fit the males and females to different habits of life, or the males to +struggle with other males for the possession of the females. +</p> + +<p> +Finally, then, I conclude that the greater variability of specific characters, +or those which distinguish species from species, than of generic characters, or +those which the species possess in common;—that the frequent extreme +variability of any part which is developed in a species in an extraordinary +manner in comparison with the same part in its congeners; and the not great +degree of variability in a part, however extraordinarily it may be developed, +if it be common to a whole group of species;—that the great variability +of secondary sexual characters, and the great amount of difference in these +same characters between closely allied species;—that secondary sexual and +ordinary specific differences are generally displayed in the same parts of the +organisation,—are all principles closely connected together. All being +mainly due to the species of the same group having descended from a common +progenitor, from whom they have inherited much in common,—to parts which +have recently and largely varied being more likely still to go on varying than +parts which have long been inherited and have not varied,—to natural +selection having more or less completely, according to the lapse of time, +overmastered the tendency to reversion and to further variability,—to +sexual selection being less rigid than ordinary selection,—and to +variations in the same parts having been accumulated by natural and sexual +selection, and thus adapted for secondary sexual, and for ordinary specific +purposes. +</p> + +<p class="p2"> +<a name="Page159"></a> +<i>Distinct species present analogous variations; and a variety of one species +often assumes some of the characters of an allied species, or reverts to some +of the characters of an early progenitor</i>.—These propositions will be +most readily understood by looking to our domestic races. The most distinct +breeds of pigeons, in countries most widely apart, present sub-varieties with +reversed feathers on the head and feathers on the feet,—characters not +possessed by the aboriginal rock-pigeon; these then are analogous variations in +two or more distinct races. The frequent presence of fourteen or even sixteen +tail-feathers in the pouter, may be considered as a variation representing the +normal structure of another race, the fantail. I presume that no one will doubt +that all such analogous variations are due to the several races of the pigeon +having inherited from a common parent the same constitution and tendency to +variation, when acted on by similar unknown influences. In the vegetable +kingdom we have a case of analogous variation, in the enlarged stems, or roots +as commonly called, of the Swedish turnip and Ruta baga, plants which several +botanists rank as varieties produced by cultivation from a common parent: if +this be not so, the case will then be one of analogous variation in two +so-called distinct species; and to these a third may be added, namely, the +common turnip. According to the ordinary view of each species having been +independently created, we should have to attribute this similarity in the +enlarged stems of these three plants, not to the vera causa of community of +descent, and a consequent tendency to vary in a like manner, but to three +separate yet closely related acts of creation. +</p> + +<p> +With pigeons, however, we have another case, namely, the occasional appearance +in all the breeds, of slaty-blue birds with two black bars on the wings, a +white +<a name="Page160"></a> +rump, a bar at the end of the tail, with the outer feathers externally edged +near their bases with white. As all these marks are characteristic of the +parent rock-pigeon, I presume that no one will doubt that this is a case of +reversion, and not of a new yet analogous variation appearing in the several +breeds. We may I think confidently come to this conclusion, because, as we have +seen, these coloured marks are eminently liable to appear in the crossed +offspring of two distinct and differently coloured breeds; and in this case +there is nothing in the external conditions of life to cause the reappearance +of the slaty-blue, with the several marks, beyond the influence of the mere act +of crossing on the laws of inheritance. +</p> + +<p> +No doubt it is a very surprising fact that characters should reappear after +having been lost for many, perhaps for hundreds of generations. But when a +breed has been crossed only once by some other breed, the offspring +occasionally show a tendency to revert in character to the foreign breed for +many generations—some say, for a dozen or even a score of generations. +After twelve generations, the proportion of blood, to use a common expression, +of any one ancestor, is only 1 in 2048; and yet, as we see, it is generally +believed that a tendency to reversion is retained by this very small proportion +of foreign blood. In a breed which has not been crossed, but in which +<i>both</i> parents have lost some character which their progenitor possessed, +the tendency, whether strong or weak, to reproduce the lost character might be, +as was formerly remarked, for all that we can see to the contrary, transmitted +for almost any number of generations. When a character which has been lost in a +breed, reappears after a great number of generations, the most probable +hypothesis is, not that the offspring suddenly takes after an ancestor some +hundred generations +<a name="Page161"></a> +distant, but that in each successive generation there has been a tendency to +reproduce the character in question, which at last, under unknown favourable +conditions, gains an ascendancy. For instance, it is probable that in each +generation of the barb-pigeon, which produces most rarely a blue and +black-barred bird, there has been a tendency in each generation in the plumage +to assume this colour. This view is hypothetical, but could be supported by +some facts; and I can see no more abstract improbability in a tendency to +produce any character being inherited for an endless number of generations, +than in quite useless or rudimentary organs being, as we all know them to be, +thus inherited. Indeed, we may sometimes observe a mere tendency to produce a +rudiment inherited: for instance, in the common snapdragon (Antirrhinum) a +rudiment of a fifth stamen so often appears, that this plant must have an +inherited tendency to produce it. +</p> + +<p> +As all the species of the same genus are supposed, on my theory, to have +descended from a common parent, it might be expected that they would +occasionally vary in an analogous manner; so that a variety of one species +would resemble in some of its characters another species; this other species +being on my view only a well-marked and permanent variety. But characters thus +gained would probably be of an unimportant nature, for the presence of all +important characters will be governed by natural selection, in accordance with +the diverse habits of the species, and will not be left to the mutual action of +the conditions of life and of a similar inherited constitution. It might +further be expected that the species of the same genus would occasionally +exhibit reversions to lost ancestral characters. As, however, we never know the +exact character of the common ancestor of a group, we could not distinguish +these two +<a name="Page162"></a> +cases: if, for instance, we did not know that the rock-pigeon was not +feather-footed or turn-crowned, we could not have told, whether these +characters in our domestic breeds were reversions or only analogous variations; +but we might have inferred that the blueness was a case of reversion, from the +number of the markings, which are correlated with the blue tint, and which it +does not appear probable would all appear together from simple variation. More +especially we might have inferred this, from the blue colour and marks so often +appearing when distinct breeds of diverse colours are crossed. Hence, though +under nature it must generally be left doubtful, what cases are reversions to +an anciently existing character, and what are new but analogous variations, yet +we ought, on my theory, sometimes to find the varying offspring of a species +assuming characters (either from reversion or from analogous variation) which +already occur in some other members of the same group. And this undoubtedly is +the case in nature. +</p> + +<p> +A considerable part of the difficulty in recognising a variable species in our +systematic works, is due to its varieties mocking, as it were, some of the +other species of the same genus. A considerable catalogue, also, could be given +of forms intermediate between two other forms, which themselves must be +doubtfully ranked as either varieties or species; and this shows, unless all +these forms be considered as independently created species, that the one in +varying has assumed some of the characters of the other, so as to produce the +intermediate form. But the best evidence is afforded by parts or organs of an +important and uniform nature occasionally varying so as to acquire, in some +degree, the character of the same part or organ in an allied species. I have +collected a long list of such cases; but +<a name="Page163"></a> +here, as before, I lie under a great disadvantage in not being able to give +them. I can only repeat that such cases certainly do occur, and seem to me very +remarkable. +</p> + +<p> +I will, however, give one curious and complex case, not indeed as affecting any +important character, but from occurring in several species of the same genus, +partly under domestication and partly under nature. It is a case apparently of +reversion. The ass not rarely has very distinct transverse bars on its legs, +like those on the legs of a zebra: it has been asserted that these are plainest +in the foal, and from inquiries which I have made, I believe this to be true. +It has also been asserted that the stripe on each shoulder is sometimes double. +The shoulder stripe is certainly very variable in length and outline. A white +ass, but <i>not</i> an albino, has been described without either spinal or +shoulder-stripe; and these stripes are sometimes very obscure, or actually +quite lost, in dark-coloured asses. The koulan of Pallas is said to have been +seen with a double shoulder-stripe. The hemionus has no shoulder-stripe; but +traces of it, as stated by Mr. Blyth and others, occasionally appear: and I +have been informed by Colonel Poole that the foals of this species are +generally striped on the legs, and faintly on the shoulder. The quagga, though +so plainly barred like a zebra over the body, is without bars on the legs; but +Dr. Gray has figured one specimen with very distinct zebra-like bars on the +hocks. +</p> + +<p> +With respect to the horse, I have collected cases in England of the spinal +stripe in horses of the most distinct breeds, and of <i>all</i> colours; +transverse bars on the legs are not rare in duns, mouse-duns, and in one +instance in a chestnut: a faint shoulder-stripe may sometimes be seen in duns, +and I have seen a trace in a +<a name="Page164"></a> +bay horse. My son made a careful examination and sketch for me of a dun Belgian +cart-horse with a double stripe on each shoulder and with leg-stripes; and a +man, whom I can implicitly trust, has examined for me a small dun Welch pony +with <i>three</i> short parallel stripes on each shoulder. +</p> + +<p> +In the north-west part of India the Kattywar breed of horses is so generally +striped, that, as I hear from Colonel Poole, who examined the breed for the +Indian Government, a horse without stripes is not considered as purely-bred. +The spine is always striped; the legs are generally barred; and the +shoulder-stripe, which is sometimes double and sometimes treble, is common; the +side of the face, moreover, is sometimes striped. The stripes are plainest in +the foal; and sometimes quite disappear in old horses. Colonel Poole has seen +both gray and bay Kattywar horses striped when first foaled. I have, also, +reason to suspect, from information given me by Mr. W. W. Edwards, that with +the English race-horse the spinal stripe is much commoner in the foal than in +the full-grown animal. Without here entering on further details, I may state +that I have collected cases of leg and shoulder stripes in horses of very +different breeds, in various countries from Britain to Eastern China; and from +Norway in the north to the Malay Archipelago in the south. In all parts of the +world these stripes occur far oftenest in duns and mouse-duns; by the term dun +a large range of colour is included, from one between brown and black to a +close approach to cream-colour. +</p> + +<p> +I am aware that Colonel Hamilton Smith, who has written on this subject, +believes that the several breeds of the horse have descended from several +aboriginal species—one of which, the dun, was striped; and that the +above-described appearances are all due to ancient +<a name="Page165"></a> +crosses with the dun stock. But I am not at all satisfied with this theory, and +should be loth to apply it to breeds so distinct as the heavy Belgian +cart-horse, Welch ponies, cobs, the lanky Kattywar race, etc., inhabiting the +most distant parts of the world. +</p> + +<p> +Now let us turn to the effects of crossing the several species of the +horse-genus. Rollin asserts, that the common mule from the ass and horse is +particularly apt to have bars on its legs. I once saw a mule with its legs so +much striped that any one at first would have thought that it must have been +the product of a zebra; and Mr. W. C. Martin, in his excellent treatise on the +horse, has given a figure of a similar mule. In four coloured drawings, which I +have seen, of hybrids between the ass and zebra, the legs were much more +plainly barred than the rest of the body; and in one of them there was a double +shoulder-stripe. In Lord Moreton’s famous hybrid from a chestnut mare and +male quagga, the hybrid, and even the pure offspring subsequently produced from +the mare by a black Arabian sire, were much more plainly barred across the legs +than is even the pure quagga. Lastly, and this is another most remarkable case, +a hybrid has been figured by Dr. Gray (and he informs me that he knows of a +second case) from the ass and the hemionus; and this hybrid, though the ass +seldom has stripes on its legs and the hemionus has none and has not even a +shoulder-stripe, nevertheless had all four legs barred, and had three short +shoulder-stripes, like those on the dun Welch pony, and even had some +zebra-like stripes on the sides of its face. With respect to this last fact, I +was so convinced that not even a stripe of colour appears from what would +commonly be called an accident, that I was led solely from the occurrence of +the face-stripes on this hybrid from the ass and hemionus, +<a name="Page166"></a> +to ask Colonel Poole whether such face-stripes ever occur in the eminently +striped Kattywar breed of horses, and was, as we have seen, answered in the +affirmative. +</p> + +<p> +What now are we to say to these several facts? We see several very distinct +species of the horse-genus becoming, by simple variation, striped on the legs +like a zebra, or striped on the shoulders like an ass. In the horse we see this +tendency strong whenever a dun tint appears—a tint which approaches to +that of the general colouring of the other species of the genus. The appearance +of the stripes is not accompanied by any change of form or by any other new +character. We see this tendency to become striped most strongly displayed in +hybrids from between several of the most distinct species. Now observe the case +of the several breeds of pigeons: they are descended from a pigeon (including +two or three sub-species or geographical races) of a bluish colour, with +certain bars and other marks; and when any breed assumes by simple variation a +bluish tint, these bars and other marks invariably reappear; but without any +other change of form or character. When the oldest and truest breeds of various +colours are crossed, we see a strong tendency for the blue tint and bars and +marks to reappear in the mongrels. I have stated that the most probable +hypothesis to account for the reappearance of very ancient characters, +is—that there is a <i>tendency</i> in the young of each successive +generation to produce the long-lost character, and that this tendency, from +unknown causes, sometimes prevails. And we have just seen that in several +species of the horse-genus the stripes are either plainer or appear more +commonly in the young than in the old. Call the breeds of pigeons, some of +which have bred true for centuries, species; and how exactly parallel is the +case with that of the species of the horse-genus! +<a name="Page167"></a> +For myself, I venture confidently to look back thousands on thousands of +generations, and I see an animal striped like a zebra, but perhaps otherwise +very differently constructed, the common parent of our domestic horse, whether +or not it be descended from one or more wild stocks, of the ass, the hemionus, +quagga, and zebra. +</p> + +<p> +He who believes that each equine species was independently created, will, I +presume, assert that each species has been created with a tendency to vary, +both under nature and under domestication, in this particular manner, so as +often to become striped like other species of the genus; and that each has been +created with a strong tendency, when crossed with species inhabiting distant +quarters of the world, to produce hybrids resembling in their stripes, not +their own parents, but other species of the genus. To admit this view is, as it +seems to me, to reject a real for an unreal, or at least for an unknown, cause. +It makes the works of God a mere mockery and deception; I would almost as soon +believe with the old and ignorant cosmogonists, that fossil shells had never +lived, but had been created in stone so as to mock the shells now living on the +sea-shore. +</p> + +<p class="p2"> +<i>Summary</i>.—Our ignorance of the laws of variation is profound. Not +in one case out of a hundred can we pretend to assign any reason why this or +that part differs, more or less, from the same part in the parents. But +whenever we have the means of instituting a comparison, the same laws appear to +have acted in producing the lesser differences between varieties of the same +species, and the greater differences between species of the same genus. The +external conditions of life, as climate and food, etc., seem to have induced +some slight modifications. Habit in producing constitutional differences, +<a name="Page168"></a> +and use in strengthening, and disuse in weakening and diminishing organs, seem +to have been more potent in their effects. Homologous parts tend to vary in the +same way, and homologous parts tend to cohere. Modifications in hard parts and +in external parts sometimes affect softer and internal parts. When one part is +largely developed, perhaps it tends to draw nourishment from the adjoining +parts; and every part of the structure which can be saved without detriment to +the individual, will be saved. Changes of structure at an early age will +generally affect parts subsequently developed; and there are very many other +correlations of growth, the nature of which we are utterly unable to +understand. Multiple parts are variable in number and in structure, perhaps +arising from such parts not having been closely specialised to any particular +function, so that their modifications have not been closely checked by natural +selection. It is probably from this same cause that organic beings low in the +scale of nature are more variable than those which have their whole +organisation more specialised, and are higher in the scale. Rudimentary organs, +from being useless, will be disregarded by natural selection, and hence +probably are variable. Specific characters—that is, the characters which +have come to differ since the several species of the same genus branched off +from a common parent—are more variable than generic characters, or those +which have long been inherited, and have not differed within this same period. +In these remarks we have referred to special parts or organs being still +variable, because they have recently varied and thus come to differ; but we +have also seen in the second Chapter that the same principle applies to the +whole individual; for in a district where many species of any genus are +found—that is, where there has been much former +<a name="Page169"></a> +variation and differentiation, or where the manufactory of new specific forms +has been actively at work—there, on an average, we now find most +varieties or incipient species. Secondary sexual characters are highly +variable, and such characters differ much in the species of the same group. +Variability in the same parts of the organisation has generally been taken +advantage of in giving secondary sexual differences to the sexes of the same +species, and specific differences to the several species of the same genus. Any +part or organ developed to an extraordinary size or in an extraordinary manner, +in comparison with the same part or organ in the allied species, must have gone +through an extraordinary amount of modification since the genus arose; and thus +we can understand why it should often still be variable in a much higher degree +than other parts; for variation is a long-continued and slow process, and +natural selection will in such cases not as yet have had time to overcome the +tendency to further variability and to reversion to a less modified state. But +when a species with any extraordinarily-developed organ has become the parent +of many modified descendants—which on my view must be a very slow +process, requiring a long lapse of time—in this case, natural selection +may readily have succeeded in giving a fixed character to the organ, in however +extraordinary a manner it may be developed. Species inheriting nearly the same +constitution from a common parent and exposed to similar influences will +naturally tend to present analogous variations, and these same species may +occasionally revert to some of the characters of their ancient progenitors. +Although new and important modifications may not arise from reversion and +analogous variation, such modifications will add to the beautiful and +harmonious diversity of nature. +</p> + +<p> +<a name="Page170"></a> +Whatever the cause may be of each slight difference in the offspring from their +parents—and a cause for each must exist—it is the steady +accumulation, through natural selection, of such differences, when beneficial +to the individual, that gives rise to all the more important modifications of +structure, by which the innumerable beings on the face of this earth are +enabled to struggle with each other, and the best adapted to survive. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page171"></a><a name="chap06"></a>CHAPTER VI.<br /> +DIFFICULTIES ON THEORY.</h2> + +<p class="letter"> +Difficulties on the theory of descent with modification. Transitions. Absence +or rarity of transitional varieties. Transitions in habits of life. Diversified +habits in the same species. Species with habits widely different from those of +their allies. Organs of extreme perfection. Means of transition. Cases of +difficulty. Natura non facit saltum. Organs of small importance. Organs not in +all cases absolutely perfect. The law of Unity of Type and of the Conditions of +Existence embraced by the theory of Natural Selection. +</p> + +<p> +Long before having arrived at this part of my work, a crowd of difficulties +will have occurred to the reader. Some of them are so grave that to this day I +can never reflect on them without being staggered; but, to the best of my +judgment, the greater number are only apparent, and those that are real are +not, I think, fatal to my theory. +</p> + +<p> +These difficulties and objections may be classed under the following +heads:— +</p> + +<p> +Firstly, why, if species have descended from other species by insensibly fine +gradations, do we not everywhere see innumerable transitional forms? Why is not +all nature in confusion instead of the species being, as we see them, well +defined? +</p> + +<p> +Secondly, is it possible that an animal having, for instance, the structure and +habits of a bat, could have been formed by the modification of some animal with +wholly different habits? Can we believe that natural selection could produce, +on the one hand, organs of trifling importance, such as the tail of a giraffe, +which serves as a fly-flapper, and, on the other hand, organs of +<a name="Page172"></a> +such wonderful structure, as the eye, of which we hardly as yet fully +understand the inimitable perfection? +</p> + +<p> +Thirdly, can instincts be acquired and modified through natural selection? What +shall we say to so marvellous an instinct as that which leads the bee to make +cells, which have practically anticipated the discoveries of profound +mathematicians? +</p> + +<p> +Fourthly, how can we account for species, when crossed, being sterile and +producing sterile offspring, whereas, when varieties are crossed, their +fertility is unimpaired? +</p> + +<p> +The two first heads shall be here discussed—Instinct and Hybridism in +separate chapters. +</p> + +<p class="p2"> +<i>On the absence or rarity of transitional varieties.</i>—As natural +selection acts solely by the preservation of profitable modifications, each new +form will tend in a fully-stocked country to take the place of, and finally to +exterminate, its own less improved parent or other less-favoured forms with +which it comes into competition. Thus extinction and natural selection will, as +we have seen, go hand in hand. Hence, if we look at each species as descended +from some other unknown form, both the parent and all the transitional +varieties will generally have been exterminated by the very process of +formation and perfection of the new form. +</p> + +<p> +But, as by this theory innumerable transitional forms must have existed, why do +we not find them embedded in countless numbers in the crust of the earth? It +will be much more convenient to discuss this question in the chapter on the +Imperfection of the geological record; and I will here only state that I +believe the answer mainly lies in the record being incomparably less perfect +than is generally supposed; the imperfection of the record being chiefly due to +organic beings not inhabiting +<a name="Page173"></a> +profound depths of the sea, and to their remains being embedded and preserved +to a future age only in masses of sediment sufficiently thick and extensive to +withstand an enormous amount of future degradation; and such fossiliferous +masses can be accumulated only where much sediment is deposited on the shallow +bed of the sea, whilst it slowly subsides. These contingencies will concur only +rarely, and after enormously long intervals. Whilst the bed of the sea is +stationary or is rising, or when very little sediment is being deposited, there +will be blanks in our geological history. The crust of the earth is a vast +museum; but the natural collections have been made only at intervals of time +immensely remote. +</p> + +<p> +But it may be urged that when several closely-allied species inhabit the same +territory we surely ought to find at the present time many transitional forms. +Let us take a simple case: in travelling from north to south over a continent, +we generally meet at successive intervals with closely allied or representative +species, evidently filling nearly the same place in the natural economy of the +land. These representative species often meet and interlock; and as the one +becomes rarer and rarer, the other becomes more and more frequent, till the one +replaces the other. But if we compare these species where they intermingle, +they are generally as absolutely distinct from each other in every detail of +structure as are specimens taken from the metropolis inhabited by each. By my +theory these allied species have descended from a common parent; and during the +process of modification, each has become adapted to the conditions of life of +its own region, and has supplanted and exterminated its original parent and all +the transitional varieties between its past and present states. Hence we ought +not to expect at the +<a name="Page174"></a> +present time to meet with numerous transitional varieties in each region, +though they must have existed there, and may be embedded there in a fossil +condition. But in the intermediate region, having intermediate conditions of +life, why do we not now find closely-linking intermediate varieties? This +difficulty for a long time quite confounded me. But I think it can be in large +part explained. +</p> + +<p> +In the first place we should be extremely cautious in inferring, because an +area is now continuous, that it has been continuous during a long period. +Geology would lead us to believe that almost every continent has been broken up +into islands even during the later tertiary periods; and in such islands +distinct species might have been separately formed without the possibility of +intermediate varieties existing in the intermediate zones. By changes in the +form of the land and of climate, marine areas now continuous must often have +existed within recent times in a far less continuous and uniform condition than +at present. But I will pass over this way of escaping from the difficulty; for +I believe that many perfectly defined species have been formed on strictly +continuous areas; though I do not doubt that the formerly broken condition of +areas now continuous has played an important part in the formation of new +species, more especially with freely-crossing and wandering animals. +</p> + +<p> +In looking at species as they are now distributed over a wide area, we +generally find them tolerably numerous over a large territory, then becoming +somewhat abruptly rarer and rarer on the confines, and finally disappearing. +Hence the neutral territory between two representative species is generally +narrow in comparison with the territory proper to each. We see the same fact in +ascending mountains, and sometimes +<a name="Page175"></a> +it is quite remarkable how abruptly, as Alph. De Candolle has observed, a +common alpine species disappears. The same fact has been noticed by Forbes in +sounding the depths of the sea with the dredge. To those who look at climate +and the physical conditions of life as the all-important elements of +distribution, these facts ought to cause surprise, as climate and height or +depth graduate away insensibly. But when we bear in mind that almost every +species, even in its metropolis, would increase immensely in numbers, were it +not for other competing species; that nearly all either prey on or serve as +prey for others; in short, that each organic being is either directly or +indirectly related in the most important manner to other organic beings, we +must see that the range of the inhabitants of any country by no means +exclusively depends on insensibly changing physical conditions, but in large +part on the presence of other species, on which it depends, or by which it is +destroyed, or with which it comes into competition; and as these species are +already defined objects (however they may have become so), not blending one +into another by insensible gradations, the range of any one species, depending +as it does on the range of others, will tend to be sharply defined. Moreover, +each species on the confines of its range, where it exists in lessened numbers, +will, during fluctuations in the number of its enemies or of its prey, or in +the seasons, be extremely liable to utter extermination; and thus its +geographical range will come to be still more sharply defined. +</p> + +<p> +If I am right in believing that allied or representative species, when +inhabiting a continuous area, are generally so distributed that each has a wide +range, with a comparatively narrow neutral territory between them, in which +they become rather suddenly rarer and rarer; then, as varieties do not +essentially differ from species, +<a name="Page176"></a> +the same rule will probably apply to both; and if we in imagination adapt a +varying species to a very large area, we shall have to adapt two varieties to +two large areas, and a third variety to a narrow intermediate zone. The +intermediate variety, consequently, will exist in lesser numbers from +inhabiting a narrow and lesser area; and practically, as far as I can make out, +this rule holds good with varieties in a state of nature. I have met with +striking instances of the rule in the case of varieties intermediate between +well-marked varieties in the genus Balanus. And it would appear from +information given me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, that +generally when varieties intermediate between two other forms occur, they are +much rarer numerically than the forms which they connect. Now, if we may trust +these facts and inferences, and therefore conclude that varieties linking two +other varieties together have generally existed in lesser numbers than the +forms which they connect, then, I think, we can understand why intermediate +varieties should not endure for very long periods;—why as a general rule +they should be exterminated and disappear, sooner than the forms which they +originally linked together. +</p> + +<p> +For any form existing in lesser numbers would, as already remarked, run a +greater chance of being exterminated than one existing in large numbers; and in +this particular case the intermediate form would be eminently liable to the +inroads of closely allied forms existing on both sides of it. But a far more +important consideration, as I believe, is that, during the process of further +modification, by which two varieties are supposed on my theory to be converted +and perfected into two distinct species, the two which exist in larger numbers +from inhabiting larger areas, will have a great advantage over the intermediate +variety, which exists +<a name="Page177"></a> +in smaller numbers in a narrow and intermediate zone. For forms existing in +larger numbers will always have a better chance, within any given period, of +presenting further favourable variations for natural selection to seize on, +than will the rarer forms which exist in lesser numbers. Hence, the more common +forms, in the race for life, will tend to beat and supplant the less common +forms, for these will be more slowly modified and improved. It is the same +principle which, as I believe, accounts for the common species in each country, +as shown in the second chapter, presenting on an average a greater number of +well-marked varieties than do the rarer species. I may illustrate what I mean +by supposing three varieties of sheep to be kept, one adapted to an extensive +mountainous region; a second to a comparatively narrow, hilly tract; and a +third to wide plains at the base; and that the inhabitants are all trying with +equal steadiness and skill to improve their stocks by selection; the chances in +this case will be strongly in favour of the great holders on the mountains or +on the plains improving their breeds more quickly than the small holders on the +intermediate narrow, hilly tract; and consequently the improved mountain or +plain breed will soon take the place of the less improved hill breed; and thus +the two breeds, which originally existed in greater numbers, will come into +close contact with each other, without the interposition of the supplanted, +intermediate hill-variety. +</p> + +<p> +To sum up, I believe that species come to be tolerably well-defined objects, +and do not at any one period present an inextricable chaos of varying and +intermediate links: firstly, because new varieties are very slowly formed, for +variation is a very slow process, and natural selection can do nothing until +favourable variations chance to occur, and until a place in the natural polity +<a name="Page178"></a> +of the country can be better filled by some modification of some one or more of +its inhabitants. And such new places will depend on slow changes of climate, or +on the occasional immigration of new inhabitants, and, probably, in a still +more important degree, on some of the old inhabitants becoming slowly modified, +with the new forms thus produced and the old ones acting and reacting on each +other. So that, in any one region and at any one time, we ought only to see a +few species presenting slight modifications of structure in some degree +permanent; and this assuredly we do see. +</p> + +<p> +Secondly, areas now continuous must often have existed within the recent period +in isolated portions, in which many forms, more especially amongst the classes +which unite for each birth and wander much, may have separately been rendered +sufficiently distinct to rank as representative species. In this case, +intermediate varieties between the several representative species and their +common parent, must formerly have existed in each broken portion of the land, +but these links will have been supplanted and exterminated during the process +of natural selection, so that they will no longer exist in a living state. +</p> + +<p> +Thirdly, when two or more varieties have been formed in different portions of a +strictly continuous area, intermediate varieties will, it is probable, at first +have been formed in the intermediate zones, but they will generally have had a +short duration. For these intermediate varieties will, from reasons already +assigned (namely from what we know of the actual distribution of closely allied +or representative species, and likewise of acknowledged varieties), exist in +the intermediate zones in lesser numbers than the varieties which they tend to +connect. From this cause alone the intermediate +<a name="Page179"></a> +varieties will be liable to accidental extermination; and during the process of +further modification through natural selection, they will almost certainly be +beaten and supplanted by the forms which they connect; for these from existing +in greater numbers will, in the aggregate, present more variation, and thus be +further improved through natural selection and gain further advantages. +</p> + +<p> +Lastly, looking not to any one time, but to all time, if my theory be true, +numberless intermediate varieties, linking most closely all the species of the +same group together, must assuredly have existed; but the very process of +natural selection constantly tends, as has been so often remarked, to +exterminate the parent forms and the intermediate links. Consequently evidence +of their former existence could be found only amongst fossil remains, which are +preserved, as we shall in a future chapter attempt to show, in an extremely +imperfect and intermittent record. +</p> + +<p class="p2"> +<i>On the origin and transitions of organic beings with peculiar habits and +structure</i>.—It has been asked by the opponents of such views as I +hold, how, for instance, a land carnivorous animal could have been converted +into one with aquatic habits; for how could the animal in its transitional +state have subsisted? It would be easy to show that within the same group +carnivorous animals exist having every intermediate grade between truly aquatic +and strictly terrestrial habits; and as each exists by a struggle for life, it +is clear that each is well adapted in its habits to its place in nature. Look +at the Mustela vison of North America, which has webbed feet and which +resembles an otter in its fur, short legs, and form of tail; during summer this +animal dives for and preys on fish, but during the long winter +<a name="Page180"></a> +it leaves the frozen waters, and preys like other polecats on mice and land +animals. If a different case had been taken, and it had been asked how an +insectivorous quadruped could possibly have been converted into a flying bat, +the question would have been far more difficult, and I could have given no +answer. Yet I think such difficulties have very little weight. +</p> + +<p> +Here, as on other occasions, I lie under a heavy disadvantage, for out of the +many striking cases which I have collected, I can give only one or two +instances of transitional habits and structures in closely allied species of +the same genus; and of diversified habits, either constant or occasional, in +the same species. And it seems to me that nothing less than a long list of such +cases is sufficient to lessen the difficulty in any particular case like that +of the bat. +</p> + +<p> +Look at the family of squirrels; here we have the finest gradation from animals +with their tails only slightly flattened, and from others, as Sir J. Richardson +has remarked, with the posterior part of their bodies rather wide and with the +skin on their flanks rather full, to the so-called flying squirrels; and flying +squirrels have their limbs and even the base of the tail united by a broad +expanse of skin, which serves as a parachute and allows them to glide through +the air to an astonishing distance from tree to tree. We cannot doubt that each +structure is of use to each kind of squirrel in its own country, by enabling it +to escape birds or beasts of prey, or to collect food more quickly, or, as +there is reason to believe, by lessening the danger from occasional falls. But +it does not follow from this fact that the structure of each squirrel is the +best that it is possible to conceive under all natural conditions. Let the +climate and vegetation change, let other competing rodents or new beasts of +prey immigrate, or old ones +<a name="Page181"></a> +become modified, and all analogy would lead us to believe that some at least of +the squirrels would decrease in numbers or become exterminated, unless they +also became modified and improved in structure in a corresponding manner. +Therefore, I can see no difficulty, more especially under changing conditions +of life, in the continued preservation of individuals with fuller and fuller +flank-membranes, each modification being useful, each being propagated, until +by the accumulated effects of this process of natural selection, a perfect +so-called flying squirrel was produced. +</p> + +<p> +Now look at the Galeopithecus or flying lemur, which formerly was falsely +ranked amongst bats. It has an extremely wide flank-membrane, stretching from +the corners of the jaw to the tail, and including the limbs and the elongated +fingers: the flank membrane is, also, furnished with an extensor muscle. +Although no graduated links of structure, fitted for gliding through the air, +now connect the Galeopithecus with the other Lemuridæ, yet I can see no +difficulty in supposing that such links formerly existed, and that each had +been formed by the same steps as in the case of the less perfectly gliding +squirrels; and that each grade of structure had been useful to its possessor. +Nor can I see any insuperable difficulty in further believing it possible that +the membrane-connected fingers and fore-arm of the Galeopithecus might be +greatly lengthened by natural selection; and this, as far as the organs of +flight are concerned, would convert it into a bat. In bats which have the +wing-membrane extended from the top of the shoulder to the tail, including the +hind-legs, we perhaps see traces of an apparatus originally constructed for +gliding through the air rather than for flight. +</p> + +<p> +If about a dozen genera of birds had become extinct or were unknown, who would +have ventured to have +<a name="Page182"></a> +surmised that birds might have existed which used their wings solely as +flappers, like the logger-headed duck (Micropterus of Eyton); as fins in the +water and front legs on the land, like the penguin; as sails, like the ostrich; +and functionally for no purpose, like the Apteryx. Yet the structure of each of +these birds is good for it, under the conditions of life to which it is +exposed, for each has to live by a struggle; but it is not necessarily the best +possible under all possible conditions. It must not be inferred from these +remarks that any of the grades of wing-structure here alluded to, which perhaps +may all have resulted from disuse, indicate the natural steps by which birds +have acquired their perfect power of flight; but they serve, at least, to show +what diversified means of transition are possible. +</p> + +<p> +Seeing that a few members of such water-breathing classes as the Crustacea and +Mollusca are adapted to live on the land, and seeing that we have flying birds +and mammals, flying insects of the most diversified types, and formerly had +flying reptiles, it is conceivable that flying-fish, which now glide far +through the air, slightly rising and turning by the aid of their fluttering +fins, might have been modified into perfectly winged animals. If this had been +effected, who would have ever imagined that in an early transitional state they +had been inhabitants of the open ocean, and had used their incipient organs of +flight exclusively, as far as we know, to escape being devoured by other fish? +</p> + +<p> +When we see any structure highly perfected for any particular habit, as the +wings of a bird for flight, we should bear in mind that animals displaying +early transitional grades of the structure will seldom continue to exist to the +present day, for they will have been supplanted by the very process of +perfection through natural selection. Furthermore, we may conclude that +transitional +<a name="Page183"></a> +grades between structures fitted for very different habits of life will rarely +have been developed at an early period in great numbers and under many +subordinate forms. Thus, to return to our imaginary illustration of the +flying-fish, it does not seem probable that fishes capable of true flight would +have been developed under many subordinate forms, for taking prey of many kinds +in many ways, on the land and in the water, until their organs of flight had +come to a high stage of perfection, so as to have given them a decided +advantage over other animals in the battle for life. Hence the chance of +discovering species with transitional grades of structure in a fossil condition +will always be less, from their having existed in lesser numbers, than in the +case of species with fully developed structures. +</p> + +<p> +I will now give two or three instances of diversified and of changed habits in +the individuals of the same species. When either case occurs, it would be easy +for natural selection to fit the animal, by some modification of its structure, +for its changed habits, or exclusively for one of its several different habits. +But it is difficult to tell, and immaterial for us, whether habits generally +change first and structure afterwards; or whether slight modifications of +structure lead to changed habits; both probably often change almost +simultaneously. Of cases of changed habits it will suffice merely to allude to +that of the many British insects which now feed on exotic plants, or +exclusively on artificial substances. Of diversified habits innumerable +instances could be given: I have often watched a tyrant flycatcher (Saurophagus +sulphuratus) in South America, hovering over one spot and then proceeding to +another, like a kestrel, and at other times standing stationary on the margin +of water, and then dashing like a kingfisher at a fish. In our own country the +larger titmouse (Parus major) may be +<a name="Page184"></a> +seen climbing branches, almost like a creeper; it often, like a shrike, kills +small birds by blows on the head; and I have many times seen and heard it +hammering the seeds of the yew on a branch, and thus breaking them like a +nuthatch. In North America the black bear was seen by Hearne swimming for hours +with widely open mouth, thus catching, like a whale, insects in the water. Even +in so extreme a case as this, if the supply of insects were constant, and if +better adapted competitors did not already exist in the country, I can see no +difficulty in a race of bears being rendered, by natural selection, more and +more aquatic in their structure and habits, with larger and larger mouths, till +a creature was produced as monstrous as a whale. +</p> + +<p> +As we sometimes see individuals of a species following habits widely different +from those both of their own species and of the other species of the same +genus, we might expect, on my theory, that such individuals would occasionally +have given rise to new species, having anomalous habits, and with their +structure either slightly or considerably modified from that of their proper +type. And such instances do occur in nature. Can a more striking instance of +adaptation be given than that of a woodpecker for climbing trees and for +seizing insects in the chinks of the bark? Yet in North America there are +woodpeckers which feed largely on fruit, and others with elongated wings which +chase insects on the wing; and on the plains of La Plata, where not a tree +grows, there is a woodpecker, which in every essential part of its +organisation, even in its colouring, in the harsh tone of its voice, and +undulatory flight, told me plainly of its close blood-relationship to our +common species; yet it is a woodpecker which never climbs a tree! +</p> + +<p> +Petrels are the most aërial and oceanic of birds, yet in the quiet Sounds of +Tierra del Fuego, the Puffinuria +<a name="Page185"></a> +berardi, in its general habits, in its astonishing power of diving, its manner +of swimming, and of flying when unwillingly it takes flight, would be mistaken +by any one for an auk or grebe; nevertheless, it is essentially a petrel, but +with many parts of its organisation profoundly modified. On the other hand, the +acutest observer by examining the dead body of the water-ouzel would never have +suspected its sub-aquatic habits; yet this anomalous member of the strictly +terrestrial thrush family wholly subsists by diving,—grasping the stones +with its feet and using its wings under water. +</p> + +<p> +He who believes that each being has been created as we now see it, must +occasionally have felt surprise when he has met with an animal having habits +and structure not at all in agreement. What can be plainer than that the webbed +feet of ducks and geese are formed for swimming? yet there are upland geese +with webbed feet which rarely or never go near the water; and no one except +Audubon has seen the frigate-bird, which has all its four toes webbed, alight +on the surface of the sea. On the other hand, grebes and coots are eminently +aquatic, although their toes are only bordered by membrane. What seems plainer +than that the long toes of grallatores are formed for walking over swamps and +floating plants, yet the water-hen is nearly as aquatic as the coot; and the +landrail nearly as terrestrial as the quail or partridge. In such cases, and +many others could be given, habits have changed without a corresponding change +of structure. The webbed feet of the upland goose may be said to have become +rudimentary in function, though not in structure. In the frigate-bird, the +deeply-scooped membrane between the toes shows that structure has begun to +change. +</p> + +<p> +He who believes in separate and innumerable acts of creation will say, that in +these cases it has pleased the +<a name="Page186"></a> +Creator to cause a being of one type to take the place of one of another type; +but this seems to me only restating the fact in dignified language. He who +believes in the struggle for existence and in the principle of natural +selection, will acknowledge that every organic being is constantly endeavouring +to increase in numbers; and that if any one being vary ever so little, either +in habits or structure, and thus gain an advantage over some other inhabitant +of the country, it will seize on the place of that inhabitant, however +different it may be from its own place. Hence it will cause him no surprise +that there should be geese and frigate-birds with webbed feet, either living on +the dry land or most rarely alighting on the water; that there should be +long-toed corncrakes living in meadows instead of in swamps; that there should +be woodpeckers where not a tree grows; that there should be diving thrushes, +and petrels with the habits of auks. +</p> + +<p class="p2"> +<i>Organs of extreme perfection and complication</i>.—To suppose that the +eye, with all its inimitable contrivances for adjusting the focus to different +distances, for admitting different amounts of light, and for the correction of +spherical and chromatic aberration, could have been formed by natural +selection, seems, I freely confess, absurd in the highest possible degree. Yet +reason tells me, that if numerous gradations from a perfect and complex eye to +one very imperfect and simple, each grade being useful to its possessor, can be +shown to exist; if further, the eye does vary ever so slightly, and the +variations be inherited, which is certainly the case; and if any variation or +modification in the organ be ever useful to an animal under changing conditions +of life, then the difficulty of believing that a perfect and complex eye could +be formed by natural +<a name="Page187"></a> +selection, though insuperable by our imagination, can hardly be considered +real. How a nerve comes to be sensitive to light, hardly concerns us more than +how life itself first originated; but I may remark that several facts make me +suspect that any sensitive nerve may be rendered sensitive to light, and +likewise to those coarser vibrations of the air which produce sound. +</p> + +<p> +In looking for the gradations by which an organ in any species has been +perfected, we ought to look exclusively to its lineal ancestors; but this is +scarcely ever possible, and we are forced in each case to look to species of +the same group, that is to the collateral descendants from the same original +parent-form, in order to see what gradations are possible, and for the chance +of some gradations having been transmitted from the earlier stages of descent, +in an unaltered or little altered condition. Amongst existing Vertebrata, we +find but a small amount of gradation in the structure of the eye, and from +fossil species we can learn nothing on this head. In this great class we should +probably have to descend far beneath the lowest known fossiliferous stratum to +discover the earlier stages, by which the eye has been perfected. +</p> + +<p> +In the Articulata we can commence a series with an optic nerve merely coated +with pigment, and without any other mechanism; and from this low stage, +numerous gradations of structure, branching off in two fundamentally different +lines, can be shown to exist, until we reach a moderately high stage of +perfection. In certain crustaceans, for instance, there is a double cornea, the +inner one divided into facets, within each of which there is a lens-shaped +swelling. In other crustaceans the transparent cones which are coated by +pigment, and which properly act only by excluding lateral pencils of light, are +convex at their upper ends +<a name="Page188"></a> +and must act by convergence; and at their lower ends there seems to be an +imperfect vitreous substance. With these facts, here far too briefly and +imperfectly given, which show that there is much graduated diversity in the +eyes of living crustaceans, and bearing in mind how small the number of living +animals is in proportion to those which have become extinct, I can see no very +great difficulty (not more than in the case of many other structures) in +believing that natural selection has converted the simple apparatus of an optic +nerve merely coated with pigment and invested by transparent membrane, into an +optical instrument as perfect as is possessed by any member of the great +Articulate class. +</p> + +<p> +He who will go thus far, if he find on finishing this treatise that large +bodies of facts, otherwise inexplicable, can be explained by the theory of +descent, ought not to hesitate to go further, and to admit that a structure +even as perfect as the eye of an eagle might be formed by natural selection, +although in this case he does not know any of the transitional grades. His +reason ought to conquer his imagination; though I have felt the difficulty far +too keenly to be surprised at any degree of hesitation in extending the +principle of natural selection to such startling lengths. +</p> + +<p> +It is scarcely possible to avoid comparing the eye to a telescope. We know that +this instrument has been perfected by the long-continued efforts of the highest +human intellects; and we naturally infer that the eye has been formed by a +somewhat analogous process. But may not this inference be presumptuous? Have we +any right to assume that the Creator works by intellectual powers like those of +man? If we must compare the eye to an optical instrument, we ought in +imagination to take a thick layer of transparent tissue, with a nerve sensitive +to light beneath, and then suppose every +<a name="Page189"></a> +part of this layer to be continually changing slowly in density, so as to +separate into layers of different densities and thicknesses, placed at +different distances from each other, and with the surfaces of each layer slowly +changing in form. Further we must suppose that there is a power always intently +watching each slight accidental alteration in the transparent layers; and +carefully selecting each alteration which, under varied circumstances, may in +any way, or in any degree, tend to produce a distincter image. We must suppose +each new state of the instrument to be multiplied by the million; and each to +be preserved till a better be produced, and then the old ones to be destroyed. +In living bodies, variation will cause the slight alterations, generation will +multiply them almost infinitely, and natural selection will pick out with +unerring skill each improvement. Let this process go on for millions on +millions of years; and during each year on millions of individuals of many +kinds; and may we not believe that a living optical instrument might thus be +formed as superior to one of glass, as the works of the Creator are to those of +man? +</p> + +<p> +If it could be demonstrated that any complex organ existed, which could not +possibly have been formed by numerous, successive, slight modifications, my +theory would absolutely break down. But I can find out no such case. No doubt +many organs exist of which we do not know the transitional grades, more +especially if we look to much-isolated species, round which, according to my +theory, there has been much extinction. Or again, if we look to an organ common +to all the members of a large class, for in this latter case the organ must +have been first formed at an extremely remote period, since which all the many +members of the class have been developed; and in order to discover the early +transitional grades through which the organ has +<a name="Page190"></a> +passed, we should have to look to very ancient ancestral forms, long since +become extinct. +</p> + +<p> +We should be extremely cautious in concluding that an organ could not have been +formed by transitional gradations of some kind. Numerous cases could be given +amongst the lower animals of the same organ performing at the same time wholly +distinct functions; thus the alimentary canal respires, digests, and excretes +in the larva of the dragon-fly and in the fish Cobites. In the Hydra, the +animal may be turned inside out, and the exterior surface will then digest and +the stomach respire. In such cases natural selection might easily specialise, +if any advantage were thus gained, a part or organ, which had performed two +functions, for one function alone, and thus wholly change its nature by +insensible steps. Two distinct organs sometimes perform simultaneously the same +function in the same individual; to give one instance, there are fish with +gills or branchiæ that breathe the air dissolved in the water, at the same +time that they breathe free air in their swimbladders, this latter organ having +a ductus pneumaticus for its supply, and being divided by highly vascular +partitions. In these cases, one of the two organs might with ease be modified +and perfected so as to perform all the work by itself, being aided during the +process of modification by the other organ; and then this other organ might be +modified for some other and quite distinct purpose, or be quite obliterated. +</p> + +<p> +The illustration of the swimbladder in fishes is a good one, because it shows +us clearly the highly important fact that an organ originally constructed for +one purpose, namely flotation, may be converted into one for a wholly different +purpose, namely respiration. The swimbladder has, also, been worked in as an +accessory to the auditory organs of certain fish, or, for I do not know which +<a name="Page191"></a> +view is now generally held, a part of the auditory apparatus has been worked in +as a complement to the swimbladder. All physiologists admit that the +swimbladder is homologous, or “ideally similar,” in position and +structure with the lungs of the higher vertebrate animals: hence there seems to +me to be no great difficulty in believing that natural selection has actually +converted a swimbladder into a lung, or organ used exclusively for respiration. +</p> + +<p> +I can, indeed, hardly doubt that all vertebrate animals having true lungs have +descended by ordinary generation from an ancient prototype, of which we know +nothing, furnished with a floating apparatus or swimbladder. We can thus, as I +infer from Professor Owen’s interesting description of these parts, +understand the strange fact that every particle of food and drink which we +swallow has to pass over the orifice of the trachea, with some risk of falling +into the lungs, notwithstanding the beautiful contrivance by which the glottis +is closed. In the higher Vertebrata the branchiæ have wholly +disappeared—the slits on the sides of the neck and the loop-like course +of the arteries still marking in the embryo their former position. But it is +conceivable that the now utterly lost branchiæ might have been gradually worked +in by natural selection for some quite distinct purpose: in the same manner as, +on the view entertained by some naturalists that the branchiæ and dorsal scales +of Annelids are homologous with the wings and wing-covers of insects, it is +probable that organs which at a very ancient period served for respiration have +been actually converted into organs of flight. +</p> + +<p> +In considering transitions of organs, it is so important to bear in mind the +probability of conversion from one function to another, that I will give one +more instance. Pedunculated cirripedes have two minute folds of skin, +<a name="Page192"></a> +called by me the ovigerous frena, which serve, through the means of a sticky +secretion, to retain the eggs until they are hatched within the sack. These +cirripedes have no branchiæ, the whole surface of the body and sack, including +the small frena, serving for respiration. The Balanidæ or sessile cirripedes, +on the other hand, have no ovigerous frena, the eggs lying loose at the bottom +of the sack, in the well-enclosed shell; but they have large folded branchiæ. +Now I think no one will dispute that the ovigerous frena in the one family are +strictly homologous with the branchiæ of the other family; indeed, they +graduate into each other. Therefore I do not doubt that little folds of skin, +which originally served as ovigerous frena, but which, likewise, very slightly +aided the act of respiration, have been gradually converted by natural +selection into branchiæ, simply through an increase in their size and the +obliteration of their adhesive glands. If all pedunculated cirripedes had +become extinct, and they have already suffered far more extinction than have +sessile cirripedes, who would ever have imagined that the branchiæ in this +latter family had originally existed as organs for preventing the ova from +being washed out of the sack? +</p> + +<p> +Although we must be extremely cautious in concluding that any organ could not +possibly have been produced by successive transitional gradations, yet, +undoubtedly, grave cases of difficulty occur, some of which will be discussed +in my future work. +</p> + +<p> +One of the gravest is that of neuter insects, which are often very differently +constructed from either the males or fertile females; but this case will be +treated of in the next chapter. The electric organs of fishes offer another +case of special difficulty; it is impossible to conceive by what steps these +wondrous organs have been produced; but, as Owen and others have remarked, +<a name="Page193"></a> +their intimate structure closely resembles that of common muscle; and as it has +lately been shown that Rays have an organ closely analogous to the electric +apparatus, and yet do not, as Matteuchi asserts, discharge any electricity, we +must own that we are far too ignorant to argue that no transition of any kind +is possible. +</p> + +<p> +The electric organs offer another and even more serious difficulty; for they +occur in only about a dozen fishes, of which several are widely remote in their +affinities. Generally when the same organ appears in several members of the +same class, especially if in members having very different habits of life, we +may attribute its presence to inheritance from a common ancestor; and its +absence in some of the members to its loss through disuse or natural selection. +But if the electric organs had been inherited from one ancient progenitor thus +provided, we might have expected that all electric fishes would have been +specially related to each other. Nor does geology at all lead to the belief +that formerly most fishes had electric organs, which most of their modified +descendants have lost. The presence of luminous organs in a few insects, +belonging to different families and orders, offers a parallel case of +difficulty. Other cases could be given; for instance in plants, the very +curious contrivance of a mass of pollen-grains, borne on a foot-stalk with a +sticky gland at the end, is the same in Orchis and Asclepias,—genera +almost as remote as possible amongst flowering plants. In all these cases of +two very distinct species furnished with apparently the same anomalous organ, +it should be observed that, although the general appearance and function of the +organ may be the same, yet some fundamental difference can generally be +detected. I am inclined to believe that in nearly the same way as two men have +sometimes independently hit on +<a name="Page194"></a> +the very same invention, so natural selection, working for the good of each +being and taking advantage of analogous variations, has sometimes modified in +very nearly the same manner two parts in two organic beings, which owe but +little of their structure in common to inheritance from the same ancestor. +</p> + +<p> +Although in many cases it is most difficult to conjecture by what transitions +an organ could have arrived at its present state; yet, considering that the +proportion of living and known forms to the extinct and unknown is very small, +I have been astonished how rarely an organ can be named, towards which no +transitional grade is known to lead. The truth of this remark is indeed shown +by that old canon in natural history of “Natura non facit saltum.” +We meet with this admission in the writings of almost every experienced +naturalist; or, as Milne Edwards has well expressed it, nature is prodigal in +variety, but niggard in innovation. Why, on the theory of Creation, should this +be so? Why should all the parts and organs of many independent beings, each +supposed to have been separately created for its proper place in nature, be so +invariably linked together by graduated steps? Why should not Nature have taken +a leap from structure to structure? On the theory of natural selection, we can +clearly understand why she should not; for natural selection can act only by +taking advantage of slight successive variations; she can never take a leap, +but must advance by the shortest and slowest steps. +</p> + +<p class="p2"> +<i>Organs of little apparent importance</i>.—As natural selection acts by +life and death,—by the preservation of individuals with any favourable +variation, and by the destruction of those with any unfavourable deviation of +structure,—I have sometimes felt much difficulty in +<a name="Page195"></a> +understanding the origin of simple parts, of which the importance does not seem +sufficient to cause the preservation of successively varying individuals. I +have sometimes felt as much difficulty, though of a very different kind, on +this head, as in the case of an organ as perfect and complex as the eye. +</p> + +<p> +In the first place, we are much too ignorant in regard to the whole economy of +any one organic being, to say what slight modifications would be of importance +or not. In a former chapter I have given instances of most trifling characters, +such as the down on fruit and the colour of the flesh, which, from determining +the attacks of insects or from being correlated with constitutional +differences, might assuredly be acted on by natural selection. The tail of the +giraffe looks like an artificially constructed fly-flapper; and it seems at +first incredible that this could have been adapted for its present purpose by +successive slight modifications, each better and better, for so trifling an +object as driving away flies; yet we should pause before being too positive +even in this case, for we know that the distribution and existence of cattle +and other animals in South America absolutely depends on their power of +resisting the attacks of insects: so that individuals which could by any means +defend themselves from these small enemies, would be able to range into new +pastures and thus gain a great advantage. It is not that the larger quadrupeds +are actually destroyed (except in some rare cases) by the flies, but they are +incessantly harassed and their strength reduced, so that they are more subject +to disease, or not so well enabled in a coming dearth to search for food, or to +escape from beasts of prey. +</p> + +<p> +Organs now of trifling importance have probably in some cases been of high +importance to an early progenitor, and, after having been slowly perfected at a +<a name="Page196"></a> +former period, have been transmitted in nearly the same state, although now +become of very slight use; and any actually injurious deviations in their +structure will always have been checked by natural selection. Seeing how +important an organ of locomotion the tail is in most aquatic animals, its +general presence and use for many purposes in so many land animals, which in +their lungs or modified swim-bladders betray their aquatic origin, may perhaps +be thus accounted for. A well-developed tail having been formed in an aquatic +animal, it might subsequently come to be worked in for all sorts of purposes, +as a fly-flapper, an organ of prehension, or as an aid in turning, as with the +dog, though the aid must be slight, for the hare, with hardly any tail, can +double quickly enough. +</p> + +<p> +In the second place, we may sometimes attribute importance to characters which +are really of very little importance, and which have originated from quite +secondary causes, independently of natural selection. We should remember that +climate, food, etc., probably have some little direct influence on the +organisation; that characters reappear from the law of reversion; that +correlation of growth will have had a most important influence in modifying +various structures; and finally, that sexual selection will often have largely +modified the external characters of animals having a will, to give one male an +advantage in fighting with another or in charming the females. Moreover when a +modification of structure has primarily arisen from the above or other unknown +causes, it may at first have been of no advantage to the species, but may +subsequently have been taken advantage of by the descendants of the species +under new conditions of life and with newly acquired habits. +</p> + +<p> +To give a few instances to illustrate these latter +<a name="Page197"></a> +remarks. If green woodpeckers alone had existed, and we did not know that there +were many black and pied kinds, I dare say that we should have thought that the +green colour was a beautiful adaptation to hide this tree-frequenting bird from +its enemies; and consequently that it was a character of importance and might +have been acquired through natural selection; as it is, I have no doubt that +the colour is due to some quite distinct cause, probably to sexual selection. A +trailing bamboo in the Malay Archipelago climbs the loftiest trees by the aid +of exquisitely constructed hooks clustered around the ends of the branches, and +this contrivance, no doubt, is of the highest service to the plant; but as we +see nearly similar hooks on many trees which are not climbers, the hooks on the +bamboo may have arisen from unknown laws of growth, and have been subsequently +taken advantage of by the plant undergoing further modification and becoming a +climber. The naked skin on the head of a vulture is generally looked at as a +direct adaptation for wallowing in putridity; and so it may be, or it may +possibly be due to the direct action of putrid matter; but we should be very +cautious in drawing any such inference, when we see that the skin on the head +of the clean-feeding male turkey is likewise naked. The sutures in the skulls +of young mammals have been advanced as a beautiful adaptation for aiding +parturition, and no doubt they facilitate, or may be indispensable for this +act; but as sutures occur in the skulls of young birds and reptiles, which have +only to escape from a broken egg, we may infer that this structure has arisen +from the laws of growth, and has been taken advantage of in the parturition of +the higher animals. +</p> + +<p> +We are profoundly ignorant of the causes producing slight and unimportant +variations; and we are immediately +<a name="Page198"></a> +made conscious of this by reflecting on the differences in the breeds of our +domesticated animals in different countries,—more especially in the less +civilized countries where there has been but little artificial selection. +Careful observers are convinced that a damp climate affects the growth of the +hair, and that with the hair the horns are correlated. Mountain breeds always +differ from lowland breeds; and a mountainous country would probably affect the +hind limbs from exercising them more, and possibly even the form of the pelvis; +and then by the law of homologous variation, the front limbs and even the head +would probably be affected. The shape, also, of the pelvis might affect by +pressure the shape of the head of the young in the womb. The laborious +breathing necessary in high regions would, we have some reason to believe, +increase the size of the chest; and again correlation would come into play. +Animals kept by savages in different countries often have to struggle for their +own subsistence, and would be exposed to a certain extent to natural selection, +and individuals with slightly different constitutions would succeed best under +different climates; and there is reason to believe that constitution and colour +are correlated. A good observer, also, states that in cattle susceptibility to +the attacks of flies is correlated with colour, as is the liability to be +poisoned by certain plants; so that colour would be thus subjected to the +action of natural selection. But we are far too ignorant to speculate on the +relative importance of the several known and unknown laws of variation; and I +have here alluded to them only to show that, if we are unable to account for +the characteristic differences of our domestic breeds, which nevertheless we +generally admit to have arisen through ordinary generation, we ought not to lay +too much stress on our +<a name="Page199"></a> +ignorance of the precise cause of the slight analogous differences between +species. I might have adduced for this same purpose the differences between the +races of man, which are so strongly marked; I may add that some little light +can apparently be thrown on the origin of these differences, chiefly through +sexual selection of a particular kind, but without here entering on copious +details my reasoning would appear frivolous. +</p> + +<p> +The foregoing remarks lead me to say a few words on the protest lately made by +some naturalists, against the utilitarian doctrine that every detail of +structure has been produced for the good of its possessor. They believe that +very many structures have been created for beauty in the eyes of man, or for +mere variety. This doctrine, if true, would be absolutely fatal to my theory. +Yet I fully admit that many structures are of no direct use to their +possessors. Physical conditions probably have had some little effect on +structure, quite independently of any good thus gained. Correlation of growth +has no doubt played a most important part, and a useful modification of one +part will often have entailed on other parts diversified changes of no direct +use. So again characters which formerly were useful, or which formerly had +arisen from correlation of growth, or from other unknown cause, may reappear +from the law of reversion, though now of no direct use. The effects of sexual +selection, when displayed in beauty to charm the females, can be called useful +only in rather a forced sense. But by far the most important consideration is +that the chief part of the organisation of every being is simply due to +inheritance; and consequently, though each being assuredly is well fitted for +its place in nature, many structures now have no direct relation to the habits +of life of each species. Thus, we can hardly believe that the webbed feet of +the upland +<a name="Page200"></a> +goose or of the frigate-bird are of special use to these birds; we cannot +believe that the same bones in the arm of the monkey, in the fore leg of the +horse, in the wing of the bat, and in the flipper of the seal, are of special +use to these animals. We may safely attribute these structures to inheritance. +But to the progenitor of the upland goose and of the frigate-bird, webbed feet +no doubt were as useful as they now are to the most aquatic of existing birds. +So we may believe that the progenitor of the seal had not a flipper, but a foot +with five toes fitted for walking or grasping; and we may further venture to +believe that the several bones in the limbs of the monkey, horse, and bat, +which have been inherited from a common progenitor, were formerly of more +special use to that progenitor, or its progenitors, than they now are to these +animals having such widely diversified habits. Therefore we may infer that +these several bones might have been acquired through natural selection, +subjected formerly, as now, to the several laws of inheritance, reversion, +correlation of growth, etc. Hence every detail of structure in every living +creature (making some little allowance for the direct action of physical +conditions) may be viewed, either as having been of special use to some +ancestral form, or as being now of special use to the descendants of this +form—either directly, or indirectly through the complex laws of growth. +</p> + +<p> +Natural selection cannot possibly produce any modification in any one species +exclusively for the good of another species; though throughout nature one +species incessantly takes advantage of, and profits by, the structure of +another. But natural selection can and does often produce structures for the +direct injury of other species, as we see in the fang of the adder, and in the +ovipositor of the ichneumon, by which its eggs are deposited +<a name="Page201"></a> +in the living bodies of other insects. If it could be proved that any part of +the structure of any one species had been formed for the exclusive good of +another species, it would annihilate my theory, for such could not have been +produced through natural selection. Although many statements may be found in +works on natural history to this effect, I cannot find even one which seems to +me of any weight. It is admitted that the rattlesnake has a poison-fang for its +own defence and for the destruction of its prey; but some authors suppose that +at the same time this snake is furnished with a rattle for its own injury, +namely, to warn its prey to escape. I would almost as soon believe that the cat +curls the end of its tail when preparing to spring, in order to warn the doomed +mouse. But I have not space here to enter on this and other such cases. +</p> + +<p> +Natural selection will never produce in a being anything injurious to itself, +for natural selection acts solely by and for the good of each. No organ will be +formed, as Paley has remarked, for the purpose of causing pain or for doing an +injury to its possessor. If a fair balance be struck between the good and evil +caused by each part, each will be found on the whole advantageous. After the +lapse of time, under changing conditions of life, if any part comes to be +injurious, it will be modified; or if it be not so, the being will become +extinct, as myriads have become extinct. +</p> + +<p> +Natural selection tends only to make each organic being as perfect as, or +slightly more perfect than, the other inhabitants of the same country with +which it has to struggle for existence. And we see that this is the degree of +perfection attained under nature. The endemic productions of New Zealand, for +instance, are perfect one compared with another; but they are now rapidly +yielding before the advancing legions of plants +<a name="Page202"></a> +and animals introduced from Europe. Natural selection will not produce absolute +perfection, nor do we always meet, as far as we can judge, with this high +standard under nature. The correction for the aberration of light is said, on +high authority, not to be perfect even in that most perfect organ, the eye. If +our reason leads us to admire with enthusiasm a multitude of inimitable +contrivances in nature, this same reason tells us, though we may easily err on +both sides, that some other contrivances are less perfect. Can we consider the +sting of the wasp or of the bee as perfect, which, when used against many +attacking animals, cannot be withdrawn, owing to the backward serratures, and +so inevitably causes the death of the insect by tearing out its viscera? +</p> + +<p> +If we look at the sting of the bee, as having originally existed in a remote +progenitor as a boring and serrated instrument, like that in so many members of +the same great order, and which has been modified but not perfected for its +present purpose, with the poison originally adapted to cause galls subsequently +intensified, we can perhaps understand how it is that the use of the sting +should so often cause the insect’s own death: for if on the whole the +power of stinging be useful to the community, it will fulfil all the +requirements of natural selection, though it may cause the death of some few +members. If we admire the truly wonderful power of scent by which the males of +many insects find their females, can we admire the production for this single +purpose of thousands of drones, which are utterly useless to the community for +any other end, and which are ultimately slaughtered by their industrious and +sterile sisters? It may be difficult, but we ought to admire the savage +instinctive hatred of the queen-bee, which urges her instantly to destroy the +<a name="Page203"></a> +young queens her daughters as soon as born, or to perish herself in the combat; +for undoubtedly this is for the good of the community; and maternal love or +maternal hatred, though the latter fortunately is most rare, is all the same to +the inexorable principle of natural selection. If we admire the several +ingenious contrivances, by which the flowers of the orchis and of many other +plants are fertilised through insect agency, can we consider as equally perfect +the elaboration by our fir-trees of dense clouds of pollen, in order that a few +granules may be wafted by a chance breeze on to the ovules? +</p> + +<p class="p2"> +<i>Summary of Chapter</i>.—We have in this chapter discussed some of the +difficulties and objections which may be urged against my theory. Many of them +are very grave; but I think that in the discussion light has been thrown on +several facts, which on the theory of independent acts of creation are utterly +obscure. We have seen that species at any one period are not indefinitely +variable, and are not linked together by a multitude of intermediate +gradations, partly because the process of natural selection will always be very +slow, and will act, at any one time, only on a very few forms; and partly +because the very process of natural selection almost implies the continual +supplanting and extinction of preceding and intermediate gradations. Closely +allied species, now living on a continuous area, must often have been formed +when the area was not continuous, and when the conditions of life did not +insensibly graduate away from one part to another. When two varieties are +formed in two districts of a continuous area, an intermediate variety will +often be formed, fitted for an intermediate zone; but from reasons assigned, +the intermediate variety will usually exist in lesser numbers than +<a name="Page204"></a> +the two forms which it connects; consequently the two latter, during the course +of further modification, from existing in greater numbers, will have a great +advantage over the less numerous intermediate variety, and will thus generally +succeed in supplanting and exterminating it. +</p> + +<p> +We have seen in this chapter how cautious we should be in concluding that the +most different habits of life could not graduate into each other; that a bat, +for instance, could not have been formed by natural selection from an animal +which at first could only glide through the air. +</p> + +<p> +We have seen that a species may under new conditions of life change its habits, +or have diversified habits, with some habits very unlike those of its nearest +congeners. Hence we can understand, bearing in mind that each organic being is +trying to live wherever it can live, how it has arisen that there are upland +geese with webbed feet, ground woodpeckers, diving thrushes, and petrels with +the habits of auks. +</p> + +<p> +Although the belief that an organ so perfect as the eye could have been formed +by natural selection, is more than enough to stagger any one; yet in the case +of any organ, if we know of a long series of gradations in complexity, each +good for its possessor, then, under changing conditions of life, there is no +logical impossibility in the acquirement of any conceivable degree of +perfection through natural selection. In the cases in which we know of no +intermediate or transitional states, we should be very cautious in concluding +that none could have existed, for the homologies of many organs and their +intermediate states show that wonderful metamorphoses in function are at least +possible. For instance, a swim-bladder has apparently been converted into an +air-breathing lung. The same organ having performed +<a name="Page205"></a> +simultaneously very different functions, and then having been specialised for +one function; and two very distinct organs having performed at the same time +the same function, the one having been perfected whilst aided by the other, +must often have largely facilitated transitions. +</p> + +<p> +We are far too ignorant, in almost every case, to be enabled to assert that any +part or organ is so unimportant for the welfare of a species, that +modifications in its structure could not have been slowly accumulated by means +of natural selection. But we may confidently believe that many modifications, +wholly due to the laws of growth, and at first in no way advantageous to a +species, have been subsequently taken advantage of by the still further +modified descendants of this species. We may, also, believe that a part +formerly of high importance has often been retained (as the tail of an aquatic +animal by its terrestrial descendants), though it has become of such small +importance that it could not, in its present state, have been acquired by +natural selection,—a power which acts solely by the preservation of +profitable variations in the struggle for life. +</p> + +<p> +Natural selection will produce nothing in one species for the exclusive good or +injury of another; though it may well produce parts, organs, and excretions +highly useful or even indispensable, or highly injurious to another species, +but in all cases at the same time useful to the owner. Natural selection in +each well-stocked country, must act chiefly through the competition of the +inhabitants one with another, and consequently will produce perfection, or +strength in the battle for life, only according to the standard of that +country. Hence the inhabitants of one country, generally the smaller one, will +often yield, as we see they do yield, to the inhabitants of another and +generally larger country. For in +<a name="Page206"></a> +the larger country there will have existed more individuals, and more +diversified forms, and the competition will have been severer, and thus the +standard of perfection will have been rendered higher. Natural selection will +not necessarily produce absolute perfection; nor, as far as we can judge by our +limited faculties, can absolute perfection be everywhere found. +</p> + +<p> +On the theory of natural selection we can clearly understand the full meaning +of that old canon in natural history, “Natura non facit saltum.” +This canon, if we look only to the present inhabitants of the world, is not +strictly correct, but if we include all those of past times, it must by my +theory be strictly true. +</p> + +<p> +It is generally acknowledged that all organic beings have been formed on two +great laws—Unity of Type, and the Conditions of Existence. By unity of +type is meant that fundamental agreement in structure, which we see in organic +beings of the same class, and which is quite independent of their habits of +life. On my theory, unity of type is explained by unity of descent. The +expression of conditions of existence, so often insisted on by the illustrious +Cuvier, is fully embraced by the principle of natural selection. For natural +selection acts by either now adapting the varying parts of each being to its +organic and inorganic conditions of life; or by having adapted them during +long-past periods of time: the adaptations being aided in some cases by use and +disuse, being slightly affected by the direct action of the external conditions +of life, and being in all cases subjected to the several laws of growth. Hence, +in fact, the law of the Conditions of Existence is the higher law; as it +includes, through the inheritance of former adaptations, that of Unity of Type. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page207"></a><a name="chap07"></a>CHAPTER VII.<br /> +INSTINCT.</h2> + +<p class="letter"> +Instincts comparable with habits, but different in their origin. Instincts +graduated. Aphides and ants. Instincts variable. Domestic instincts, their +origin. Natural instincts of the cuckoo, ostrich, and parasitic bees. +Slave-making ants. Hive-bee, its cell-making instinct. Difficulties on the +theory of the Natural Selection of instincts. Neuter or sterile insects. +Summary. +</p> + +<p> +The subject of instinct might have been worked into the previous chapters; but +I have thought that it would be more convenient to treat the subject +separately, especially as so wonderful an instinct as that of the hive-bee +making its cells will probably have occurred to many readers, as a difficulty +sufficient to overthrow my whole theory. I must premise, that I have nothing to +do with the origin of the primary mental powers, any more than I have with that +of life itself. We are concerned only with the diversities of instinct and of +the other mental qualities of animals within the same class. +</p> + +<p> +I will not attempt any definition of instinct. It would be easy to show that +several distinct mental actions are commonly embraced by this term; but every +one understands what is meant, when it is said that instinct impels the cuckoo +to migrate and to lay her eggs in other birds’ nests. An action, which we +ourselves should require experience to enable us to perform, when performed by +an animal, more especially by a very young one, without any experience, and +when performed by many individuals in the same way, without their knowing for +what purpose it is performed, is usually said to be instinctive. +<a name="Page208"></a> +But I could show that none of these characters of instinct are universal. A +little dose, as Pierre Huber expresses it, of judgment or reason, often comes +into play, even in animals very low in the scale of nature. +</p> + +<p> +Frederick Cuvier and several of the older metaphysicians have compared instinct +with habit. This comparison gives, I think, a remarkably accurate notion of the +frame of mind under which an instinctive action is performed, but not of its +origin. How unconsciously many habitual actions are performed, indeed not +rarely in direct opposition to our conscious will! yet they may be modified by +the will or reason. Habits easily become associated with other habits, and with +certain periods of time and states of the body. When once acquired, they often +remain constant throughout life. Several other points of resemblance between +instincts and habits could be pointed out. As in repeating a well-known song, +so in instincts, one action follows another by a sort of rhythm; if a person be +interrupted in a song, or in repeating anything by rote, he is generally forced +to go back to recover the habitual train of thought: so P. Huber found it was +with a caterpillar, which makes a very complicated hammock; for if he took a +caterpillar which had completed its hammock up to, say, the sixth stage of +construction, and put it into a hammock completed up only to the third stage, +the caterpillar simply re-performed the fourth, fifth, and sixth stages of +construction. If, however, a caterpillar were taken out of a hammock made up, +for instance, to the third stage, and were put into one finished up to the +sixth stage, so that much of its work was already done for it, far from feeling +the benefit of this, it was much embarrassed, and, in order to complete its +hammock, seemed forced to start from the third stage, where it had left off, +and thus tried to complete the already finished work. +</p> + +<p> +<a name="Page209"></a> +If we suppose any habitual action to become inherited—and I think it can +be shown that this does sometimes happen—then the resemblance between +what originally was a habit and an instinct becomes so close as not to be +distinguished. If Mozart, instead of playing the pianoforte at three years old +with wonderfully little practice, had played a tune with no practice at all, he +might truly be said to have done so instinctively. But it would be the most +serious error to suppose that the greater number of instincts have been +acquired by habit in one generation, and then transmitted by inheritance to +succeeding generations. It can be clearly shown that the most wonderful +instincts with which we are acquainted, namely, those of the hive-bee and of +many ants, could not possibly have been thus acquired. +</p> + +<p> +It will be universally admitted that instincts are as important as corporeal +structure for the welfare of each species, under its present conditions of +life. Under changed conditions of life, it is at least possible that slight +modifications of instinct might be profitable to a species; and if it can be +shown that instincts do vary ever so little, then I can see no difficulty in +natural selection preserving and continually accumulating variations of +instinct to any extent that may be profitable. It is thus, as I believe, that +all the most complex and wonderful instincts have originated. As modifications +of corporeal structure arise from, and are increased by, use or habit, and are +diminished or lost by disuse, so I do not doubt it has been with instincts. But +I believe that the effects of habit are of quite subordinate importance to the +effects of the natural selection of what may be called accidental variations of +instincts;—that is of variations produced by the same unknown causes +which produce slight deviations of bodily structure. +</p> + +<p> +No complex instinct can possibly be produced through +<a name="Page210"></a> +natural selection, except by the slow and gradual accumulation of numerous, +slight, yet profitable, variations. Hence, as in the case of corporeal +structures, we ought to find in nature, not the actual transitional gradations +by which each complex instinct has been acquired—for these could be found +only in the lineal ancestors of each species—but we ought to find in the +collateral lines of descent some evidence of such gradations; or we ought at +least to be able to show that gradations of some kind are possible; and this we +certainly can do. I have been surprised to find, making allowance for the +instincts of animals having been but little observed except in Europe and North +America, and for no instinct being known amongst extinct species, how very +generally gradations, leading to the most complex instincts, can be discovered. +The canon of “Natura non facit saltum” applies with almost equal +force to instincts as to bodily organs. Changes of instinct may sometimes be +facilitated by the same species having different instincts at different periods +of life, or at different seasons of the year, or when placed under different +circumstances, etc.; in which case either one or the other instinct might be +preserved by natural selection. And such instances of diversity of instinct in +the same species can be shown to occur in nature. +</p> + +<p> +Again as in the case of corporeal structure, and conformably with my theory, +the instinct of each species is good for itself, but has never, as far as we +can judge, been produced for the exclusive good of others. One of the strongest +instances of an animal apparently performing an action for the sole good of +another, with which I am acquainted, is that of aphides voluntarily yielding +their sweet excretion to ants: that they do so voluntarily, the following facts +show. I removed all the ants from a group of about a dozen aphides on a +dock-plant, +<a name="Page211"></a> +and prevented their attendance during several hours. After this interval, I +felt sure that the aphides would want to excrete. I watched them for some time +through a lens, but not one excreted; I then tickled and stroked them with a +hair in the same manner, as well as I could, as the ants do with their +antennæ; but not one excreted. Afterwards I allowed an ant to visit them, and +it immediately seemed, by its eager way of running about, to be well aware what +a rich flock it had discovered; it then began to play with its antennæ on the +abdomen first of one aphis and then of another; and each aphis, as soon as it +felt the antennæ, immediately lifted up its abdomen and excreted a limpid drop +of sweet juice, which was eagerly devoured by the ant. Even the quite young +aphides behaved in this manner, showing that the action was instinctive, and +not the result of experience. But as the excretion is extremely viscid, it is +probably a convenience to the aphides to have it removed; and therefore +probably the aphides do not instinctively excrete for the sole good of the +ants. Although I do not believe that any animal in the world performs an action +for the exclusive good of another of a distinct species, yet each species tries +to take advantage of the instincts of others, as each takes advantage of the +weaker bodily structure of others. So again, in some few cases, certain +instincts cannot be considered as absolutely perfect; but as details on this +and other such points are not indispensable, they may be here passed over. +</p> + +<p> +As some degree of variation in instincts under a state of nature, and the +inheritance of such variations, are indispensable for the action of natural +selection, as many instances as possible ought to have been here given; but +want of space prevents me. I can only assert, that instincts certainly do +vary—for instance, +<a name="Page212"></a> +the migratory instinct, both in extent and direction, and in its total loss. So +it is with the nests of birds, which vary partly in dependence on the +situations chosen, and on the nature and temperature of the country inhabited, +but often from causes wholly unknown to us: Audubon has given several +remarkable cases of differences in nests of the same species in the northern +and southern United States. Fear of any particular enemy is certainly an +instinctive quality, as may be seen in nestling birds, though it is +strengthened by experience, and by the sight of fear of the same enemy in other +animals. But fear of man is slowly acquired, as I have elsewhere shown, by +various animals inhabiting desert islands; and we may see an instance of this, +even in England, in the greater wildness of all our large birds than of our +small birds; for the large birds have been most persecuted by man. We may +safely attribute the greater wildness of our large birds to this cause; for in +uninhabited islands large birds are not more fearful than small; and the +magpie, so wary in England, is tame in Norway, as is the hooded crow in Egypt. +</p> + +<p> +That the general disposition of individuals of the same species, born in a +state of nature, is extremely diversified, can be shown by a multitude of +facts. Several cases also, could be given, of occasional and strange habits in +certain species, which might, if advantageous to the species, give rise, +through natural selection, to quite new instincts. But I am well aware that +these general statements, without facts given in detail, can produce but a +feeble effect on the reader’s mind. I can only repeat my assurance, that +I do not speak without good evidence. +</p> + +<p> +The possibility, or even probability, of inherited variations of instinct in a +state of nature will be strengthened by briefly considering a few cases under +<a name="Page213"></a> +domestication. We shall thus also be enabled to see the respective parts which +habit and the selection of so-called accidental variations have played in +modifying the mental qualities of our domestic animals. A number of curious and +authentic instances could be given of the inheritance of all shades of +disposition and tastes, and likewise of the oddest tricks, associated with +certain frames of mind or periods of time. But let us look to the familiar case +of the several breeds of dogs: it cannot be doubted that young pointers (I have +myself seen a striking instance) will sometimes point and even back other dogs +the very first time that they are taken out; retrieving is certainly in some +degree inherited by retrievers; and a tendency to run round, instead of at, a +flock of sheep, by shepherd-dogs. I cannot see that these actions, performed +without experience by the young, and in nearly the same manner by each +individual, performed with eager delight by each breed, and without the end +being known,—for the young pointer can no more know that he points to aid +his master, than the white butterfly knows why she lays her eggs on the leaf of +the cabbage,—I cannot see that these actions differ essentially from true +instincts. If we were to see one kind of wolf, when young and without any +training, as soon as it scented its prey, stand motionless like a statue, and +then slowly crawl forward with a peculiar gait; and another kind of wolf +rushing round, instead of at, a herd of deer, and driving them to a distant +point, we should assuredly call these actions instinctive. Domestic instincts, +as they may be called, are certainly far less fixed or invariable than natural +instincts; but they have been acted on by far less rigorous selection, and have +been transmitted for an incomparably shorter period, under less fixed +conditions of life. +</p> + +<p> +How strongly these domestic instincts, habits, and dispositions +<a name="Page214"></a> +are inherited, and how curiously they become mingled, is well shown when +different breeds of dogs are crossed. Thus it is known that a cross with a +bull-dog has affected for many generations the courage and obstinacy of +greyhounds; and a cross with a greyhound has given to a whole family of +shepherd-dogs a tendency to hunt hares. These domestic instincts, when thus +tested by crossing, resemble natural instincts, which in a like manner become +curiously blended together, and for a long period exhibit traces of the +instincts of either parent: for example, Le Roy describes a dog, whose +great-grandfather was a wolf, and this dog showed a trace of its wild parentage +only in one way, by not coming in a straight line to his master when called. +</p> + +<p> +Domestic instincts are sometimes spoken of as actions which have become +inherited solely from long-continued and compulsory habit, but this, I think, +is not true. No one would ever have thought of teaching, or probably could have +taught, the tumbler-pigeon to tumble,—an action which, as I have +witnessed, is performed by young birds, that have never seen a pigeon tumble. +We may believe that some one pigeon showed a slight tendency to this strange +habit, and that the long-continued selection of the best individuals in +successive generations made tumblers what they now are; and near Glasgow there +are house-tumblers, as I hear from Mr. Brent, which cannot fly eighteen inches +high without going head over heels. It may be doubted whether any one would +have thought of training a dog to point, had not some one dog naturally shown a +tendency in this line; and this is known occasionally to happen, as I once saw +in a pure terrier. When the first tendency was once displayed, methodical +selection and the inherited effects of compulsory training in each successive +generation would soon complete the work; and unconscious +<a name="Page215"></a> +selection is still at work, as each man tries to procure, without intending to +improve the breed, dogs which will stand and hunt best. On the other hand, +habit alone in some cases has sufficed; no animal is more difficult to tame +than the young of the wild rabbit; scarcely any animal is tamer than the young +of the tame rabbit; but I do not suppose that domestic rabbits have ever been +selected for tameness; and I presume that we must attribute the whole of the +inherited change from extreme wildness to extreme tameness, simply to habit and +long-continued close confinement. +</p> + +<p> +Natural instincts are lost under domestication: a remarkable instance of this +is seen in those breeds of fowls which very rarely or never become +“broody,” that is, never wish to sit on their eggs. Familiarity +alone prevents our seeing how universally and largely the minds of our domestic +animals have been modified by domestication. It is scarcely possible to doubt +that the love of man has become instinctive in the dog. All wolves, foxes, +jackals, and species of the cat genus, when kept tame, are most eager to attack +poultry, sheep, and pigs; and this tendency has been found incurable in dogs +which have been brought home as puppies from countries, such as Tierra del +Fuego and Australia, where the savages do not keep these domestic animals. How +rarely, on the other hand, do our civilised dogs, even when quite young, +require to be taught not to attack poultry, sheep, and pigs! No doubt they +occasionally do make an attack, and are then beaten; and if not cured, they are +destroyed; so that habit, with some degree of selection, has probably concurred +in civilising by inheritance our dogs. On the other hand, young chickens have +lost, wholly by habit, that fear of the dog and cat which no doubt was +originally instinctive in them, in the same way as it is so plainly instinctive +in +<a name="Page216"></a> +young pheasants, though reared under a hen. It is not that chickens have lost +all fear, but fear only of dogs and cats, for if the hen gives the +danger-chuckle, they will run (more especially young turkeys) from under her, +and conceal themselves in the surrounding grass or thickets; and this is +evidently done for the instinctive purpose of allowing, as we see in wild +ground-birds, their mother to fly away. But this instinct retained by our +chickens has become useless under domestication, for the mother-hen has almost +lost by disuse the power of flight. +</p> + +<p> +Hence, we may conclude, that domestic instincts have been acquired and natural +instincts have been lost partly by habit, and partly by man selecting and +accumulating during successive generations, peculiar mental habits and actions, +which at first appeared from what we must in our ignorance call an accident. In +some cases compulsory habit alone has sufficed to produce such inherited mental +changes; in other cases compulsory habit has done nothing, and all has been the +result of selection, pursued both methodically and unconsciously; but in most +cases, probably, habit and selection have acted together. +</p> + +<p> +We shall, perhaps, best understand how instincts in a state of nature have +become modified by selection, by considering a few cases. I will select only +three, out of the several which I shall have to discuss in my future +work,—namely, the instinct which leads the cuckoo to lay her eggs in +other birds’ nests; the slave-making instinct of certain ants; and the +comb-making power of the hive-bee: these two latter instincts have generally, +and most justly, been ranked by naturalists as the most wonderful of all known +instincts. +</p> + +<p> +It is now commonly admitted that the more immediate and final cause of the +cuckoo’s instinct is, that +<a name="Page217"></a> +she lays her eggs, not daily, but at intervals +of two or three days; so that, if she were to make her own nest and sit on her +own eggs, those first laid would have to be left for some time unincubated, or +there would be eggs and young birds of different ages in the same nest. If this +were the case, the process of laying and hatching might be inconveniently long, +more especially as she has to migrate at a very early period; and the first +hatched young would probably have to be fed by the male alone. But the American +cuckoo is in this predicament; for she makes her own nest and has eggs and +young successively hatched, all at the same time. It has been asserted that the +American cuckoo occasionally lays her eggs in other birds’ nests; but I +hear on the high authority of Dr. Brewer, that this is a mistake. Nevertheless, +I could give several instances of various birds which have been known +occasionally to lay their eggs in other birds’ nests. Now let us suppose +that the ancient progenitor of our European cuckoo had the habits of the +American cuckoo; but that occasionally she laid an egg in another bird’s +nest. If the old bird profited by this occasional habit, or if the young were +made more vigorous by advantage having been taken of the mistaken maternal +instinct of another bird, than by their own mother’s care, encumbered as +she can hardly fail to be by having eggs and young of different ages at the +same time; then the old birds or the fostered young would gain an advantage. +And analogy would lead me to believe, that the young thus reared would be apt +to follow by inheritance the occasional and aberrant habit of their mother, and +in their turn would be apt to lay their eggs in other birds’ nests, and +thus be successful in rearing their young. By a continued process of this +nature, I believe that the strange instinct of our cuckoo could be, and has +been, +<a name="Page218"></a> +generated. I may add that, according to Dr. Gray and to some other +observers, the European cuckoo has not utterly lost all maternal love and care +for her own offspring. +</p> + +<p> +The occasional habit of birds laying their eggs in other birds’ nests, +either of the same or of a distinct species, is not very uncommon with the +Gallinaceæ; and this perhaps explains the origin of a singular instinct in the +allied group of ostriches. For several hen ostriches, at least in the case of +the American species, unite and lay first a few eggs in one nest and then in +another; and these are hatched by the males. This instinct may probably be +accounted for by the fact of the hens laying a large number of eggs; but, as in +the case of the cuckoo, at intervals of two or three days. This instinct, +however, of the American ostrich has not as yet been perfected; for a +surprising number of eggs lie strewed over the plains, so that in one +day’s hunting I picked up no less than twenty lost and wasted eggs. +</p> + +<p> +Many bees are parasitic, and always lay their eggs in the nests of bees of +other kinds. This case is more remarkable than that of the cuckoo; for these +bees have not only their instincts but their structure modified in accordance +with their parasitic habits; for they do not possess the pollen-collecting +apparatus which would be necessary if they had to store food for their own +young. Some species, likewise, of Sphegidæ (wasp-like insects) are parasitic +on other species; and M. Fabre has lately shown good reason for believing that +although the Tachytes nigra generally makes its own burrow and stores it with +paralysed prey for its own larvæ to feed on, yet that when this insect finds a +burrow already made and stored by another sphex, it takes advantage of the +prize, and becomes for the occasion parasitic. In this case, as with the +supposed case of the cuckoo, I can +<a name="Page219"></a> +see no difficulty in natural selection making an occasional habit permanent, if +of advantage to the species, and if the insect whose nest and stored food are +thus feloniously appropriated, be not thus exterminated. +</p> + +<p class="p2"> +<i>Slave-making instinct</i>.—This remarkable instinct was first +discovered in the Formica (Polyerges) rufescens by Pierre Huber, a better +observer even than his celebrated father. This ant is absolutely dependent on +its slaves; without their aid, the species would certainly become extinct in a +single year. The males and fertile females do no work. The workers or sterile +females, though most energetic and courageous in capturing slaves, do no other +work. They are incapable of making their own nests, or of feeding their own +larvæ. When the old nest is found inconvenient, and they have to migrate, it +is the slaves which determine the migration, and actually carry their masters +in their jaws. So utterly helpless are the masters, that when Huber shut up +thirty of them without a slave, but with plenty of the food which they like +best, and with their larvæ and pupæ to stimulate them to work, they did +nothing; they could not even feed themselves, and many perished of hunger. +Huber then introduced a single slave (F. fusca), and she instantly set to work, +fed and saved the survivors; made some cells and tended the larvæ, and put all +to rights. What can be more extraordinary than these well-ascertained facts? If +we had not known of any other slave-making ant, it would have been hopeless to +have speculated how so wonderful an instinct could have been perfected. +</p> + +<p> +Formica sanguinea was likewise first discovered by P. Huber to be a +slave-making ant. This species is found in the southern parts of England, and +its habits have been attended to by Mr. F. Smith, of the British +<a name="Page220"></a> +Museum, to whom I am much indebted for information on this and other subjects. +Although fully trusting to the statements of Huber and Mr. Smith, I tried to +approach the subject in a sceptical frame of mind, as any one may well be +excused for doubting the truth of so extraordinary and odious an instinct as +that of making slaves. Hence I will give the observations which I have myself +made, in some little detail. I opened fourteen nests of F. sanguinea, and found +a few slaves in all. Males and fertile females of the slave-species are found +only in their own proper communities, and have never been observed in the nests +of F. sanguinea. The slaves are black and not above half the size of their red +masters, so that the contrast in their appearance is very great. When the nest +is slightly disturbed, the slaves occasionally come out, and like their masters +are much agitated and defend the nest: when the nest is much disturbed and the +larvæ and pupæ are exposed, the slaves work energetically with their masters in +carrying them away to a place of safety. Hence, it is clear, that the slaves +feel quite at home. During the months of June and July, on three successive +years, I have watched for many hours several nests in Surrey and Sussex, and +never saw a slave either leave or enter a nest. As, during these months, the +slaves are very few in number, I thought that they might behave differently +when more numerous; but Mr. Smith informs me that he has watched the nests at +various hours during May, June and August, both in Surrey and Hampshire, and +has never seen the slaves, though present in large numbers in August, either +leave or enter the nest. Hence he considers them as strictly household slaves. +The masters, on the other hand, may be constantly seen bringing in materials +for the nest, and food of all kinds. During the present year, however, in the +month +<a name="Page221"></a> +of July, I came across a community with an unusually large stock of slaves, and +I observed a few slaves mingled with their masters leaving the nest, and +marching along the same road to a tall Scotch-fir-tree, twenty-five yards +distant, which they ascended together, probably in search of aphides or cocci. +According to Huber, who had ample opportunities for observation, in Switzerland +the slaves habitually work with their masters in making the nest, and they +alone open and close the doors in the morning and evening; and, as Huber +expressly states, their principal office is to search for aphides. This +difference in the usual habits of the masters and slaves in the two countries, +probably depends merely on the slaves being captured in greater numbers in +Switzerland than in England. +</p> + +<p> +One day I fortunately chanced to witness a migration from one nest to another, +and it was a most interesting spectacle to behold the masters carefully +carrying, as Huber has described, their slaves in their jaws. Another day my +attention was struck by about a score of the slave-makers haunting the same +spot, and evidently not in search of food; they approached and were vigorously +repulsed by an independent community of the slave species (F. fusca); sometimes +as many as three of these ants clinging to the legs of the slave-making F. +sanguinea. The latter ruthlessly killed their small opponents, and carried +their dead bodies as food to their nest, twenty-nine yards distant; but they +were prevented from getting any pupæ to rear as slaves. I then dug up a small +parcel of the pupæ of F. fusca from another nest, and put them down on a bare +spot near the place of combat; they were eagerly seized, and carried off by the +tyrants, who perhaps fancied that, after all, they had been victorious in their +late combat. +</p> + +<p> +<a name="Page222"></a> +At the same time I laid on the same place a small parcel of the pupæ of +another species, F. flava, with a few of these little yellow ants still +clinging to the fragments of the nest. This species is sometimes, though +rarely, made into slaves, as has been described by Mr. Smith. Although so small +a species, it is very courageous, and I have seen it ferociously attack other +ants. In one instance I found to my surprise an independent community of F. +flava under a stone beneath a nest of the slave-making F. sanguinea; and when I +had accidentally disturbed both nests, the little ants attacked their big +neighbours with surprising courage. Now I was curious to ascertain whether F. +sanguinea could distinguish the pupæ of F. fusca, which they habitually make +into slaves, from those of the little and furious F. flava, which they rarely +capture, and it was evident that they did at once distinguish them: for we have +seen that they eagerly and instantly seized the pupæ of F. fusca, whereas they +were much terrified when they came across the pupæ, or even the earth from the +nest of F. flava, and quickly ran away; but in about a quarter of an hour, +shortly after all the little yellow ants had crawled away, they took heart and +carried off the pupæ. +</p> + +<p> +One evening I visited another community of F. sanguinea, and found a number of +these ants entering their nest, carrying the dead bodies of F. fusca (showing +that it was not a migration) and numerous pupæ. I traced the returning file +burthened with booty, for about forty yards, to a very thick clump of heath, +whence I saw the last individual of F. sanguinea emerge, carrying a pupa; but I +was not able to find the desolated nest in the thick heath. The nest, however, +must have been close at hand, for two or three individuals of F. fusca were +rushing about in the greatest agitation, and one was +<a name="Page223"></a> +perched motionless with its own pupa in its mouth on the top of a spray of +heath over its ravaged home. +</p> + +<p> +Such are the facts, though they did not need confirmation by me, in regard to +the wonderful instinct of making slaves. Let it be observed what a contrast the +instinctive habits of F. sanguinea present with those of the F. rufescens. The +latter does not build its own nest, does not determine its own migrations, does +not collect food for itself or its young, and cannot even feed itself: it is +absolutely dependent on its numerous slaves. Formica sanguinea, on the other +hand, possesses much fewer slaves, and in the early part of the summer +extremely few. The masters determine when and where a new nest shall be formed, +and when they migrate, the masters carry the slaves. Both in Switzerland and +England the slaves seem to have the exclusive care of the larvæ, and the +masters alone go on slave-making expeditions. In Switzerland the slaves and +masters work together, making and bringing materials for the nest: both, but +chiefly the slaves, tend, and milk as it may be called, their aphides; and thus +both collect food for the community. In England the masters alone usually leave +the nest to collect building materials and food for themselves, their slaves +and larvæ. So that the masters in this country receive much less service from +their slaves than they do in Switzerland. +</p> + +<p> +By what steps the instinct of F. sanguinea originated I will not pretend to +conjecture. But as ants, which are not slave-makers, will, as I have seen, +carry off pupæ of other species, if scattered near their nests, it is possible +that pupæ originally stored as food might become developed; and the ants thus +unintentionally reared would then follow their proper instincts, and do what +work they could. If their presence proved useful to the species which had +seized them—if it were more advantageous +<a name="Page224"></a> +to this species to capture workers than to procreate them—the habit of +collecting pupæ originally for food might by natural selection be strengthened +and rendered permanent for the very different purpose of raising slaves. When +the instinct was once acquired, if carried out to a much less extent even than +in our British F. sanguinea, which, as we have seen, is less aided by its +slaves than the same species in Switzerland, I can see no difficulty in natural +selection increasing and modifying the instinct—always supposing each +modification to be of use to the species—until an ant was formed as +abjectly dependent on its slaves as is the Formica rufescens. +</p> + +<p class="p2"> +<i>Cell-making instinct of the Hive-Bee</i>.—I will not here enter on +minute details on this subject, but will merely give an outline of the +conclusions at which I have arrived. He must be a dull man who can examine the +exquisite structure of a comb, so beautifully adapted to its end, without +enthusiastic admiration. We hear from mathematicians that bees have practically +solved a recondite problem, and have made their cells of the proper shape to +hold the greatest possible amount of honey, with the least possible consumption +of precious wax in their construction. It has been remarked that a skilful +workman, with fitting tools and measures, would find it very difficult to make +cells of wax of the true form, though this is perfectly effected by a crowd of +bees working in a dark hive. Grant whatever instincts you please, and it seems +at first quite inconceivable how they can make all the necessary angles and +planes, or even perceive when they are correctly made. But the difficulty is +not nearly so great as it at first appears: all this beautiful work can be +shown, I think, to follow from a few very simple instincts. +</p> + +<p> +<a name="Page225"></a> +I was led to investigate this subject by Mr. Waterhouse, who has shown that the +form of the cell stands in close relation to the presence of adjoining cells; +and the following view may, perhaps, be considered only as a modification of +his theory. Let us look to the great principle of gradation, and see whether +Nature does not reveal to us her method of work. At one end of a short series +we have humble-bees, which use their old cocoons to hold honey, sometimes +adding to them short tubes of wax, and likewise making separate and very +irregular rounded cells of wax. At the other end of the series we have the +cells of the hive-bee, placed in a double layer: each cell, as is well known, +is an hexagonal prism, with the basal edges of its six sides bevelled so as to +join on to a pyramid, formed of three rhombs. These rhombs have certain angles, +and the three which form the pyramidal base of a single cell on one side of the +comb, enter into the composition of the bases of three adjoining cells on the +opposite side. In the series between the extreme perfection of the cells of the +hive-bee and the simplicity of those of the humble-bee, we have the cells of +the Mexican Melipona domestica, carefully described and figured by Pierre +Huber. The Melipona itself is intermediate in structure between the hive and +humble bee, but more nearly related to the latter: it forms a nearly regular +waxen comb of cylindrical cells, in which the young are hatched, and, in +addition, some large cells of wax for holding honey. These latter cells are +nearly spherical and of nearly equal sizes, and are aggregated into an +irregular mass. But the important point to notice, is that these cells are +always made at that degree of nearness to each other, that they would have +intersected or broken into each other, if the spheres had been completed; but +this is never permitted, the bees building perfectly flat walls of wax between +the spheres +<a name="Page226"></a> +which thus tend to intersect. Hence each cell consists of an outer spherical +portion and of two, three, or more perfectly flat surfaces, according as the +cell adjoins two, three or more other cells. When one cell comes into contact +with three other cells, which, from the spheres being nearly of the same size, +is very frequently and necessarily the case, the three flat surfaces are united +into a pyramid; and this pyramid, as Huber has remarked, is manifestly a gross +imitation of the three-sided pyramidal basis of the cell of the hive-bee. As in +the cells of the hive-bee, so here, the three plane surfaces in any one cell +necessarily enter into the construction of three adjoining cells. It is obvious +that the Melipona saves wax by this manner of building; for the flat walls +between the adjoining cells are not double, but are of the same thickness as +the outer spherical portions, and yet each flat portion forms a part of two +cells. +</p> + +<p> +Reflecting on this case, it occurred to me that if the Melipona had made its +spheres at some given distance from each other, and had made them of equal +sizes and had arranged them symmetrically in a double layer, the resulting +structure would probably have been as perfect as the comb of the hive-bee. +Accordingly I wrote to Professor Miller, of Cambridge, and this geometer has +kindly read over the following statement, drawn up from his information, and +tells me that it is strictly correct:— +</p> + +<p> +If a number of equal spheres be described with their centres placed in two +parallel layers; with the centre of each sphere at the distance of radius x the +square root of 2 or radius x 1.41421 (or at some lesser distance), from the +centres of the six surrounding spheres in the same layer; and at the same +distance from the centres of the adjoining spheres in the other and parallel +layer; then, if planes of intersection between the several spheres in +<a name="Page227"></a> +both layers be formed, there will result a double layer of hexagonal prisms +united together by pyramidal bases formed of three rhombs; and the rhombs and +the sides of the hexagonal prisms will have every angle identically the same +with the best measurements which have been made of the cells of the hive-bee. +</p> + +<p> +Hence we may safely conclude that if we could slightly modify the instincts +already possessed by the Melipona, and in themselves not very wonderful, this +bee would make a structure as wonderfully perfect as that of the hive-bee. We +must suppose the Melipona to make her cells truly spherical, and of equal +sizes; and this would not be very surprising, seeing that she already does so +to a certain extent, and seeing what perfectly cylindrical burrows in wood many +insects can make, apparently by turning round on a fixed point. We must suppose +the Melipona to arrange her cells in level layers, as she already does her +cylindrical cells; and we must further suppose, and this is the greatest +difficulty, that she can somehow judge accurately at what distance to stand +from her fellow-labourers when several are making their spheres; but she is +already so far enabled to judge of distance, that she always describes her +spheres so as to intersect largely; and then she unites the points of +intersection by perfectly flat surfaces. We have further to suppose, but this +is no difficulty, that after hexagonal prisms have been formed by the +intersection of adjoining spheres in the same layer, she can prolong the +hexagon to any length requisite to hold the stock of honey; in the same way as +the rude humble-bee adds cylinders of wax to the circular mouths of her old +cocoons. By such modifications of instincts in themselves not very +wonderful,—hardly more wonderful than those which guide a bird to make +its nest,—I believe that the hive-bee +<a name="Page228"></a> +has acquired, through natural selection, her inimitable architectural powers. +</p> + +<p> +But this theory can be tested by experiment. Following the example of Mr. +Tegetmeier, I separated two combs, and put between them a long, thick, square +strip of wax: the bees instantly began to excavate minute circular pits in it; +and as they deepened these little pits, they made them wider and wider until +they were converted into shallow basins, appearing to the eye perfectly true or +parts of a sphere, and of about the diameter of a cell. It was most interesting +to me to observe that wherever several bees had begun to excavate these basins +near together, they had begun their work at such a distance from each other, +that by the time the basins had acquired the above stated width (<i>i.e.</i> +about the width of an ordinary cell), and were in depth about one sixth of the +diameter of the sphere of which they formed a part, the rims of the basins +intersected or broke into each other. As soon as this occurred, the bees ceased +to excavate, and began to build up flat walls of wax on the lines of +intersection between the basins, so that each hexagonal prism was built upon +the festooned edge of a smooth basin, instead of on the straight edges of a +three-sided pyramid as in the case of ordinary cells. +</p> + +<p> +I then put into the hive, instead of a thick, square piece of wax, a thin and +narrow, knife-edged ridge, coloured with vermilion. The bees instantly began on +both sides to excavate little basins near to each other, in the same way as +before; but the ridge of wax was so thin, that the bottoms of the basins, if +they had been excavated to the same depth as in the former experiment, would +have broken into each other from the opposite sides. The bees, however, did not +suffer this to happen, and they stopped their excavations in due +<a name="Page229"></a> +time; so that the basins, as soon as they had been a little deepened, came to +have flat bottoms; and these flat bottoms, formed by thin little plates of the +vermilion wax having been left ungnawed, were situated, as far as the eye could +judge, exactly along the planes of imaginary intersection between the basins on +the opposite sides of the ridge of wax. In parts, only little bits, in other +parts, large portions of a rhombic plate had been left between the opposed +basins, but the work, from the unnatural state of things, had not been neatly +performed. The bees must have worked at very nearly the same rate on the +opposite sides of the ridge of vermilion wax, as they circularly gnawed away +and deepened the basins on both sides, in order to have succeeded in thus +leaving flat plates between the basins, by stopping work along the intermediate +planes or planes of intersection. +</p> + +<p> +Considering how flexible thin wax is, I do not see that there is any difficulty +in the bees, whilst at work on the two sides of a strip of wax, perceiving when +they have gnawed the wax away to the proper thinness, and then stopping their +work. In ordinary combs it has appeared to me that the bees do not always +succeed in working at exactly the same rate from the opposite sides; for I have +noticed half-completed rhombs at the base of a just-commenced cell, which were +slightly concave on one side, where I suppose that the bees had excavated too +quickly, and convex on the opposed side, where the bees had worked less +quickly. In one well-marked instance, I put the comb back into the hive, and +allowed the bees to go on working for a short time, and again examined the +cell, and I found that the rhombic plate had been completed, and had become +<i>perfectly flat:</i> it was absolutely impossible, from the extreme thinness +of the little rhombic plate, that they could have effected +<a name="Page230"></a> +this by gnawing away the convex side; and I suspect that the bees in such cases +stand in the opposed cells and push and bend the ductile and warm wax (which as +I have tried is easily done) into its proper intermediate plane, and thus +flatten it. +</p> + +<p> +From the experiment of the ridge of vermilion wax, we can clearly see that if +the bees were to build for themselves a thin wall of wax, they could make their +cells of the proper shape, by standing at the proper distance from each other, +by excavating at the same rate, and by endeavouring to make equal spherical +hollows, but never allowing the spheres to break into each other. Now bees, as +may be clearly seen by examining the edge of a growing comb, do make a rough, +circumferential wall or rim all round the comb; and they gnaw into this from +the opposite sides, always working circularly as they deepen each cell. They do +not make the whole three-sided pyramidal base of any one cell at the same time, +but only the one rhombic plate which stands on the extreme growing margin, or +the two plates, as the case may be; and they never complete the upper edges of +the rhombic plates, until the hexagonal walls are commenced. Some of these +statements differ from those made by the justly celebrated elder Huber, but I +am convinced of their accuracy; and if I had space, I could show that they are +conformable with my theory. +</p> + +<p> +Huber’s statement that the very first cell is excavated out of a little +parallel-sided wall of wax, is not, as far as I have seen, strictly correct; +the first commencement having always been a little hood of wax; but I will not +here enter on these details. We see how important a part excavation plays in +the construction of the cells; but it would be a great error to suppose that +the bees cannot build up a rough wall of wax in the proper +<a name="Page231"></a> +position—that is, along the plane of intersection between two adjoining +spheres. I have several specimens showing clearly that they can do this. Even +in the rude circumferential rim or wall of wax round a growing comb, flexures +may sometimes be observed, corresponding in position to the planes of the +rhombic basal plates of future cells. But the rough wall of wax has in every +case to be finished off, by being largely gnawed away on both sides. The manner +in which the bees build is curious; they always make the first rough wall from +ten to twenty times thicker than the excessively thin finished wall of the +cell, which will ultimately be left. We shall understand how they work, by +supposing masons first to pile up a broad ridge of cement, and then to begin +cutting it away equally on both sides near the ground, till a smooth, very thin +wall is left in the middle; the masons always piling up the cut-away cement, +and adding fresh cement, on the summit of the ridge. We shall thus have a thin +wall steadily growing upward; but always crowned by a gigantic coping. From all +the cells, both those just commenced and those completed, being thus crowned by +a strong coping of wax, the bees can cluster and crawl over the comb without +injuring the delicate hexagonal walls, which are only about one four-hundredth +of an inch in thickness; the plates of the pyramidal basis being about twice as +thick. By this singular manner of building, strength is continually given to +the comb, with the utmost ultimate economy of wax. +</p> + +<p> +It seems at first to add to the difficulty of understanding how the cells are +made, that a multitude of bees all work together; one bee after working a short +time at one cell going to another, so that, as Huber has stated, a score of +individuals work even at the commencement of the first cell. I was able +practically to show this fact, by covering the edges of the hexagonal walls +<a name="Page232"></a> +of a single cell, or the extreme margin of the circumferential rim of a growing +comb, with an extremely thin layer of melted vermilion wax; and I invariably +found that the colour was most delicately diffused by the bees—as +delicately as a painter could have done with his brush—by atoms of the +coloured wax having been taken from the spot on which it had been placed, and +worked into the growing edges of the cells all round. The work of construction +seems to be a sort of balance struck between many bees, all instinctively +standing at the same relative distance from each other, all trying to sweep +equal spheres, and then building up, or leaving ungnawed, the planes of +intersection between these spheres. It was really curious to note in cases of +difficulty, as when two pieces of comb met at an angle, how often the bees +would entirely pull down and rebuild in different ways the same cell, sometimes +recurring to a shape which they had at first rejected. +</p> + +<p> +When bees have a place on which they can stand in their proper positions for +working,—for instance, on a slip of wood, placed directly under the +middle of a comb growing downwards so that the comb has to be built over one +face of the slip—in this case the bees can lay the foundations of one +wall of a new hexagon, in its strictly proper place, projecting beyond the +other completed cells. It suffices that the bees should be enabled to stand at +their proper relative distances from each other and from the walls of the last +completed cells, and then, by striking imaginary spheres, they can build up a +wall intermediate between two adjoining spheres; but, as far as I have seen, +they never gnaw away and finish off the angles of a cell till a large part both +of that cell and of the adjoining cells has been built. This capacity in bees +of laying down under certain circumstances a rough wall in its proper place +between two just-commenced +<a name="Page233"></a> +cells, is important, as it bears on a fact, which seems at first quite +subversive of the foregoing theory; namely, that the cells on the extreme +margin of wasp-combs are sometimes strictly hexagonal; but I have not space +here to enter on this subject. Nor does there seem to me any great difficulty +in a single insect (as in the case of a queen-wasp) making hexagonal cells, if +she work alternately on the inside and outside of two or three cells commenced +at the same time, always standing at the proper relative distance from the +parts of the cells just begun, sweeping spheres or cylinders, and building up +intermediate planes. It is even conceivable that an insect might, by fixing on +a point at which to commence a cell, and then moving outside, first to one +point, and then to five other points, at the proper relative distances from the +central point and from each other, strike the planes of intersection, and so +make an isolated hexagon: but I am not aware that any such case has been +observed; nor would any good be derived from a single hexagon being built, as +in its construction more materials would be required than for a cylinder. +</p> + +<p> +As natural selection acts only by the accumulation of slight modifications of +structure or instinct, each profitable to the individual under its conditions +of life, it may reasonably be asked, how a long and graduated succession of +modified architectural instincts, all tending towards the present perfect plan +of construction, could have profited the progenitors of the hive-bee? I think +the answer is not difficult: it is known that bees are often hard pressed to +get sufficient nectar; and I am informed by Mr. Tegetmeier that it has been +experimentally found that no less than from twelve to fifteen pounds of dry +sugar are consumed by a hive of bees for the secretion of each pound of wax; so +that a prodigious quantity of fluid nectar must be collected and consumed by +the bees in a hive for +<a name="Page234"></a> +the secretion of the wax necessary for the construction of their combs. +Moreover, many bees have to remain idle for many days during the process of +secretion. A large store of honey is indispensable to support a large stock of +bees during the winter; and the security of the hive is known mainly to depend +on a large number of bees being supported. Hence the saving of wax by largely +saving honey must be a most important element of success in any family of bees. +Of course the success of any species of bee may be dependent on the number of +its parasites or other enemies, or on quite distinct causes, and so be +altogether independent of the quantity of honey which the bees could collect. +But let us suppose that this latter circumstance determined, as it probably +often does determine, the numbers of a humble-bee which could exist in a +country; and let us further suppose that the community lived throughout the +winter, and consequently required a store of honey: there can in this case be +no doubt that it would be an advantage to our humble-bee, if a slight +modification of her instinct led her to make her waxen cells near together, so +as to intersect a little; for a wall in common even to two adjoining cells, +would save some little wax. Hence it would continually be more and more +advantageous to our humble-bee, if she were to make her cells more and more +regular, nearer together, and aggregated into a mass, like the cells of the +Melipona; for in this case a large part of the bounding surface of each cell +would serve to bound other cells, and much wax would be saved. Again, from the +same cause, it would be advantageous to the Melipona, if she were to make her +cells closer together, and more regular in every way than at present; for then, +as we have seen, the spherical surfaces would wholly disappear, and would all +be replaced by plane surfaces; and the Melipona +<a name="Page235"></a> +would make a comb as perfect as that of the hive-bee. Beyond this stage of +perfection in architecture, natural selection could not lead; for the comb of +the hive-bee, as far as we can see, is absolutely perfect in economising wax. +</p> + +<p> +Thus, as I believe, the most wonderful of all known instincts, that of the +hive-bee, can be explained by natural selection having taken advantage of +numerous, successive, slight modifications of simpler instincts; natural +selection having by slow degrees, more and more perfectly, led the bees to +sweep equal spheres at a given distance from each other in a double layer, and +to build up and excavate the wax along the planes of intersection. The bees, of +course, no more knowing that they swept their spheres at one particular +distance from each other, than they know what are the several angles of the +hexagonal prisms and of the basal rhombic plates. The motive power of the +process of natural selection having been economy of wax; that individual swarm +which wasted least honey in the secretion of wax, having succeeded best, and +having transmitted by inheritance its newly acquired economical instinct to new +swarms, which in their turn will have had the best chance of succeeding in the +struggle for existence. +</p> + +<p class="p2"> +No doubt many instincts of very difficult explanation could be opposed to the +theory of natural selection,—cases, in which we cannot see how an +instinct could possibly have originated; cases, in which no intermediate +gradations are known to exist; cases of instinct of apparently such trifling +importance, that they could hardly have been acted on by natural selection; +cases of instincts almost identically the same in animals so remote in the +scale of nature, that we cannot account +<a name="Page236"></a> +for their similarity by inheritance from a common parent, and must therefore +believe that they have been acquired by independent acts of natural selection. +I will not here enter on these several cases, but will confine myself to one +special difficulty, which at first appeared to me insuperable, and actually +fatal to my whole theory. I allude to the neuters or sterile females in +insect-communities: for these neuters often differ widely in instinct and in +structure from both the males and fertile females, and yet, from being sterile, +they cannot propagate their kind. +</p> + +<p> +The subject well deserves to be discussed at great length, but I will here take +only a single case, that of working or sterile ants. How the workers have been +rendered sterile is a difficulty; but not much greater than that of any other +striking modification of structure; for it can be shown that some insects and +other articulate animals in a state of nature occasionally become sterile; and +if such insects had been social, and it had been profitable to the community +that a number should have been annually born capable of work, but incapable of +procreation, I can see no very great difficulty in this being effected by +natural selection. But I must pass over this preliminary difficulty. The great +difficulty lies in the working ants differing widely from both the males and +the fertile females in structure, as in the shape of the thorax and in being +destitute of wings and sometimes of eyes, and in instinct. As far as instinct +alone is concerned, the prodigious difference in this respect between the +workers and the perfect females, would have been far better exemplified by the +hive-bee. If a working ant or other neuter insect had been an animal in the +ordinary state, I should have unhesitatingly assumed that all its characters +had been slowly acquired through natural selection; namely, by an individual +<a name="Page237"></a> +having been born with some slight profitable modification of structure, this +being inherited by its offspring, which again varied and were again selected, +and so onwards. But with the working ant we have an insect differing greatly +from its parents, yet absolutely sterile; so that it could never have +transmitted successively acquired modifications of structure or instinct to its +progeny. It may well be asked how is it possible to reconcile this case with +the theory of natural selection? +</p> + +<p> +First, let it be remembered that we have innumerable instances, both in our +domestic productions and in those in a state of nature, of all sorts of +differences of structure which have become correlated to certain ages, and to +either sex. We have differences correlated not only to one sex, but to that +short period alone when the reproductive system is active, as in the nuptial +plumage of many birds, and in the hooked jaws of the male salmon. We have even +slight differences in the horns of different breeds of cattle in relation to an +artificially imperfect state of the male sex; for oxen of certain breeds have +longer horns than in other breeds, in comparison with the horns of the bulls or +cows of these same breeds. Hence I can see no real difficulty in any character +having become correlated with the sterile condition of certain members of +insect-communities: the difficulty lies in understanding how such correlated +modifications of structure could have been slowly accumulated by natural +selection. +</p> + +<p> +This difficulty, though appearing insuperable, is lessened, or, as I believe, +disappears, when it is remembered that selection may be applied to the family, +as well as to the individual, and may thus gain the desired end. Thus, a +well-flavoured vegetable is cooked, and the individual is destroyed; but the +horticulturist sows seeds of the same stock, and confidently expects to +<a name="Page238"></a> +get nearly the same variety; breeders of cattle wish the flesh and fat to be +well marbled together; the animal has been slaughtered, but the breeder goes +with confidence to the same family. I have such faith in the powers of +selection, that I do not doubt that a breed of cattle, always yielding oxen +with extraordinarily long horns, could be slowly formed by carefully watching +which individual bulls and cows, when matched, produced oxen with the longest +horns; and yet no one ox could ever have propagated its kind. Thus I believe it +has been with social insects: a slight modification of structure, or instinct, +correlated with the sterile condition of certain members of the community, has +been advantageous to the community: consequently the fertile males and females +of the same community flourished, and transmitted to their fertile offspring a +tendency to produce sterile members having the same modification. And I believe +that this process has been repeated, until that prodigious amount of difference +between the fertile and sterile females of the same species has been produced, +which we see in many social insects. +</p> + +<p> +But we have not as yet touched on the climax of the difficulty; namely, the +fact that the neuters of several ants differ, not only from the fertile females +and males, but from each other, sometimes to an almost incredible degree, and +are thus divided into two or even three castes. The castes, moreover, do not +generally graduate into each other, but are perfectly well defined; being as +distinct from each other, as are any two species of the same genus, or rather +as any two genera of the same family. Thus in Eciton, there are working and +soldier neuters, with jaws and instincts extraordinarily different: in +Cryptocerus, the workers of one caste alone carry a wonderful sort of shield on +their heads, the use of which is quite unknown: in the Mexican Myrmecocystus, +<a name="Page239"></a> +the workers of one caste never leave the nest; they are fed by the workers of +another caste, and they have an enormously developed abdomen which secretes a +sort of honey, supplying the place of that excreted by the aphides, or the +domestic cattle as they may be called, which our European ants guard or +imprison. +</p> + +<p> +It will indeed be thought that I have an overweening confidence in the +principle of natural selection, when I do not admit that such wonderful and +well-established facts at once annihilate my theory. In the simpler case of +neuter insects all of one caste or of the same kind, which have been rendered +by natural selection, as I believe to be quite possible, different from the +fertile males and females,—in this case, we may safely conclude from the +analogy of ordinary variations, that each successive, slight, profitable +modification did not probably at first appear in all the individual neuters in +the same nest, but in a few alone; and that by the long-continued selection of +the fertile parents which produced most neuters with the profitable +modification, all the neuters ultimately came to have the desired character. On +this view we ought occasionally to find neuter-insects of the same species, in +the same nest, presenting gradations of structure; and this we do find, even +often, considering how few neuter-insects out of Europe have been carefully +examined. Mr. F. Smith has shown how surprisingly the neuters of several +British ants differ from each other in size and sometimes in colour; and that +the extreme forms can sometimes be perfectly linked together by individuals +taken out of the same nest: I have myself compared perfect gradations of this +kind. It often happens that the larger or the smaller sized workers are the +most numerous; or that both large and small are numerous, with those of an +intermediate size scanty in numbers. Formica flava has larger and +<a name="Page240"></a> +smaller workers, with some of intermediate size; and, in this species, as Mr. +F. Smith has observed, the larger workers have simple eyes (ocelli), which +though small can be plainly distinguished, whereas the smaller workers have +their ocelli rudimentary. Having carefully dissected several specimens of these +workers, I can affirm that the eyes are far more rudimentary in the smaller +workers than can be accounted for merely by their proportionally lesser size; +and I fully believe, though I dare not assert so positively, that the workers +of intermediate size have their ocelli in an exactly intermediate condition. So +that we here have two bodies of sterile workers in the same nest, differing not +only in size, but in their organs of vision, yet connected by some few members +in an intermediate condition. I may digress by adding, that if the smaller +workers had been the most useful to the community, and those males and females +had been continually selected, which produced more and more of the smaller +workers, until all the workers had come to be in this condition; we should then +have had a species of ant with neuters very nearly in the same condition with +those of Myrmica. For the workers of Myrmica have not even rudiments of ocelli, +though the male and female ants of this genus have well-developed ocelli. +</p> + +<p> +I may give one other case: so confidently did I expect to find gradations in +important points of structure between the different castes of neuters in the +same species, that I gladly availed myself of Mr. F. Smith’s offer of +numerous specimens from the same nest of the driver ant (Anomma) of West +Africa. The reader will perhaps best appreciate the amount of difference in +these workers, by my giving not the actual measurements, but a strictly +accurate illustration: the difference was the same as if we were to see a set +of workmen building +<a name="Page241"></a> +a house of whom many were five feet four inches high, and many sixteen feet +high; but we must suppose that the larger workmen had heads four instead of +three times as big as those of the smaller men, and jaws nearly five times as +big. The jaws, moreover, of the working ants of the several sizes differed +wonderfully in shape, and in the form and number of the teeth. But the +important fact for us is, that though the workers can be grouped into castes of +different sizes, yet they graduate insensibly into each other, as does the +widely-different structure of their jaws. I speak confidently on this latter +point, as Mr. Lubbock made drawings for me with the camera lucida of the jaws +which I had dissected from the workers of the several sizes. +</p> + +<p> +With these facts before me, I believe that natural selection, by acting on the +fertile parents, could form a species which should regularly produce neuters, +either all of large size with one form of jaw, or all of small size with jaws +having a widely different structure; or lastly, and this is our climax of +difficulty, one set of workers of one size and structure, and simultaneously +another set of workers of a different size and structure;—a graduated +series having been first formed, as in the case of the driver ant, and then the +extreme forms, from being the most useful to the community, having been +produced in greater and greater numbers through the natural selection of the +parents which generated them; until none with an intermediate structure were +produced. +</p> + +<p> +Thus, as I believe, the wonderful fact of two distinctly defined castes of +sterile workers existing in the same nest, both widely different from each +other and from their parents, has originated. We can see how useful their +production may have been to a social community of insects, on the same +principle that the division of +<a name="Page242"></a> +labour is useful to civilised man. As ants work by inherited instincts and by +inherited tools or weapons, and not by acquired knowledge and manufactured +instruments, a perfect division of labour could be effected with them only by +the workers being sterile; for had they been fertile, they would have +intercrossed, and their instincts and structure would have become blended. And +nature has, as I believe, effected this admirable division of labour in the +communities of ants, by the means of natural selection. But I am bound to +confess, that, with all my faith in this principle, I should never have +anticipated that natural selection could have been efficient in so high a +degree, had not the case of these neuter insects convinced me of the fact. I +have, therefore, discussed this case, at some little but wholly insufficient +length, in order to show the power of natural selection, and likewise because +this is by far the most serious special difficulty, which my theory has +encountered. The case, also, is very interesting, as it proves that with +animals, as with plants, any amount of modification in structure can be +effected by the accumulation of numerous, slight, and as we must call them +accidental, variations, which are in any manner profitable, without exercise or +habit having come into play. For no amount of exercise, or habit, or volition, +in the utterly sterile members of a community could possibly have affected the +structure or instincts of the fertile members, which alone leave descendants. I +am surprised that no one has advanced this demonstrative case of neuter +insects, against the well-known doctrine of Lamarck. +</p> + +<p class="p2"> +<i>Summary</i>.—I have endeavoured briefly in this chapter to show that +the mental qualities of our domestic animals vary, and that the variations are +inherited. Still more briefly I have attempted to show that instincts +<a name="Page243"></a> +vary slightly in a state of nature. No one will dispute that instincts are of +the highest importance to each animal. Therefore I can see no difficulty, under +changing conditions of life, in natural selection accumulating slight +modifications of instinct to any extent, in any useful direction. In some cases +habit or use and disuse have probably come into play. I do not pretend that the +facts given in this chapter strengthen in any great degree my theory; but none +of the cases of difficulty, to the best of my judgment, annihilate it. On the +other hand, the fact that instincts are not always absolutely perfect and are +liable to mistakes;—that no instinct has been produced for the exclusive +good of other animals, but that each animal takes advantage of the instincts of +others;—that the canon in natural history, of “natura non facit +saltum” is applicable to instincts as well as to corporeal structure, and +is plainly explicable on the foregoing views, but is otherwise +inexplicable,—all tend to corroborate the theory of natural selection. +</p> + +<p> +This theory is, also, strengthened by some few other facts in regard to +instincts; as by that common case of closely allied, but certainly distinct, +species, when inhabiting distant parts of the world and living under +considerably different conditions of life, yet often retaining nearly the same +instincts. For instance, we can understand on the principle of inheritance, how +it is that the thrush of South America lines its nest with mud, in the same +peculiar manner as does our British thrush: how it is that the male wrens +(Troglodytes) of North America, build “cock-nests,” to roost in, +like the males of our distinct Kitty-wrens,—a habit wholly unlike that of +any other known bird. Finally, it may not be a logical deduction, but to my +imagination it is far more satisfactory to look at such instincts as the young +<a name="Page244"></a> +cuckoo ejecting its foster-brothers,—ants making slaves,—the larvæ +of ichneumonidæ feeding within the live bodies of caterpillars,—not as +specially endowed or created instincts, but as small consequences of one +general law, leading to the advancement of all organic beings, namely, +multiply, vary, let the strongest live and the weakest die. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page245"></a><a name="chap08"></a>CHAPTER VIII.<br /> +HYBRIDISM.</h2> + +<p class="letter"> +Distinction between the sterility of first crosses and of hybrids. Sterility +various in degree, not universal, affected by close interbreeding, removed by +domestication. Laws governing the sterility of hybrids. Sterility not a special +endowment, but incidental on other differences. Causes of the sterility of +first crosses and of hybrids. Parallelism between the effects of changed +conditions of life and crossing. Fertility of varieties when crossed and of +their mongrel offspring not universal. Hybrids and mongrels compared +independently of their fertility. Summary. +</p> + +<p> +The view generally entertained by naturalists is that species, when +intercrossed, have been specially endowed with the quality of sterility, in +order to prevent the confusion of all organic forms. This view certainly seems +at first probable, for species within the same country could hardly have kept +distinct had they been capable of crossing freely. The importance of the fact +that hybrids are very generally sterile, has, I think, been much underrated by +some late writers. On the theory of natural selection the case is especially +important, inasmuch as the sterility of hybrids could not possibly be of any +advantage to them, and therefore could not have been acquired by the continued +preservation of successive profitable degrees of sterility. I hope, however, to +be able to show that sterility is not a specially acquired or endowed quality, +but is incidental on other acquired differences. +</p> + +<p> +In treating this subject, two classes of facts, to a large extent fundamentally +different, have generally been confounded together; namely, the sterility of +two +<a name="Page246"></a> +species when first crossed, and the sterility of the hybrids produced from +them. +</p> + +<p> +Pure species have of course their organs of reproduction in a perfect +condition, yet when intercrossed they produce either few or no offspring. +Hybrids, on the other hand, have their reproductive organs functionally +impotent, as may be clearly seen in the state of the male element in both +plants and animals; though the organs themselves are perfect in structure, as +far as the microscope reveals. In the first case the two sexual elements which +go to form the embryo are perfect; in the second case they are either not at +all developed, or are imperfectly developed. This distinction is important, +when the cause of the sterility, which is common to the two cases, has to be +considered. The distinction has probably been slurred over, owing to the +sterility in both cases being looked on as a special endowment, beyond the +province of our reasoning powers. +</p> + +<p> +The fertility of varieties, that is of the forms known or believed to have +descended from common parents, when intercrossed, and likewise the fertility of +their mongrel offspring, is, on my theory, of equal importance with the +sterility of species; for it seems to make a broad and clear distinction +between varieties and species. +</p> + +<p> +First, for the sterility of species when crossed and of their hybrid offspring. +It is impossible to study the several memoirs and works of those two +conscientious and admirable observers, Kölreuter and Gärtner, who almost +devoted their lives to this subject, without being deeply impressed with the +high generality of some degree of sterility. Kölreuter makes the rule +universal; but then he cuts the knot, for in ten cases in which he found two +forms, considered by most authors as distinct species, quite fertile together, +he +<a name="Page247"></a> +unhesitatingly ranks them as varieties. Gärtner, also, makes the rule equally +universal; and he disputes the entire fertility of Kölreuter’s ten cases. +But in these and in many other cases, Gärtner is obliged carefully to count the +seeds, in order to show that there is any degree of sterility. He always +compares the maximum number of seeds produced by two species when crossed and +by their hybrid offspring, with the average number produced by both pure +parent-species in a state of nature. But a serious cause of error seems to me +to be here introduced: a plant to be hybridised must be castrated, and, what is +often more important, must be secluded in order to prevent pollen being brought +to it by insects from other plants. Nearly all the plants experimentised on by +Gärtner were potted, and apparently were kept in a chamber in his house. That +these processes are often injurious to the fertility of a plant cannot be +doubted; for Gärtner gives in his table about a score of cases of plants which +he castrated, and artificially fertilised with their own pollen, and (excluding +all cases such as the Leguminosæ, in which there is an acknowledged difficulty +in the manipulation) half of these twenty plants had their fertility in some +degree impaired. Moreover, as Gärtner during several years repeatedly crossed +the primrose and cowslip, which we have such good reason to believe to be +varieties, and only once or twice succeeded in getting fertile seed; as he +found the common red and blue pimpernels (Anagallis arvensis and coerulea), +which the best botanists rank as varieties, absolutely sterile together; and as +he came to the same conclusion in several other analogous cases; it seems to me +that we may well be permitted to doubt whether many other species are really so +sterile, when intercrossed, as Gärtner believes. +</p> + +<p> +<a name="Page248"></a> +It is certain, on the one hand, that the sterility of various species when +crossed is so different in degree and graduates away so insensibly, and, on the +other hand, that the fertility of pure species is so easily affected by various +circumstances, that for all practical purposes it is most difficult to say +where perfect fertility ends and sterility begins. I think no better evidence +of this can be required than that the two most experienced observers who have +ever lived, namely, Kölreuter and Gärtner, should have arrived at diametrically +opposite conclusions in regard to the very same species. It is also most +instructive to compare—but I have not space here to enter on +details—the evidence advanced by our best botanists on the question +whether certain doubtful forms should be ranked as species or varieties, with +the evidence from fertility adduced by different hybridisers, or by the same +author, from experiments made during different years. It can thus be shown that +neither sterility nor fertility affords any clear distinction between species +and varieties; but that the evidence from this source graduates away, and is +doubtful in the same degree as is the evidence derived from other +constitutional and structural differences. +</p> + +<p> +In regard to the sterility of hybrids in successive generations; though Gärtner +was enabled to rear some hybrids, carefully guarding them from a cross with +either pure parent, for six or seven, and in one case for ten generations, yet +he asserts positively that their fertility never increased, but generally +greatly decreased. I do not doubt that this is usually the case, and that the +fertility often suddenly decreases in the first few generations. Nevertheless I +believe that in all these experiments the fertility has been diminished by an +independent cause, namely, from close interbreeding. I have collected so large +a body of facts, showing +<a name="Page249"></a> +that close interbreeding lessens fertility, and, on the other hand, that an +occasional cross with a distinct individual or variety increases fertility, +that I cannot doubt the correctness of this almost universal belief amongst +breeders. Hybrids are seldom raised by experimentalists in great numbers; and +as the parent-species, or other allied hybrids, generally grow in the same +garden, the visits of insects must be carefully prevented during the flowering +season: hence hybrids will generally be fertilised during each generation by +their own individual pollen; and I am convinced that this would be injurious to +their fertility, already lessened by their hybrid origin. I am strengthened in +this conviction by a remarkable statement repeatedly made by Gärtner, namely, +that if even the less fertile hybrids be artificially fertilised with hybrid +pollen of the same kind, their fertility, notwithstanding the frequent ill +effects of manipulation, sometimes decidedly increases, and goes on increasing. +Now, in artificial fertilisation pollen is as often taken by chance (as I know +from my own experience) from the anthers of another flower, as from the anthers +of the flower itself which is to be fertilised; so that a cross between two +flowers, though probably on the same plant, would be thus effected. Moreover, +whenever complicated experiments are in progress, so careful an observer as +Gärtner would have castrated his hybrids, and this would have insured in each +generation a cross with the pollen from a distinct flower, either from the same +plant or from another plant of the same hybrid nature. And thus, the strange +fact of the increase of fertility in the successive generations of +<i>artificially fertilised</i> hybrids may, I believe, be accounted for by +close interbreeding having been avoided. +</p> + +<p> +Now let us turn to the results arrived at by the third most experienced +hybridiser, namely, the Honourable and +<a name="Page250"></a> +Reverend W. Herbert. He is as emphatic in his conclusion that some hybrids are +perfectly fertile—as fertile as the pure parent-species—as are +Kölreuter and Gärtner that some degree of sterility between distinct species is +a universal law of nature. He experimentised on some of the very same species +as did Gärtner. The difference in their results may, I think, be in part +accounted for by Herbert’s great horticultural skill, and by his having +hothouses at his command. Of his many important statements I will here give +only a single one as an example, namely, that “every ovule in a pod of +Crinum capense fertilised by C. revolutum produced a plant, which (he says) I +never saw to occur in a case of its natural fecundation.” So that we here +have perfect, or even more than commonly perfect, fertility in a first cross +between two distinct species. +</p> + +<p> +This case of the Crinum leads me to refer to a most singular fact, namely, that +there are individual plants, as with certain species of Lobelia, and with all +the species of the genus Hippeastrum, which can be far more easily fertilised +by the pollen of another and distinct species, than by their own pollen. For +these plants have been found to yield seed to the pollen of a distinct species, +though quite sterile with their own pollen, notwithstanding that their own +pollen was found to be perfectly good, for it fertilised distinct species. So +that certain individual plants and all the individuals of certain species can +actually be hybridised much more readily than they can be self-fertilised! For +instance, a bulb of Hippeastrum aulicum produced four flowers; three were +fertilised by Herbert with their own pollen, and the fourth was subsequently +fertilised by the pollen of a compound hybrid descended from three other and +distinct species: the result was that “the ovaries of the three first +flowers soon ceased to grow, and after a +<a name="Page251"></a> +few days perished entirely, whereas the pod impregnated by the pollen of the +hybrid made vigorous growth and rapid progress to maturity, and bore good seed, +which vegetated freely.” In a letter to me, in 1839, Mr. Herbert told me +that he had then tried the experiment during five years, and he continued to +try it during several subsequent years, and always with the same result. This +result has, also, been confirmed by other observers in the case of Hippeastrum +with its sub-genera, and in the case of some other genera, as Lobelia, +Passiflora and Verbascum. Although the plants in these experiments appeared +perfectly healthy, and although both the ovules and pollen of the same flower +were perfectly good with respect to other species, yet as they were +functionally imperfect in their mutual self-action, we must infer that the +plants were in an unnatural state. Nevertheless these facts show on what slight +and mysterious causes the lesser or greater fertility of species when crossed, +in comparison with the same species when self-fertilised, sometimes depends. +</p> + +<p> +The practical experiments of horticulturists, though not made with scientific +precision, deserve some notice. It is notorious in how complicated a manner the +species of Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron, etc., have +been crossed, yet many of these hybrids seed freely. For instance, Herbert +asserts that a hybrid from Calceolaria integrifolia and plantaginea, species +most widely dissimilar in general habit, “reproduced itself as perfectly +as if it had been a natural species from the mountains of Chile.” I have +taken some pains to ascertain the degree of fertility of some of the complex +crosses of Rhododendrons, and I am assured that many of them are perfectly +fertile. Mr. C. Noble, for instance, informs me that he raises stocks for +grafting from a hybrid +<a name="Page252"></a> +between Rhododendron Ponticum and Catawbiense, and that this hybrid +“seeds as freely as it is possible to imagine.” Had hybrids, when +fairly treated, gone on decreasing in fertility in each successive generation, +as Gärtner believes to be the case, the fact would have been notorious to +nurserymen. Horticulturists raise large beds of the same hybrids, and such +alone are fairly treated, for by insect agency the several individuals of the +same hybrid variety are allowed to freely cross with each other, and the +injurious influence of close interbreeding is thus prevented. Any one may +readily convince himself of the efficiency of insect-agency by examining the +flowers of the more sterile kinds of hybrid rhododendrons, which produce no +pollen, for he will find on their stigmas plenty of pollen brought from other +flowers. +</p> + +<p> +In regard to animals, much fewer experiments have been carefully tried than +with plants. If our systematic arrangements can be trusted, that is if the +genera of animals are as distinct from each other, as are the genera of plants, +then we may infer that animals more widely separated in the scale of nature can +be more easily crossed than in the case of plants; but the hybrids themselves +are, I think, more sterile. I doubt whether any case of a perfectly fertile +hybrid animal can be considered as thoroughly well authenticated. It should, +however, be borne in mind that, owing to few animals breeding freely under +confinement, few experiments have been fairly tried: for instance, the +canary-bird has been crossed with nine other finches, but as not one of these +nine species breeds freely in confinement, we have no right to expect that the +first crosses between them and the canary, or that their hybrids, should be +perfectly fertile. Again, with respect to the fertility in successive +generations of the more fertile +<a name="Page253"></a> +hybrid animals, I hardly know of an instance in which two families of the same +hybrid have been raised at the same time from different parents, so as to avoid +the ill effects of close interbreeding. On the contrary, brothers and sisters +have usually been crossed in each successive generation, in opposition to the +constantly repeated admonition of every breeder. And in this case, it is not at +all surprising that the inherent sterility in the hybrids should have gone on +increasing. If we were to act thus, and pair brothers and sisters in the case +of any pure animal, which from any cause had the least tendency to sterility, +the breed would assuredly be lost in a very few generations. +</p> + +<p> +Although I do not know of any thoroughly well-authenticated cases of perfectly +fertile hybrid animals, I have some reason to believe that the hybrids from +Cervulus vaginalis and Reevesii, and from Phasianus colchicus with P. torquatus +and with P. versicolor are perfectly fertile. The hybrids from the common and +Chinese geese (A. cygnoides), species which are so different that they are +generally ranked in distinct genera, have often bred in this country with +either pure parent, and in one single instance they have bred <i>inter se</i>. +This was effected by Mr. Eyton, who raised two hybrids from the same parents +but from different hatches; and from these two birds he raised no less than +eight hybrids (grandchildren of the pure geese) from one nest. In India, +however, these cross-bred geese must be far more fertile; for I am assured by +two eminently capable judges, namely Mr. Blyth and Capt. Hutton, that whole +flocks of these crossed geese are kept in various parts of the country; and as +they are kept for profit, where neither pure parent-species exists, they must +certainly be highly fertile. +</p> + +<p> +A doctrine which originated with Pallas, has been +<a name="Page254"></a> +largely accepted by modern naturalists; namely, that most of our domestic +animals have descended from two or more aboriginal species, since commingled by +intercrossing. On this view, the aboriginal species must either at first have +produced quite fertile hybrids, or the hybrids must have become in subsequent +generations quite fertile under domestication. This latter alternative seems to +me the most probable, and I am inclined to believe in its truth, although it +rests on no direct evidence. I believe, for instance, that our dogs have +descended from several wild stocks; yet, with perhaps the exception of certain +indigenous domestic dogs of South America, all are quite fertile together; and +analogy makes me greatly doubt, whether the several aboriginal species would at +first have freely bred together and have produced quite fertile hybrids. So +again there is reason to believe that our European and the humped Indian cattle +are quite fertile together; but from facts communicated to me by Mr. Blyth, I +think they must be considered as distinct species. On this view of the origin +of many of our domestic animals, we must either give up the belief of the +almost universal sterility of distinct species of animals when crossed; or we +must look at sterility, not as an indelible characteristic, but as one capable +of being removed by domestication. +</p> + +<p> +Finally, looking to all the ascertained facts on the intercrossing of plants +and animals, it may be concluded that some degree of sterility, both in first +crosses and in hybrids,is an extremely general result; but that it cannot, +under our present state of knowledge, be considered as absolutely universal. +</p> + +<p class="p2"> +<i>Laws governing the Sterility of first Crosses and of Hybrids</i>.—We +will now consider a little more in detail the +<a name="Page255"></a> +circumstances and rules governing the sterility of first crosses and of +hybrids. Our chief object will be to see whether or not the rules indicate that +species have specially been endowed with this quality, in order to prevent +their crossing and blending together in utter confusion. The following rules +and conclusions are chiefly drawn up from Gärtner’s admirable work on the +hybridisation of plants. I have taken much pains to ascertain how far the rules +apply to animals, and considering how scanty our knowledge is in regard to +hybrid animals, I have been surprised to find how generally the same rules +apply to both kingdoms. +</p> + +<p> +It has been already remarked, that the degree of fertility, both of first +crosses and of hybrids, graduates from zero to perfect fertility. It is +surprising in how many curious ways this gradation can be shown to exist; but +only the barest outline of the facts can here be given. When pollen from a +plant of one family is placed on the stigma of a plant of a distinct family, it +exerts no more influence than so much inorganic dust. From this absolute zero +of fertility, the pollen of different species of the same genus applied to the +stigma of some one species, yields a perfect gradation in the number of seeds +produced, up to nearly complete or even quite complete fertility; and, as we +have seen, in certain abnormal cases, even to an excess of fertility, beyond +that which the plant’s own pollen will produce. So in hybrids themselves, +there are some which never have produced, and probably never would produce, +even with the pollen of either pure parent, a single fertile seed: but in some +of these cases a first trace of fertility may be detected, by the pollen of one +of the pure parent-species causing the flower of the hybrid to wither earlier +than it otherwise would have done; and the early withering of the flower is +well known to be a sign +<a name="Page256"></a> +of incipient fertilisation. From this extreme degree of sterility we have +self-fertilised hybrids producing a greater and greater number of seeds up to +perfect fertility. +</p> + +<p> +Hybrids from two species which are very difficult to cross, and which rarely +produce any offspring, are generally very sterile; but the parallelism between +the difficulty of making a first cross, and the sterility of the hybrids thus +produced—two classes of facts which are generally confounded +together—is by no means strict. There are many cases, in which two pure +species can be united with unusual facility, and produce numerous +hybrid-offspring, yet these hybrids are remarkably sterile. On the other hand, +there are species which can be crossed very rarely, or with extreme difficulty, +but the hybrids, when at last produced, are very fertile. Even within the +limits of the same genus, for instance in Dianthus, these two opposite cases +occur. +</p> + +<p> +The fertility, both of first crosses and of hybrids, is more easily affected by +unfavourable conditions, than is the fertility of pure species. But the degree +of fertility is likewise innately variable; for it is not always the same when +the same two species are crossed under the same circumstances, but depends in +part upon the constitution of the individuals which happen to have been chosen +for the experiment. So it is with hybrids, for their degree of fertility is +often found to differ greatly in the several individuals raised from seed out +of the same capsule and exposed to exactly the same conditions. +</p> + +<p> +By the term systematic affinity is meant, the resemblance between species in +structure and in constitution, more especially in the structure of parts which +are of high physiological importance and which differ little in the allied +species. Now the fertility of first crosses +<a name="Page257"></a> +between species, and of the hybrids produced from them, is largely governed by +their systematic affinity. This is clearly shown by hybrids never having been +raised between species ranked by systematists in distinct families; and on the +other hand, by very closely allied species generally uniting with facility. But +the correspondence between systematic affinity and the facility of crossing is +by no means strict. A multitude of cases could be given of very closely allied +species which will not unite, or only with extreme difficulty; and on the other +hand of very distinct species which unite with the utmost facility. In the same +family there may be a genus, as Dianthus, in which very many species can most +readily be crossed; and another genus, as Silene, in which the most persevering +efforts have failed to produce between extremely close species a single hybrid. +Even within the limits of the same genus, we meet with this same difference; +for instance, the many species of Nicotiana have been more largely crossed than +the species of almost any other genus; but Gärtner found that N. acuminata, +which is not a particularly distinct species, obstinately failed to fertilise, +or to be fertilised by, no less than eight other species of Nicotiana. Very +many analogous facts could be given. +</p> + +<p> +No one has been able to point out what kind, or what amount, of difference in +any recognisable character is sufficient to prevent two species crossing. It +can be shown that plants most widely different in habit and general appearance, +and having strongly marked differences in every part of the flower, even in the +pollen, in the fruit, and in the cotyledons, can be crossed. Annual and +perennial plants, deciduous and evergreen trees, plants inhabiting different +stations and fitted for extremely different climates, can often be crossed with +ease. +</p> + +<p> +<a name="Page258"></a> +By a reciprocal cross between two species, I mean the case, for instance, of a +stallion-horse being first crossed with a female-ass, and then a male-ass with +a mare: these two species may then be said to have been reciprocally crossed. +There is often the widest possible difference in the facility of making +reciprocal crosses. Such cases are highly important, for they prove that the +capacity in any two species to cross is often completely independent of their +systematic affinity, or of any recognisable difference in their whole +organisation. On the other hand, these cases clearly show that the capacity for +crossing is connected with constitutional differences imperceptible by us, and +confined to the reproductive system. This difference in the result of +reciprocal crosses between the same two species was long ago observed by +Kölreuter. To give an instance: Mirabilis jalappa can easily be fertilised by +the pollen of M. longiflora, and the hybrids thus produced are sufficiently +fertile; but Kölreuter tried more than two hundred times, during eight +following years, to fertilise reciprocally M. longiflora with the pollen of M. +jalappa, and utterly failed. Several other equally striking cases could be +given. Thuret has observed the same fact with certain sea-weeds or Fuci. +Gärtner, moreover, found that this difference of facility in making reciprocal +crosses is extremely common in a lesser degree. He has observed it even between +forms so closely related (as Matthiola annua and glabra) that many botanists +rank them only as varieties. It is also a remarkable fact, that hybrids raised +from reciprocal crosses, though of course compounded of the very same two +species, the one species having first been used as the father and then as the +mother, generally differ in fertility in a small, and occasionally in a high +degree. +</p> + +<p> +Several other singular rules could be given from +<a name="Page259"></a> +Gärtner: for instance, some species have a remarkable power of crossing with +other species; other species of the same genus have a remarkable power of +impressing their likeness on their hybrid offspring; but these two powers do +not at all necessarily go together. There are certain hybrids which instead of +having, as is usual, an intermediate character between their two parents, +always closely resemble one of them; and such hybrids, though externally so +like one of their pure parent-species, are with rare exceptions extremely +sterile. So again amongst hybrids which are usually intermediate in structure +between their parents, exceptional and abnormal individuals sometimes are born, +which closely resemble one of their pure parents; and these hybrids are almost +always utterly sterile, even when the other hybrids raised from seed from the +same capsule have a considerable degree of fertility. These facts show how +completely fertility in the hybrid is independent of its external resemblance +to either pure parent. +</p> + +<p> +Considering the several rules now given, which govern the fertility of first +crosses and of hybrids, we see that when forms, which must be considered as +good and distinct species, are united, their fertility graduates from zero to +perfect fertility, or even to fertility under certain conditions in excess. +That their fertility, besides being eminently susceptible to favourable and +unfavourable conditions, is innately variable. That it is by no means always +the same in degree in the first cross and in the hybrids produced from this +cross. That the fertility of hybrids is not related to the degree in which they +resemble in external appearance either parent. And lastly, that the facility of +making a first cross between any two species is not always governed by their +systematic affinity or +<a name="Page260"></a> +degree of resemblance to each other. This latter statement is clearly proved by +reciprocal crosses between the same two species, for according as the one +species or the other is used as the father or the mother, there is generally +some difference, and occasionally the widest possible difference, in the +facility of effecting an union. The hybrids, moreover, produced from reciprocal +crosses often differ in fertility. +</p> + +<p> +Now do these complex and singular rules indicate that species have been endowed +with sterility simply to prevent their becoming confounded in nature? I think +not. For why should the sterility be so extremely different in degree, when +various species are crossed, all of which we must suppose it would be equally +important to keep from blending together? Why should the degree of sterility be +innately variable in the individuals of the same species? Why should some +species cross with facility, and yet produce very sterile hybrids; and other +species cross with extreme difficulty, and yet produce fairly fertile hybrids? +Why should there often be so great a difference in the result of a reciprocal +cross between the same two species? Why, it may even be asked, has the +production of hybrids been permitted? to grant to species the special power of +producing hybrids, and then to stop their further propagation by different +degrees of sterility, not strictly related to the facility of the first union +between their parents, seems to be a strange arrangement. +</p> + +<p> +The foregoing rules and facts, on the other hand, appear to me clearly to +indicate that the sterility both of first crosses and of hybrids is simply +incidental or dependent on unknown differences, chiefly in the reproductive +systems, of the species which are crossed. The differences being of so peculiar +and limited a nature, +<a name="Page261"></a> +that, in reciprocal crosses between two species the male sexual element of the +one will often freely act on the female sexual element of the other, but not in +a reversed direction. It will be advisable to explain a little more fully by an +example what I mean by sterility being incidental on other differences, and not +a specially endowed quality. As the capacity of one plant to be grafted or +budded on another is so entirely unimportant for its welfare in a state of +nature, I presume that no one will suppose that this capacity is a +<i>specially</i> endowed quality, but will admit that it is incidental on +differences in the laws of growth of the two plants. We can sometimes see the +reason why one tree will not take on another, from differences in their rate of +growth, in the hardness of their wood, in the period of the flow or nature of +their sap, etc.; but in a multitude of cases we can assign no reason whatever. +Great diversity in the size of two plants, one being woody and the other +herbaceous, one being evergreen and the other deciduous, and adaptation to +widely different climates, does not always prevent the two grafting together. +As in hybridisation, so with grafting, the capacity is limited by systematic +affinity, for no one has been able to graft trees together belonging to quite +distinct families; and, on the other hand, closely allied species, and +varieties of the same species, can usually, but not invariably, be grafted with +ease. But this capacity, as in hybridisation, is by no means absolutely +governed by systematic affinity. Although many distinct genera within the same +family have been grafted together, in other cases species of the same genus +will not take on each other. The pear can be grafted far more readily on the +quince, which is ranked as a distinct genus, than on the apple, which is a +member of the same genus. Even different varieties of the pear take +<a name="Page262"></a> +with different degrees of facility on the quince; so do different varieties of +the apricot and peach on certain varieties of the plum. +</p> + +<p> +As Gärtner found that there was sometimes an innate difference in different +<i>individuals</i> of the same two species in crossing; so Sagaret believes +this to be the case with different individuals of the same two species in being +grafted together. As in reciprocal crosses, the facility of effecting an union +is often very far from equal, so it sometimes is in grafting; the common +gooseberry, for instance, cannot be grafted on the currant, whereas the currant +will take, though with difficulty, on the gooseberry. +</p> + +<p> +We have seen that the sterility of hybrids, which have their reproductive +organs in an imperfect condition, is a very different case from the difficulty +of uniting two pure species, which have their reproductive organs perfect; yet +these two distinct cases run to a certain extent parallel. Something analogous +occurs in grafting; for Thouin found that three species of Robinia, which +seeded freely on their own roots, and which could be grafted with no great +difficulty on another species, when thus grafted were rendered barren. On the +other hand, certain species of Sorbus, when grafted on other species, yielded +twice as much fruit as when on their own roots. We are reminded by this latter +fact of the extraordinary case of Hippeastrum, Lobelia, etc., which seeded much +more freely when fertilised with the pollen of distinct species, than when +self-fertilised with their own pollen. +</p> + +<p> +We thus see, that although there is a clear and fundamental difference between +the mere adhesion of grafted stocks, and the union of the male and female +elements in the act of reproduction, yet that there is a rude degree of +parallelism in the results of grafting and +<a name="Page263"></a> +of crossing distinct species. And as we must look at the curious and complex +laws governing the facility with which trees can be grafted on each other as +incidental on unknown differences in their vegetative systems, so I believe +that the still more complex laws governing the facility of first crosses, are +incidental on unknown differences, chiefly in their reproductive systems. These +differences, in both cases, follow to a certain extent, as might have been +expected, systematic affinity, by which every kind of resemblance and +dissimilarity between organic beings is attempted to be expressed. The facts by +no means seem to me to indicate that the greater or lesser difficulty of either +grafting or crossing together various species has been a special endowment; +although in the case of crossing, the difficulty is as important for the +endurance and stability of specific forms, as in the case of grafting it is +unimportant for their welfare. +</p> + +<p class="p2"> +<i>Causes of the Sterility of first Crosses and of Hybrids</i>.—We may +now look a little closer at the probable causes of the sterility of first +crosses and of hybrids. These two cases are fundamentally different, for, as +just remarked, in the union of two pure species the male and female sexual +elements are perfect, whereas in hybrids they are imperfect. Even in first +crosses, the greater or lesser difficulty in effecting a union apparently +depends on several distinct causes. There must sometimes be a physical +impossibility in the male element reaching the ovule, as would be the case with +a plant having a pistil too long for the pollen-tubes to reach the ovarium. It +has also been observed that when pollen of one species is placed on the stigma +of a distantly allied species, though the pollen-tubes protrude, they do not +penetrate the stigmatic surface. Again, the +<a name="Page264"></a> +male element may reach the female element, but be incapable of causing an +embryo to be developed, as seems to have been the case with some of +Thuret’s experiments on Fuci. No explanation can be given of these facts, +any more than why certain trees cannot be grafted on others. Lastly, an embryo +may be developed, and then perish at an early period. This latter alternative +has not been sufficiently attended to; but I believe, from observations +communicated to me by Mr. Hewitt, who has had great experience in hybridising +gallinaceous birds, that the early death of the embryo is a very frequent cause +of sterility in first crosses. I was at first very unwilling to believe in this +view; as hybrids, when once born, are generally healthy and long-lived, as we +see in the case of the common mule. Hybrids, however, are differently +circumstanced before and after birth: when born and living in a country where +their two parents can live, they are generally placed under suitable conditions +of life. But a hybrid partakes of only half of the nature and constitution of +its mother, and therefore before birth, as long as it is nourished within its +mother’s womb or within the egg or seed produced by the mother, it may be +exposed to conditions in some degree unsuitable, and consequently be liable to +perish at an early period; more especially as all very young beings seem +eminently sensitive to injurious or unnatural conditions of life. +</p> + +<p> +In regard to the sterility of hybrids, in which the sexual elements are +imperfectly developed, the case is very different. I have more than once +alluded to a large body of facts, which I have collected, showing that when +animals and plants are removed from their natural conditions, they are +extremely liable to have their reproductive systems seriously affected. This, +in fact, is +<a name="Page265"></a> +the great bar to the domestication of animals. Between the sterility thus +superinduced and that of hybrids, there are many points of similarity. In both +cases the sterility is independent of general health, and is often accompanied +by excess of size or great luxuriance. In both cases, the sterility occurs in +various degrees; in both, the male element is the most liable to be affected; +but sometimes the female more than the male. In both, the tendency goes to a +certain extent with systematic affinity, for whole groups of animals and plants +are rendered impotent by the same unnatural conditions; and whole groups of +species tend to produce sterile hybrids. On the other hand, one species in a +group will sometimes resist great changes of conditions with unimpaired +fertility; and certain species in a group will produce unusually fertile +hybrids. No one can tell, till he tries, whether any particular animal will +breed under confinement or any plant seed freely under culture; nor can he +tell, till he tries, whether any two species of a genus will produce more or +less sterile hybrids. Lastly, when organic beings are placed during several +generations under conditions not natural to them, they are extremely liable to +vary, which is due, as I believe, to their reproductive systems having been +specially affected, though in a lesser degree than when sterility ensues. So it +is with hybrids, for hybrids in successive generations are eminently liable to +vary, as every experimentalist has observed. +</p> + +<p> +Thus we see that when organic beings are placed under new and unnatural +conditions, and when hybrids are produced by the unnatural crossing of two +species, the reproductive system, independently of the general state of health, +is affected by sterility in a very similar manner. In the one case, the +conditions of life have been disturbed, though often in so slight a degree as +to +<a name="Page266"></a> +be inappreciable by us; in the other case, or that of hybrids, the external +conditions have remained the same, but the organisation has been disturbed by +two different structures and constitutions having been blended into one. For it +is scarcely possible that two organisations should be compounded into one, +without some disturbance occurring in the development, or periodical action, or +mutual relation of the different parts and organs one to another, or to the +conditions of life. When hybrids are able to breed <i>inter se</i>, they +transmit to their offspring from generation to generation the same compounded +organisation, and hence we need not be surprised that their sterility, though +in some degree variable, rarely diminishes. +</p> + +<p> +It must, however, be confessed that we cannot understand, excepting on vague +hypotheses, several facts with respect to the sterility of hybrids; for +instance, the unequal fertility of hybrids produced from reciprocal crosses; or +the increased sterility in those hybrids which occasionally and exceptionally +resemble closely either pure parent. Nor do I pretend that the foregoing +remarks go to the root of the matter: no explanation is offered why an +organism, when placed under unnatural conditions, is rendered sterile. All that +I have attempted to show, is that in two cases, in some respects allied, +sterility is the common result,—in the one case from the conditions of +life having been disturbed, in the other case from the organisation having been +disturbed by two organisations having been compounded into one. +</p> + +<p> +It may seem fanciful, but I suspect that a similar parallelism extends to an +allied yet very different class of facts. It is an old and almost universal +belief, founded, I think, on a considerable body of evidence, that slight +changes in the conditions of life are beneficial to all living things. We see +this acted on by +<a name="Page267"></a> +farmers and gardeners in their frequent exchanges of seed, tubers, etc., from +one soil or climate to another, and back again. During the convalescence of +animals, we plainly see that great benefit is derived from almost any change in +the habits of life. Again, both with plants and animals, there is abundant +evidence, that a cross between very distinct individuals of the same species, +that is between members of different strains or sub-breeds, gives vigour and +fertility to the offspring. I believe, indeed, from the facts alluded to in our +fourth chapter, that a certain amount of crossing is indispensable even with +hermaphrodites; and that close interbreeding continued during several +generations between the nearest relations, especially if these be kept under +the same conditions of life, always induces weakness and sterility in the +progeny. +</p> + +<p> +Hence it seems that, on the one hand, slight changes in the conditions of life +benefit all organic beings, and on the other hand, that slight crosses, that is +crosses between the males and females of the same species which have varied and +become slightly different, give vigour and fertility to the offspring. But we +have seen that greater changes, or changes of a particular nature, often render +organic beings in some degree sterile; and that greater crosses, that is +crosses between males and females which have become widely or specifically +different, produce hybrids which are generally sterile in some degree. I cannot +persuade myself that this parallelism is an accident or an illusion. Both +series of facts seem to be connected together by some common but unknown bond, +which is essentially related to the principle of life. +</p> + +<p class="p2"> +<i>Fertility of Varieties when crossed, and of their Mongrel +offspring</i>.—It may be urged, as a most forcible argument, +<a name="Page268"></a> +that there must be some essential distinction between species and varieties, +and that there must be some error in all the foregoing remarks, inasmuch as +varieties, however much they may differ from each other in external appearance, +cross with perfect facility, and yield perfectly fertile offspring. I fully +admit that this is almost invariably the case. But if we look to varieties +produced under nature, we are immediately involved in hopeless difficulties; +for if two hitherto reputed varieties be found in any degree sterile together, +they are at once ranked by most naturalists as species. For instance, the blue +and red pimpernel, the primrose and cowslip, which are considered by many of +our best botanists as varieties, are said by Gärtner not to be quite fertile +when crossed, and he consequently ranks them as undoubted species. If we thus +argue in a circle, the fertility of all varieties produced under nature will +assuredly have to be granted. +</p> + +<p> +If we turn to varieties, produced, or supposed to have been produced, under +domestication, we are still involved in doubt. For when it is stated, for +instance, that the German Spitz dog unites more easily than other dogs with +foxes, or that certain South American indigenous domestic dogs do not readily +cross with European dogs, the explanation which will occur to everyone, and +probably the true one, is that these dogs have descended from several +aboriginally distinct species. Nevertheless the perfect fertility of so many +domestic varieties, differing widely from each other in appearance, for +instance of the pigeon or of the cabbage, is a remarkable fact; more especially +when we reflect how many species there are, which, though resembling each other +most closely, are utterly sterile when intercrossed. Several considerations, +however, render the fertility of domestic varieties less remarkable than +<a name="Page269"></a> +at first appears. It can, in the first place, be clearly shown that mere +external dissimilarity between two species does not determine their greater or +lesser degree of sterility when crossed; and we may apply the same rule to +domestic varieties. In the second place, some eminent naturalists believe that +a long course of domestication tends to eliminate sterility in the successive +generations of hybrids, which were at first only slightly sterile; and if this +be so, we surely ought not to expect to find sterility both appearing and +disappearing under nearly the same conditions of life. Lastly, and this seems +to me by far the most important consideration, new races of animals and plants +are produced under domestication by man’s methodical and unconscious +power of selection, for his own use and pleasure: he neither wishes to select, +nor could select, slight differences in the reproductive system, or other +constitutional differences correlated with the reproductive system. He supplies +his several varieties with the same food; treats them in nearly the same +manner, and does not wish to alter their general habits of life. Nature acts +uniformly and slowly during vast periods of time on the whole organisation, in +any way which may be for each creature’s own good; and thus she may, +either directly, or more probably indirectly, through correlation, modify the +reproductive system in the several descendants from any one species. Seeing +this difference in the process of selection, as carried on by man and nature, +we need not be surprised at some difference in the result. +</p> + +<p> +I have as yet spoken as if the varieties of the same species were invariably +fertile when intercrossed. But it seems to me impossible to resist the evidence +of the existence of a certain amount of sterility in the few following cases, +which I will briefly abstract. The evidence is at least as good as that from +which we believe +<a name="Page270"></a> +in the sterility of a multitude of species. The evidence is, also, derived from +hostile witnesses, who in all other cases consider fertility and sterility as +safe criterions of specific distinction. Gärtner kept during several years a +dwarf kind of maize with yellow seeds, and a tall variety with red seeds, +growing near each other in his garden; and although these plants have separated +sexes, they never naturally crossed. He then fertilised thirteen flowers of the +one with the pollen of the other; but only a single head produced any seed, and +this one head produced only five grains. Manipulation in this case could not +have been injurious, as the plants have separated sexes. No one, I believe, has +suspected that these varieties of maize are distinct species; and it is +important to notice that the hybrid plants thus raised were themselves +<i>perfectly</i> fertile; so that even Gärtner did not venture to consider the +two varieties as specifically distinct. +</p> + +<p> +Girou de Buzareingues crossed three varieties of gourd, which like the maize +has separated sexes, and he asserts that their mutual fertilisation is by so +much the less easy as their differences are greater. How far these experiments +may be trusted, I know not; but the forms experimentised on, are ranked by +Sagaret, who mainly founds his classification by the test of infertility, as +varieties. +</p> + +<p> +The following case is far more remarkable, and seems at first quite incredible; +but it is the result of an astonishing number of experiments made during many +years on nine species of Verbascum, by so good an observer and so hostile a +witness, as Gärtner: namely, that yellow and white varieties of the same +species of Verbascum when intercrossed produce less seed, than do either +coloured varieties when fertilised with pollen from their own coloured flowers. +Moreover, he asserts that when +<a name="Page271"></a> +yellow and white varieties of one species are crossed with yellow and white +varieties of a <i>distinct</i> species, more seed is produced by the crosses +between the same coloured flowers, than between those which are differently +coloured. Yet these varieties of Verbascum present no other difference besides +the mere colour of the flower; and one variety can sometimes be raised from the +seed of the other. +</p> + +<p> +From observations which I have made on certain varieties of hollyhock, I am +inclined to suspect that they present analogous facts. +</p> + +<p> +Kölreuter, whose accuracy has been confirmed by every subsequent observer, has +proved the remarkable fact, that one variety of the common tobacco is more +fertile, when crossed with a widely distinct species, than are the other +varieties. He experimentised on five forms, which are commonly reputed to be +varieties, and which he tested by the severest trial, namely, by reciprocal +crosses, and he found their mongrel offspring perfectly fertile. But one of +these five varieties, when used either as father or mother, and crossed with +the Nicotiana glutinosa, always yielded hybrids not so sterile as those which +were produced from the four other varieties when crossed with N. glutinosa. +Hence the reproductive system of this one variety must have been in some manner +and in some degree modified. +</p> + +<p> +From these facts; from the great difficulty of ascertaining the infertility of +varieties in a state of nature, for a supposed variety if infertile in any +degree would generally be ranked as species; from man selecting only external +characters in the production of the most distinct domestic varieties, and from +not wishing or being able to produce recondite and functional differences in +the reproductive system; from these several considerations and facts, I do not +think that the very general +<a name="Page272"></a> +fertility of varieties can be proved to be of universal occurrence, or to form +a fundamental distinction between varieties and species. The general fertility +of varieties does not seem to me sufficient to overthrow the view which I have +taken with respect to the very general, but not invariable, sterility of first +crosses and of hybrids, namely, that it is not a special endowment, but is +incidental on slowly acquired modifications, more especially in the +reproductive systems of the forms which are crossed. +</p> + +<p class="p2"> +<i>Hybrids and Mongrels compared, independently of their +fertility</i>.—Independently of the question of fertility, the offspring +of species when crossed and of varieties when crossed may be compared in +several other respects. Gärtner, whose strong wish was to draw a marked line of +distinction between species and varieties, could find very few and, as it seems +to me, quite unimportant differences between the so-called hybrid offspring of +species, and the so-called mongrel offspring of varieties. And, on the other +hand, they agree most closely in very many important respects. +</p> + +<p> +I shall here discuss this subject with extreme brevity. The most important +distinction is, that in the first generation mongrels are more variable than +hybrids; but Gärtner admits that hybrids from species which have long been +cultivated are often variable in the first generation; and I have myself seen +striking instances of this fact. Gärtner further admits that hybrids between +very closely allied species are more variable than those from very distinct +species; and this shows that the difference in the degree of variability +graduates away. When mongrels and the more fertile hybrids are propagated for +several generations an extreme amount of variability in their offspring is +notorious; +<a name="Page273"></a> +but some few cases both of hybrids and mongrels long retaining uniformity of +character could be given. The variability, however, in the successive +generations of mongrels is, perhaps, greater than in hybrids. +</p> + +<p> +This greater variability of mongrels than of hybrids does not seem to me at all +surprising. For the parents of mongrels are varieties, and mostly domestic +varieties (very few experiments having been tried on natural varieties), and +this implies in most cases that there has been recent variability; and +therefore we might expect that such variability would often continue and be +super-added to that arising from the mere act of crossing. The slight degree of +variability in hybrids from the first cross or in the first generation, in +contrast with their extreme variability in the succeeding generations, is a +curious fact and deserves attention. For it bears on and corroborates the view +which I have taken on the cause of ordinary variability; namely, that it is due +to the reproductive system being eminently sensitive to any change in the +conditions of life, being thus often rendered either impotent or at least +incapable of its proper function of producing offspring identical with the +parent-form. Now hybrids in the first generation are descended from species +(excluding those long cultivated) which have not had their reproductive systems +in any way affected, and they are not variable; but hybrids themselves have +their reproductive systems seriously affected, and their descendants are highly +variable. +</p> + +<p> +But to return to our comparison of mongrels and hybrids: Gärtner states that +mongrels are more liable than hybrids to revert to either parent-form; but +this, if it be true, is certainly only a difference in degree. Gärtner further +insists that when any two species, although most closely allied to each other, +are +<a name="Page274"></a> +crossed with a third species, the hybrids are widely different from each other; +whereas if two very distinct varieties of one species are crossed with another +species, the hybrids do not differ much. But this conclusion, as far as I can +make out, is founded on a single experiment; and seems directly opposed to the +results of several experiments made by Kölreuter. +</p> + +<p> +These alone are the unimportant differences, which Gärtner is able to point +out, between hybrid and mongrel plants. On the other hand, the resemblance in +mongrels and in hybrids to their respective parents, more especially in hybrids +produced from nearly related species, follows according to Gärtner the same +laws. When two species are crossed, one has sometimes a prepotent power of +impressing its likeness on the hybrid; and so I believe it to be with varieties +of plants. With animals one variety certainly often has this prepotent power +over another variety. Hybrid plants produced from a reciprocal cross, generally +resemble each other closely; and so it is with mongrels from a reciprocal +cross. Both hybrids and mongrels can be reduced to either pure parent-form, by +repeated crosses in successive generations with either parent. +</p> + +<p> +These several remarks are apparently applicable to animals; but the subject is +here excessively complicated, partly owing to the existence of secondary sexual +characters; but more especially owing to prepotency in transmitting likeness +running more strongly in one sex than in the other, both when one species is +crossed with another, and when one variety is crossed with another variety. For +instance, I think those authors are right, who maintain that the ass has a +prepotent power over the horse, so that both the mule and the hinny more +resemble the ass than the horse; but that the prepotency runs more strongly in +the male-ass than in +<a name="Page275"></a> +the female, so that the mule, which is the offspring of the male-ass and mare, +is more like an ass, than is the hinny, which is the offspring of the +female-ass and stallion. +</p> + +<p> +Much stress has been laid by some authors on the supposed fact, that mongrel +animals alone are born closely like one of their parents; but it can be shown +that this does sometimes occur with hybrids; yet I grant much less frequently +with hybrids than with mongrels. Looking to the cases which I have collected of +cross-bred animals closely resembling one parent, the resemblances seem chiefly +confined to characters almost monstrous in their nature, and which have +suddenly appeared—such as albinism, melanism, deficiency of tail or +horns, or additional fingers and toes; and do not relate to characters which +have been slowly acquired by selection. Consequently, sudden reversions to the +perfect character of either parent would be more likely to occur with mongrels, +which are descended from varieties often suddenly produced and semi-monstrous +in character, than with hybrids, which are descended from species slowly and +naturally produced. On the whole I entirely agree with Dr. Prosper Lucas, who, +after arranging an enormous body of facts with respect to animals, comes to the +conclusion, that the laws of resemblance of the child to its parents are the +same, whether the two parents differ much or little from each other, namely in +the union of individuals of the same variety, or of different varieties, or of +distinct species. +</p> + +<p> +Laying aside the question of fertility and sterility, in all other respects +there seems to be a general and close similarity in the offspring of crossed +species, and of crossed varieties. If we look at species as having been +specially created, and at varieties as having been produced by secondary laws, +this similarity would be an +<a name="Page276"></a> +astonishing fact. But it harmonises perfectly with the view that there is no +essential distinction between species and varieties. +</p> + +<p class="p2"> +<i>Summary of Chapter</i>.—First crosses between forms sufficiently +distinct to be ranked as species, and their hybrids, are very generally, but +not universally, sterile. The sterility is of all degrees, and is often so +slight that the two most careful experimentalists who have ever lived, have +come to diametrically opposite conclusions in ranking forms by this test. The +sterility is innately variable in individuals of the same species, and is +eminently susceptible of favourable and unfavourable conditions. The degree of +sterility does not strictly follow systematic affinity, but is governed by +several curious and complex laws. It is generally different, and sometimes +widely different, in reciprocal crosses between the same two species. It is not +always equal in degree in a first cross and in the hybrid produced from this +cross. +</p> + +<p> +In the same manner as in grafting trees, the capacity of one species or variety +to take on another, is incidental on generally unknown differences in their +vegetative systems, so in crossing, the greater or less facility of one species +to unite with another, is incidental on unknown differences in their +reproductive systems. There is no more reason to think that species have been +specially endowed with various degrees of sterility to prevent them crossing +and blending in nature, than to think that trees have been specially endowed +with various and somewhat analogous degrees of difficulty in being grafted +together in order to prevent them becoming inarched in our forests. +</p> + +<p> +The sterility of first crosses between pure species, which have their +reproductive systems perfect, seems +<a name="Page277"></a> +to depend on several circumstances; in some cases largely on the early death of +the embryo. The sterility of hybrids, which have their reproductive systems +imperfect, and which have had this system and their whole organisation +disturbed by being compounded of two distinct species, seems closely allied to +that sterility which so frequently affects pure species, when their natural +conditions of life have been disturbed. This view is supported by a parallelism +of another kind;—namely, that the crossing of forms only slightly +different is favourable to the vigour and fertility of their offspring; and +that slight changes in the conditions of life are apparently favourable to the +vigour and fertility of all organic beings. It is not surprising that the +degree of difficulty in uniting two species, and the degree of sterility of +their hybrid-offspring should generally correspond, though due to distinct +causes; for both depend on the amount of difference of some kind between the +species which are crossed. Nor is it surprising that the facility of effecting +a first cross, the fertility of the hybrids produced, and the capacity of being +grafted together—though this latter capacity evidently depends on widely +different circumstances—should all run, to a certain extent, parallel +with the systematic affinity of the forms which are subjected to experiment; +for systematic affinity attempts to express all kinds of resemblance between +all species. +</p> + +<p> +First crosses between forms known to be varieties, or sufficiently alike to be +considered as varieties, and their mongrel offspring, are very generally, but +not quite universally, fertile. Nor is this nearly general and perfect +fertility surprising, when we remember how liable we are to argue in a circle +with respect to varieties in a state of nature; and when we remember that the +greater number of varieties have been produced under domestication +<a name="Page278"></a> +by the selection of mere external differences, and not of differences in the +reproductive system. In all other respects, excluding fertility, there is a +close general resemblance between hybrids and mongrels. Finally, then, the +facts briefly given in this chapter do not seem to me opposed to, but even +rather to support the view, that there is no fundamental distinction between +species and varieties. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page279"></a><a name="chap09"></a>CHAPTER IX.<br /> +ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.</h2> + +<p class="letter"> +On the absence of intermediate varieties at the present day. On the nature of +extinct intermediate varieties; on their number. On the vast lapse of time, as +inferred from the rate of deposition and of denudation. On the poorness of our +palæontological collections. On the intermittence of geological formations. On +the absence of intermediate varieties in any one formation. On the sudden +appearance of groups of species. On their sudden appearance in the lowest known +fossiliferous strata. +</p> + +<p> +In the sixth chapter I enumerated the chief objections which might be justly +urged against the views maintained in this volume. Most of them have now been +discussed. One, namely the distinctness of specific forms, and their not being +blended together by innumerable transitional links, is a very obvious +difficulty. I assigned reasons why such links do not commonly occur at the +present day, under the circumstances apparently most favourable for their +presence, namely on an extensive and continuous area with graduated physical +conditions. I endeavoured to show, that the life of each species depends in a +more important manner on the presence of other already defined organic forms, +than on climate; and, therefore, that the really governing conditions of life +do not graduate away quite insensibly like heat or moisture. I endeavoured, +also, to show that intermediate varieties, from existing in lesser numbers than +the forms which they connect, will generally be beaten out and exterminated +during the course of further modification and improvement. The main cause, +however, of innumerable intermediate links not now occurring everywhere +throughout nature depends +<a name="Page280"></a> +on the very process of natural selection, through which new varieties +continually take the places of and exterminate their parent-forms. But just in +proportion as this process of extermination has acted on an enormous scale, so +must the number of intermediate varieties, which have formerly existed on the +earth, be truly enormous. Why then is not every geological formation and every +stratum full of such intermediate links? Geology assuredly does not reveal any +such finely graduated organic chain; and this, perhaps, is the most obvious and +gravest objection which can be urged against my theory. The explanation lies, +as I believe, in the extreme imperfection of the geological record. +</p> + +<p> +In the first place it should always be borne in mind what sort of intermediate +forms must, on my theory, have formerly existed. I have found it difficult, +when looking at any two species, to avoid picturing to myself, forms +<i>directly</i> intermediate between them. But this is a wholly false view; we +should always look for forms intermediate between each species and a common but +unknown progenitor; and the progenitor will generally have differed in some +respects from all its modified descendants. To give a simple illustration: the +fantail and pouter pigeons have both descended from the rock-pigeon; if we +possessed all the intermediate varieties which have ever existed, we should +have an extremely close series between both and the rock-pigeon; but we should +have no varieties directly intermediate between the fantail and pouter; none, +for instance, combining a tail somewhat expanded with a crop somewhat enlarged, +the characteristic features of these two breeds. These two breeds, moreover, +have become so much modified, that if we had no historical or indirect evidence +regarding their origin, it would not have been possible to have +<a name="Page281"></a> +determined from a mere comparison of their structure with that of the +rock-pigeon, whether they had descended from this species or from some other +allied species, such as C. oenas. +</p> + +<p> +So with natural species, if we look to forms very distinct, for instance to the +horse and tapir, we have no reason to suppose that links ever existed directly +intermediate between them, but between each and an unknown common parent. The +common parent will have had in its whole organisation much general resemblance +to the tapir and to the horse; but in some points of structure may have +differed considerably from both, even perhaps more than they differ from each +other. Hence in all such cases, we should be unable to recognise the +parent-form of any two or more species, even if we closely compared the +structure of the parent with that of its modified descendants, unless at the +same time we had a nearly perfect chain of the intermediate links. +</p> + +<p> +It is just possible by my theory, that one of two living forms might have +descended from the other; for instance, a horse from a tapir; and in this case +<i>direct</i> intermediate links will have existed between them. But such a +case would imply that one form had remained for a very long period unaltered, +whilst its descendants had undergone a vast amount of change; and the principle +of competition between organism and organism, between child and parent, will +render this a very rare event; for in all cases the new and improved forms of +life will tend to supplant the old and unimproved forms. +</p> + +<p> +By the theory of natural selection all living species have been connected with +the parent-species of each genus, by differences not greater than we see +between the varieties of the same species at the present +<a name="Page282"></a> +day; and these parent-species, now generally extinct, have in their turn been +similarly connected with more ancient species; and so on backwards, always +converging to the common ancestor of each great class. So that the number of +intermediate and transitional links, between all living and extinct species, +must have been inconceivably great. But assuredly, if this theory be true, such +have lived upon this earth. +</p> + +<p class="p2"> +<i>On the lapse of Time</i>.—Independently of our not finding fossil +remains of such infinitely numerous connecting links, it may be objected, that +time will not have sufficed for so great an amount of organic change, all +changes having been effected very slowly through natural selection. It is +hardly possible for me even to recall to the reader, who may not be a practical +geologist, the facts leading the mind feebly to comprehend the lapse of time. +He who can read Sir Charles Lyell’s grand work on the Principles of +Geology, which the future historian will recognise as having produced a +revolution in natural science, yet does not admit how incomprehensibly vast +have been the past periods of time, may at once close this volume. Not that it +suffices to study the Principles of Geology, or to read special treatises by +different observers on separate formations, and to mark how each author +attempts to give an inadequate idea of the duration of each formation or even +each stratum. A man must for years examine for himself great piles of +superimposed strata, and watch the sea at work grinding down old rocks and +making fresh sediment, before he can hope to comprehend anything of the lapse +of time, the monuments of which we see around us. +</p> + +<p> +It is good to wander along lines of sea-coast, when formed of moderately hard +rocks, and mark the +<a name="Page283"></a> +process of degradation. The tides in most cases reach the cliffs only for a +short time twice a day, and the waves eat into them only when they are charged +with sand or pebbles; for there is reason to believe that pure water can effect +little or nothing in wearing away rock. At last the base of the cliff is +undermined, huge fragments fall down, and these remaining fixed, have to be +worn away, atom by atom, until reduced in size they can be rolled about by the +waves, and then are more quickly ground into pebbles, sand, or mud. But how +often do we see along the bases of retreating cliffs rounded boulders, all +thickly clothed by marine productions, showing how little they are abraded and +how seldom they are rolled about! Moreover, if we follow for a few miles any +line of rocky cliff, which is undergoing degradation, we find that it is only +here and there, along a short length or round a promontory, that the cliffs are +at the present time suffering. The appearance of the surface and the vegetation +show that elsewhere years have elapsed since the waters washed their base. +</p> + +<p> +He who most closely studies the action of the sea on our shores, will, I +believe, be most deeply impressed with the slowness with which rocky coasts are +worn away. The observations on this head by Hugh Miller, and by that excellent +observer Mr. Smith of Jordan Hill, are most impressive. With the mind thus +impressed, let any one examine beds of conglomerate many thousand feet in +thickness, which, though probably formed at a quicker rate than many other +deposits, yet, from being formed of worn and rounded pebbles, each of which +bears the stamp of time, are good to show how slowly the mass has been +accumulated. Let him remember Lyell’s profound remark, that the thickness +and extent of sedimentary formations +<a name="Page284"></a> +are the result and measure of the degradation which the earth’s crust has +elsewhere suffered. And what an amount of degradation is implied by the +sedimentary deposits of many countries! Professor Ramsay has given me the +maximum thickness, in most cases from actual measurement, in a few cases from +estimate, of each formation in different parts of Great Britain; and this is +the result:— +</p> + +<pre xml:space="preserve"> + Feet + + Palæozoic strata (not including igneous beds)...57,154. + Secondary strata................................13,190. + Tertiary strata..................................2,240. +</pre> + +<p class="noindent"> +—making altogether 72,584 feet; that is, very nearly thirteen and +three-quarters British miles. Some of these formations, which are represented +in England by thin beds, are thousands of feet in thickness on the Continent. +Moreover, between each successive formation, we have, in the opinion of most +geologists, enormously long blank periods. So that the lofty pile of +sedimentary rocks in Britain, gives but an inadequate idea of the time which +has elapsed during their accumulation; yet what time this must have consumed! +Good observers have estimated that sediment is deposited by the great +Mississippi river at the rate of only 600 feet in a hundred thousand years. +This estimate may be quite erroneous; yet, considering over what wide spaces +very fine sediment is transported by the currents of the sea, the process of +accumulation in any one area must be extremely slow. +</p> + +<p> +But the amount of denudation which the strata have in many places suffered, +independently of the rate of accumulation of the degraded matter, probably +offers the best evidence of the lapse of time. I remember having been much +struck with the evidence of denudation, when viewing volcanic islands, which +have been +<a name="Page285"></a> +worn by the waves and pared all round into perpendicular cliffs of one or two +thousand feet in height; for the gentle slope of the lava-streams, due to their +formerly liquid state, showed at a glance how far the hard, rocky beds had once +extended into the open ocean. The same story is still more plainly told by +faults,—those great cracks along which the strata have been upheaved on +one side, or thrown down on the other, to the height or depth of thousands of +feet; for since the crust cracked, the surface of the land has been so +completely planed down by the action of the sea, that no trace of these vast +dislocations is externally visible. +</p> + +<p> +The Craven fault, for instance, extends for upwards of 30 miles, and along this +line the vertical displacement of the strata has varied from 600 to 3000 feet. +Professor Ramsay has published an account of a downthrow in Anglesea of 2300 +feet; and he informs me that he fully believes there is one in Merionethshire +of 12,000 feet; yet in these cases there is nothing on the surface to show such +prodigious movements; the pile of rocks on the one or other side having been +smoothly swept away. The consideration of these facts impresses my mind almost +in the same manner as does the vain endeavour to grapple with the idea of +eternity. +</p> + +<p> +I am tempted to give one other case, the well-known one of the denudation of +the Weald. Though it must be admitted that the denudation of the Weald has been +a mere trifle, in comparison with that which has removed masses of our +palæozoic strata, in parts ten thousand feet in thickness, as shown in +Professor Ramsay’s masterly memoir on this subject. Yet it is an +admirable lesson to stand on the North Downs and to look at the distant South +Downs; for, remembering that at no great distance to the west the northern and +southern escarpments meet and close, one can safely picture to +<a name="Page286"></a> +oneself the great dome of rocks which must have covered up the Weald within so +limited a period as since the latter part of the Chalk formation. The distance +from the northern to the southern Downs is about 22 miles, and the thickness of +the several formations is on an average about 1100 feet, as I am informed by +Professor Ramsay. But if, as some geologists suppose, a range of older rocks +underlies the Weald, on the flanks of which the overlying sedimentary deposits +might have accumulated in thinner masses than elsewhere, the above estimate +would be erroneous; but this source of doubt probably would not greatly affect +the estimate as applied to the western extremity of the district. If, then, we +knew the rate at which the sea commonly wears away a line of cliff of any given +height, we could measure the time requisite to have denuded the Weald. This, of +course, cannot be done; but we may, in order to form some crude notion on the +subject, assume that the sea would eat into cliffs 500 feet in height at the +rate of one inch in a century. This will at first appear much too small an +allowance; but it is the same as if we were to assume a cliff one yard in +height to be eaten back along a whole line of coast at the rate of one yard in +nearly every twenty-two years. I doubt whether any rock, even as soft as chalk, +would yield at this rate excepting on the most exposed coasts; though no doubt +the degradation of a lofty cliff would be more rapid from the breakage of the +fallen fragments. On the other hand, I do not believe that any line of coast, +ten or twenty miles in length, ever suffers degradation at the same time along +its whole indented length; and we must remember that almost all strata contain +harder layers or nodules, which from long resisting attrition form a breakwater +at the base. Hence, under ordinary circumstances, I conclude that for a cliff +500 feet in height, a denudation +<a name="Page287"></a> +of one inch per century for the whole length would be an ample allowance. At +this rate, on the above data, the denudation of the Weald must have required +306,662,400 years; or say three hundred million years. +</p> + +<p> +The action of fresh water on the gently inclined Wealden district, when +upraised, could hardly have been great, but it would somewhat reduce the above +estimate. On the other hand, during oscillations of level, which we know this +area has undergone, the surface may have existed for millions of years as land, +and thus have escaped the action of the sea: when deeply submerged for perhaps +equally long periods, it would, likewise, have escaped the action of the +coast-waves. So that in all probability a far longer period than 300 million +years has elapsed since the latter part of the Secondary period. +</p> + +<p> +I have made these few remarks because it is highly important for us to gain +some notion, however imperfect, of the lapse of years. During each of these +years, over the whole world, the land and the water has been peopled by hosts +of living forms. What an infinite number of generations, which the mind cannot +grasp, must have succeeded each other in the long roll of years! Now turn to +our richest geological museums, and what a paltry display we behold! +</p> + +<p class="p2"> +<i>On the poorness of our Palæontological collections</i>.—That our +palæontological collections are very imperfect, is admitted by every one. The +remark of that admirable palæontologist, the late Edward Forbes, should not be +forgotten, namely, that numbers of our fossil species are known and named from +single and often broken specimens, or from a few specimens collected on some +one spot. Only a small portion of the surface of the earth has been +geologically explored, and no part with +<a name="Page288"></a> +sufficient care, as the important discoveries made every year in Europe prove. +No organism wholly soft can be preserved. Shells and bones will decay and +disappear when left on the bottom of the sea, where sediment is not +accumulating. I believe we are continually taking a most erroneous view, when +we tacitly admit to ourselves that sediment is being deposited over nearly the +whole bed of the sea, at a rate sufficiently quick to embed and preserve fossil +remains. Throughout an enormously large proportion of the ocean, the bright +blue tint of the water bespeaks its purity. The many cases on record of a +formation conformably covered, after an enormous interval of time, by another +and later formation, without the underlying bed having suffered in the interval +any wear and tear, seem explicable only on the view of the bottom of the sea +not rarely lying for ages in an unaltered condition. The remains which do +become embedded, if in sand or gravel, will when the beds are upraised +generally be dissolved by the percolation of rain-water. I suspect that but few +of the very many animals which live on the beach between high and low watermark +are preserved. For instance, the several species of the Chthamalinæ (a +sub-family of sessile cirripedes) coat the rocks all over the world in infinite +numbers: they are all strictly littoral, with the exception of a single +Mediterranean species, which inhabits deep water and has been found fossil in +Sicily, whereas not one other species has hitherto been found in any tertiary +formation: yet it is now known that the genus Chthamalus existed during the +chalk period. The molluscan genus Chiton offers a partially analogous case. +</p> + +<p> +With respect to the terrestrial productions which lived during the Secondary +and Palæozoic periods, it is superfluous to state that our evidence from +fossil +<a name="Page289"></a> +remains is fragmentary in an extreme degree. For instance, not a land shell is +known belonging to either of these vast periods, with one exception discovered +by Sir C. Lyell in the carboniferous strata of North America. In regard to +mammiferous remains, a single glance at the historical table published in the +Supplement to Lyell’s Manual, will bring home the truth, how accidental +and rare is their preservation, far better than pages of detail. Nor is their +rarity surprising, when we remember how large a proportion of the bones of +tertiary mammals have been discovered either in caves or in lacustrine +deposits; and that not a cave or true lacustrine bed is known belonging to the +age of our secondary or palæozoic formations. +</p> + +<p> +But the imperfection in the geological record mainly results from another and +more important cause than any of the foregoing; namely, from the several +formations being separated from each other by wide intervals of time. When we +see the formations tabulated in written works, or when we follow them in +nature, it is difficult to avoid believing that they are closely consecutive. +But we know, for instance, from Sir R. Murchison’s great work on Russia, +what wide gaps there are in that country between the superimposed formations; +so it is in North America, and in many other parts of the world. The most +skilful geologist, if his attention had been exclusively confined to these +large territories, would never have suspected that during the periods which +were blank and barren in his own country, great piles of sediment, charged with +new and peculiar forms of life, had elsewhere been accumulated. And if in each +separate territory, hardly any idea can be formed of the length of time which +has elapsed between the consecutive formations, we may infer that this could +nowhere be ascertained. The frequent +<a name="Page290"></a> +and great changes in the mineralogical composition of consecutive formations, +generally implying great changes in the geography of the surrounding lands, +whence the sediment has been derived, accords with the belief of vast intervals +of time having elapsed between each formation. +</p> + +<p> +But we can, I think, see why the geological formations of each region are +almost invariably intermittent; that is, have not followed each other in close +sequence. Scarcely any fact struck me more when examining many hundred miles of +the South American coasts, which have been upraised several hundred feet within +the recent period, than the absence of any recent deposits sufficiently +extensive to last for even a short geological period. Along the whole west +coast, which is inhabited by a peculiar marine fauna, tertiary beds are so +scantily developed, that no record of several successive and peculiar marine +faunas will probably be preserved to a distant age. A little reflection will +explain why along the rising coast of the western side of South America, no +extensive formations with recent or tertiary remains can anywhere be found, +though the supply of sediment must for ages have been great, from the enormous +degradation of the coast-rocks and from muddy streams entering the sea. The +explanation, no doubt, is, that the littoral and sub-littoral deposits are +continually worn away, as soon as they are brought up by the slow and gradual +rising of the land within the grinding action of the coast-waves. +</p> + +<p> +We may, I think, safely conclude that sediment must be accumulated in extremely +thick, solid, or extensive masses, in order to withstand the incessant action +of the waves, when first upraised and during subsequent oscillations of level. +Such thick and extensive accumulations of sediment may be formed in two ways; +either, +<a name="Page291"></a> +in profound depths of the sea, in which case, judging from the researches of E. +Forbes, we may conclude that the bottom will be inhabited by extremely few +animals, and the mass when upraised will give a most imperfect record of the +forms of life which then existed; or, sediment may be accumulated to any +thickness and extent over a shallow bottom, if it continue slowly to subside. +In this latter case, as long as the rate of subsidence and supply of sediment +nearly balance each other, the sea will remain shallow and favourable for life, +and thus a fossiliferous formation thick enough, when upraised, to resist any +amount of degradation, may be formed. +</p> + +<p> +I am convinced that all our ancient formations, which are rich in fossils, have +thus been formed during subsidence. Since publishing my views on this subject +in 1845, I have watched the progress of Geology, and have been surprised to +note how author after author, in treating of this or that great formation, has +come to the conclusion that it was accumulated during subsidence. I may add, +that the only ancient tertiary formation on the west coast of South America, +which has been bulky enough to resist such degradation as it has as yet +suffered, but which will hardly last to a distant geological age, was certainly +deposited during a downward oscillation of level, and thus gained considerable +thickness. +</p> + +<p> +All geological facts tell us plainly that each area has undergone numerous slow +oscillations of level, and apparently these oscillations have affected wide +spaces. Consequently formations rich in fossils and sufficiently thick and +extensive to resist subsequent degradation, may have been formed over wide +spaces during periods of subsidence, but only where the supply of sediment was +sufficient to keep the sea shallow and to embed and +<a name="Page292"></a> +preserve the remains before they had time to decay. On the other hand, as long +as the bed of the sea remained stationary, <i>thick</i> deposits could not have +been accumulated in the shallow parts, which are the most favourable to life. +Still less could this have happened during the alternate periods of elevation; +or, to speak more accurately, the beds which were then accumulated will have +been destroyed by being upraised and brought within the limits of the +coast-action. +</p> + +<p> +Thus the geological record will almost necessarily be rendered intermittent. I +feel much confidence in the truth of these views, for they are in strict +accordance with the general principles inculcated by Sir C. Lyell; and E. +Forbes independently arrived at a similar conclusion. +</p> + +<p> +One remark is here worth a passing notice. During periods of elevation the area +of the land and of the adjoining shoal parts of the sea will be increased, and +new stations will often be formed;—all circumstances most favourable, as +previously explained, for the formation of new varieties and species; but +during such periods there will generally be a blank in the geological record. +On the other hand, during subsidence, the inhabited area and number of +inhabitants will decrease (excepting the productions on the shores of a +continent when first broken up into an archipelago), and consequently during +subsidence, though there will be much extinction, fewer new varieties or +species will be formed; and it is during these very periods of subsidence, that +our great deposits rich in fossils have been accumulated. Nature may almost be +said to have guarded against the frequent discovery of her transitional or +linking forms. +</p> + +<p> +From the foregoing considerations it cannot be doubted that the geological +record, viewed as a whole, is extremely imperfect; but if we confine our +attention to any one formation, it becomes more difficult to understand, +<a name="Page293"></a> +why we do not therein find closely graduated varieties between the allied +species which lived at its commencement and at its close. Some cases are on +record of the same species presenting distinct varieties in the upper and lower +parts of the same formation, but, as they are rare, they may be here passed +over. Although each formation has indisputably required a vast number of years +for its deposition, I can see several reasons why each should not include a +graduated series of links between the species which then lived; but I can by no +means pretend to assign due proportional weight to the following +considerations. +</p> + +<p> +Although each formation may mark a very long lapse of years, each perhaps is +short compared with the period requisite to change one species into another. I +am aware that two palæontologists, whose opinions are worthy of much +deference, namely Bronn and Woodward, have concluded that the average duration +of each formation is twice or thrice as long as the average duration of +specific forms. But insuperable difficulties, as it seems to me, prevent us +coming to any just conclusion on this head. When we see a species first +appearing in the middle of any formation, it would be rash in the extreme to +infer that it had not elsewhere previously existed. So again when we find a +species disappearing before the uppermost layers have been deposited, it would +be equally rash to suppose that it then became wholly extinct. We forget how +small the area of Europe is compared with the rest of the world; nor have the +several stages of the same formation throughout Europe been correlated with +perfect accuracy. +</p> + +<p> +With marine animals of all kinds, we may safely infer a large amount of +migration during climatal and other changes; and when we see a species first +appearing in any formation, the probability is that it +<a name="Page294"></a> +only then first immigrated into that area. It is well known, for instance, that +several species appeared somewhat earlier in the palæozoic beds of North +America than in those of Europe; time having apparently been required for their +migration from the American to the European seas. In examining the latest +deposits of various quarters of the world, it has everywhere been noted, that +some few still existing species are common in the deposit, but have become +extinct in the immediately surrounding sea; or, conversely, that some are now +abundant in the neighbouring sea, but are rare or absent in this particular +deposit. It is an excellent lesson to reflect on the ascertained amount of +migration of the inhabitants of Europe during the Glacial period, which forms +only a part of one whole geological period; and likewise to reflect on the +great changes of level, on the inordinately great change of climate, on the +prodigious lapse of time, all included within this same glacial period. Yet it +may be doubted whether in any quarter of the world, sedimentary deposits, +<i>including fossil remains</i>, have gone on accumulating within the same area +during the whole of this period. It is not, for instance, probable that +sediment was deposited during the whole of the glacial period near the mouth of +the Mississippi, within that limit of depth at which marine animals can +flourish; for we know what vast geographical changes occurred in other parts of +America during this space of time. When such beds as were deposited in shallow +water near the mouth of the Mississippi during some part of the glacial period +shall have been upraised, organic remains will probably first appear and +disappear at different levels, owing to the migration of species and to +geographical changes. And in the distant future, a geologist examining these +beds, might be tempted to conclude that the average duration of life +<a name="Page295"></a> +of the embedded fossils had been less than that of the glacial period, instead +of having been really far greater, that is extending from before the glacial +epoch to the present day. +</p> + +<p> +In order to get a perfect gradation between two forms in the upper and lower +parts of the same formation, the deposit must have gone on accumulating for a +very long period, in order to have given sufficient time for the slow process +of variation; hence the deposit will generally have to be a very thick one; and +the species undergoing modification will have had to live on the same area +throughout this whole time. But we have seen that a thick fossiliferous +formation can only be accumulated during a period of subsidence; and to keep +the depth approximately the same, which is necessary in order to enable the +same species to live on the same space, the supply of sediment must nearly have +counterbalanced the amount of subsidence. But this same movement of subsidence +will often tend to sink the area whence the sediment is derived, and thus +diminish the supply whilst the downward movement continues. In fact, this +nearly exact balancing between the supply of sediment and the amount of +subsidence is probably a rare contingency; for it has been observed by more +than one palæontologist, that very thick deposits are usually barren of +organic remains, except near their upper or lower limits. +</p> + +<p> +It would seem that each separate formation, like the whole pile of formations +in any country, has generally been intermittent in its accumulation. When we +see, as is so often the case, a formation composed of beds of different +mineralogical composition, we may reasonably suspect that the process of +deposition has been much interrupted, as a change in the currents of the sea +and a supply of sediment of a different nature will +<a name="Page296"></a> +generally have been due to geographical changes requiring much time. Nor will +the closest inspection of a formation give any idea of the time which its +deposition has consumed. Many instances could be given of beds only a few feet +in thickness, representing formations, elsewhere thousands of feet in +thickness, and which must have required an enormous period for their +accumulation; yet no one ignorant of this fact would have suspected the vast +lapse of time represented by the thinner formation. Many cases could be given +of the lower beds of a formation having been upraised, denuded, submerged, and +then re-covered by the upper beds of the same formation,—facts, showing +what wide, yet easily overlooked, intervals have occurred in its accumulation. +In other cases we have the plainest evidence in great fossilised trees, still +standing upright as they grew, of many long intervals of time and changes of +level during the process of deposition, which would never even have been +suspected, had not the trees chanced to have been preserved: thus, Messrs. +Lyell and Dawson found carboniferous beds 1400 feet thick in Nova Scotia, with +ancient root-bearing strata, one above the other, at no less than sixty-eight +different levels. Hence, when the same species occur at the bottom, middle, and +top of a formation, the probability is that they have not lived on the same +spot during the whole period of deposition, but have disappeared and +reappeared, perhaps many times, during the same geological period. So that if +such species were to undergo a considerable amount of modification during any +one geological period, a section would not probably include all the fine +intermediate gradations which must on my theory have existed between them, but +abrupt, though perhaps very slight, changes of form. +</p> + +<p> +It is all-important to remember that naturalists have +<a name="Page297"></a> +no golden rule by which to distinguish species and varieties; they grant some +little variability to each species, but when they meet with a somewhat greater +amount of difference between any two forms, they rank both as species, unless +they are enabled to connect them together by close intermediate gradations. And +this from the reasons just assigned we can seldom hope to effect in any one +geological section. Supposing B and C to be two species, and a third, A, to be +found in an underlying bed; even if A were strictly intermediate between B and +C, it would simply be ranked as a third and distinct species, unless at the +same time it could be most closely connected with either one or both forms by +intermediate varieties. Nor should it be forgotten, as before explained, that A +might be the actual progenitor of B and C, and yet might not at all necessarily +be strictly intermediate between them in all points of structure. So that we +might obtain the parent-species and its several modified descendants from the +lower and upper beds of a formation, and unless we obtained numerous +transitional gradations, we should not recognise their relationship, and should +consequently be compelled to rank them all as distinct species. +</p> + +<p> +It is notorious on what excessively slight differences many palæontologists +have founded their species; and they do this the more readily if the specimens +come from different sub-stages of the same formation. Some experienced +conchologists are now sinking many of the very fine species of D’Orbigny +and others into the rank of varieties; and on this view we do find the kind of +evidence of change which on my theory we ought to find. Moreover, if we look to +rather wider intervals, namely, to distinct but consecutive stages of the same +great formation, we find that the embedded fossils, though almost universally +ranked as specifically different, +<a name="Page298"></a> +yet are far more closely allied to each other than are the species found in +more widely separated formations; but to this subject I shall have to return in +the following chapter. +</p> + +<p> +One other consideration is worth notice: with animals and plants that can +propagate rapidly and are not highly locomotive, there is reason to suspect, as +we have formerly seen, that their varieties are generally at first local; and +that such local varieties do not spread widely and supplant their parent-forms +until they have been modified and perfected in some considerable degree. +According to this view, the chance of discovering in a formation in any one +country all the early stages of transition between any two forms, is small, for +the successive changes are supposed to have been local or confined to some one +spot. Most marine animals have a wide range; and we have seen that with plants +it is those which have the widest range, that oftenest present varieties; so +that with shells and other marine animals, it is probably those which have had +the widest range, far exceeding the limits of the known geological formations +of Europe, which have oftenest given rise, first to local varieties and +ultimately to new species; and this again would greatly lessen the chance of +our being able to trace the stages of transition in any one geological +formation. +</p> + +<p> +It should not be forgotten, that at the present day, with perfect specimens for +examination, two forms can seldom be connected by intermediate varieties and +thus proved to be the same species, until many specimens have been collected +from many places; and in the case of fossil species this could rarely be +effected by palæontologists. We shall, perhaps, best perceive the +improbability of our being enabled to connect species by numerous, fine, +intermediate, fossil links, by asking +<a name="Page299"></a> +ourselves whether, for instance, geologists at some future period will be able +to prove, that our different breeds of cattle, sheep, horses, and dogs have +descended from a single stock or from several aboriginal stocks; or, again, +whether certain sea-shells inhabiting the shores of North America, which are +ranked by some conchologists as distinct species from their European +representatives, and by other conchologists as only varieties, are really +varieties or are, as it is called, specifically distinct. This could be +effected only by the future geologist discovering in a fossil state numerous +intermediate gradations; and such success seems to me improbable in the highest +degree. +</p> + +<p> +Geological research, though it has added numerous species to existing and +extinct genera, and has made the intervals between some few groups less wide +than they otherwise would have been, yet has done scarcely anything in breaking +down the distinction between species, by connecting them together by numerous, +fine, intermediate varieties; and this not having been effected, is probably +the gravest and most obvious of all the many objections which may be urged +against my views. Hence it will be worth while to sum up the foregoing remarks, +under an imaginary illustration. The Malay Archipelago is of about the size of +Europe from the North Cape to the Mediterranean, and from Britain to Russia; +and therefore equals all the geological formations which have been examined +with any accuracy, excepting those of the United States of America. I fully +agree with Mr. Godwin-Austen, that the present condition of the Malay +Archipelago, with its numerous large islands separated by wide and shallow +seas, probably represents the former state of Europe, when most of our +formations were accumulating. The Malay Archipelago is one of the richest +regions of the +<a name="Page300"></a> +whole world in organic beings; yet if all the species were to be collected +which have ever lived there, how imperfectly would they represent the natural +history of the world! +</p> + +<p> +But we have every reason to believe that the terrestrial productions of the +archipelago would be preserved in an excessively imperfect manner in the +formations which we suppose to be there accumulating. I suspect that not many +of the strictly littoral animals, or of those which lived on naked submarine +rocks, would be embedded; and those embedded in gravel or sand, would not +endure to a distant epoch. Wherever sediment did not accumulate on the bed of +the sea, or where it did not accumulate at a sufficient rate to protect organic +bodies from decay, no remains could be preserved. +</p> + +<p> +In our archipelago, I believe that fossiliferous formations could be formed of +sufficient thickness to last to an age, as distant in futurity as the secondary +formations lie in the past, only during periods of subsidence. These periods of +subsidence would be separated from each other by enormous intervals, during +which the area would be either stationary or rising; whilst rising, each +fossiliferous formation would be destroyed, almost as soon as accumulated, by +the incessant coast-action, as we now see on the shores of South America. +During the periods of subsidence there would probably be much extinction of +life; during the periods of elevation, there would be much variation, but the +geological record would then be least perfect. +</p> + +<p> +It may be doubted whether the duration of any one great period of subsidence +over the whole or part of the archipelago, together with a contemporaneous +accumulation of sediment, would <i>exceed</i> the average duration of the same +specific forms; and these contingencies are +<a name="Page301"></a> +indispensable for the preservation of all the transitional gradations between +any two or more species. If such gradations were not fully preserved, +transitional varieties would merely appear as so many distinct species. It is, +also, probable that each great period of subsidence would be interrupted by +oscillations of level, and that slight climatal changes would intervene during +such lengthy periods; and in these cases the inhabitants of the archipelago +would have to migrate, and no closely consecutive record of their modifications +could be preserved in any one formation. +</p> + +<p> +Very many of the marine inhabitants of the archipelago now range thousands of +miles beyond its confines; and analogy leads me to believe that it would be +chiefly these far-ranging species which would oftenest produce new varieties; +and the varieties would at first generally be local or confined to one place, +but if possessed of any decided advantage, or when further modified and +improved, they would slowly spread and supplant their parent-forms. When such +varieties returned to their ancient homes, as they would differ from their +former state, in a nearly uniform, though perhaps extremely slight degree, they +would, according to the principles followed by many palæontologists, be ranked +as new and distinct species. +</p> + +<p> +If then, there be some degree of truth in these remarks, we have no right to +expect to find in our geological formations, an infinite number of those fine +transitional forms, which on my theory assuredly have connected all the past +and present species of the same group into one long and branching chain of +life. We ought only to look for a few links, some more closely, some more +distantly related to each other; and these links, let them be ever so close, if +found in different stages of the same formation, would, by most +palæontologists, +<a name="Page302"></a> +be ranked as distinct species. But I do not pretend that I should ever have +suspected how poor a record of the mutations of life, the best preserved +geological section presented, had not the difficulty of our not discovering +innumerable transitional links between the species which appeared at the +commencement and close of each formation, pressed so hardly on my theory. +</p> + +<p class="p2"> +<i>On the sudden appearance of whole groups of Allied Species</i>.—The +abrupt manner in which whole groups of species suddenly appear in certain +formations, has been urged by several palæontologists, for instance, by +Agassiz, Pictet, and by none more forcibly than by Professor Sedgwick, as a +fatal objection to the belief in the transmutation of species. If numerous +species, belonging to the same genera or families, have really started into +life all at once, the fact would be fatal to the theory of descent with slow +modification through natural selection. For the development of a group of +forms, all of which have descended from some one progenitor, must have been an +extremely slow process; and the progenitors must have lived long ages before +their modified descendants. But we continually over-rate the perfection of the +geological record, and falsely infer, because certain genera or families have +not been found beneath a certain stage, that they did not exist before that +stage. We continually forget how large the world is, compared with the area +over which our geological formations have been carefully examined; we forget +that groups of species may elsewhere have long existed and have slowly +multiplied before they invaded the ancient archipelagoes of Europe and of the +United States. We do not make due allowance for the enormous intervals of time, +which have +<a name="Page303"></a> +probably elapsed between our consecutive formations,—longer perhaps in +some cases than the time required for the accumulation of each formation. These +intervals will have given time for the multiplication of species from some one +or some few parent-forms; and in the succeeding formation such species will +appear as if suddenly created. +</p> + +<p> +I may here recall a remark formerly made, namely that it might require a long +succession of ages to adapt an organism to some new and peculiar line of life, +for instance to fly through the air; but that when this had been effected, and +a few species had thus acquired a great advantage over other organisms, a +comparatively short time would be necessary to produce many divergent forms, +which would be able to spread rapidly and widely throughout the world. +</p> + +<p> +I will now give a few examples to illustrate these remarks; and to show how +liable we are to error in supposing that whole groups of species have suddenly +been produced. I may recall the well-known fact that in geological treatises, +published not many years ago, the great class of mammals was always spoken of +as having abruptly come in at the commencement of the tertiary series. And now +one of the richest known accumulations of fossil mammals belongs to the middle +of the secondary series; and one true mammal has been discovered in the new red +sandstone at nearly the commencement of this great series. Cuvier used to urge +that no monkey occurred in any tertiary stratum; but now extinct species have +been discovered in India, South America, and in Europe even as far back as the +eocene stage. The most striking case, however, is that of the Whale family; as +these animals have huge bones, are marine, and range over the world, the fact +of not a single bone of a whale having been discovered in +<a name="Page304"></a> +any secondary formation, seemed fully to justify the belief that this great and +distinct order had been suddenly produced in the interval between the latest +secondary and earliest tertiary formation. But now we may read in the +Supplement to Lyell’s ‘Manual,’ published in 1858, clear +evidence of the existence of whales in the upper greensand, some time before +the close of the secondary period. +</p> + +<p> +I may give another instance, which from having passed under my own eyes has +much struck me. In a memoir on Fossil Sessile Cirripedes, I have stated that, +from the number of existing and extinct tertiary species; from the +extraordinary abundance of the individuals of many species all over the world, +from the Arctic regions to the equator, inhabiting various zones of depths from +the upper tidal limits to 50 fathoms; from the perfect manner in which +specimens are preserved in the oldest tertiary beds; from the ease with which +even a fragment of a valve can be recognised; from all these circumstances, I +inferred that had sessile cirripedes existed during the secondary periods, they +would certainly have been preserved and discovered; and as not one species had +been discovered in beds of this age, I concluded that this great group had been +suddenly developed at the commencement of the tertiary series. This was a sore +trouble to me, adding as I thought one more instance of the abrupt appearance +of a great group of species. But my work had hardly been published, when a +skilful palæontologist, M. Bosquet, sent me a drawing of a perfect specimen of +an unmistakeable sessile cirripede, which he had himself extracted from the +chalk of Belgium. And, as if to make the case as striking as possible, this +sessile cirripede was a Chthamalus, a very common, large, and ubiquitous genus, +of which not one specimen has as yet been found even in any tertiary +<a name="Page305"></a> +stratum. Hence we now positively know that sessile cirripedes existed during +the secondary period; and these cirripedes might have been the progenitors of +our many tertiary and existing species. +</p> + +<p> +The case most frequently insisted on by palæontologists of the apparently +sudden appearance of a whole group of species, is that of the teleostean +fishes, low down in the Chalk period. This group includes the large majority of +existing species. Lately, Professor Pictet has carried their existence one +sub-stage further back; and some palæontologists believe that certain much +older fishes, of which the affinities are as yet imperfectly known, are really +teleostean. Assuming, however, that the whole of them did appear, as Agassiz +believes, at the commencement of the chalk formation, the fact would certainly +be highly remarkable; but I cannot see that it would be an insuperable +difficulty on my theory, unless it could likewise be shown that the species of +this group appeared suddenly and simultaneously throughout the world at this +same period. It is almost superfluous to remark that hardly any fossil-fish are +known from south of the equator; and by running through Pictet’s +Palæontology it will be seen that very few species are known from several +formations in Europe. Some few families of fish now have a confined range; the +teleostean fish might formerly have had a similarly confined range, and after +having been largely developed in some one sea, might have spread widely. Nor +have we any right to suppose that the seas of the world have always been so +freely open from south to north as they are at present. Even at this day, if +the Malay Archipelago were converted into land, the tropical parts of the +Indian Ocean would form a large and perfectly enclosed basin, in which any +great group of marine animals might be multiplied; and +<a name="Page306"></a> +here they would remain confined, until some of the species became adapted to a +cooler climate, and were enabled to double the southern capes of Africa or +Australia, and thus reach other and distant seas. +</p> + +<p> +From these and similar considerations, but chiefly from our ignorance of the +geology of other countries beyond the confines of Europe and the United States; +and from the revolution in our palæontological ideas on many points, which the +discoveries of even the last dozen years have effected, it seems to me to be +about as rash in us to dogmatize on the succession of organic beings throughout +the world, as it would be for a naturalist to land for five minutes on some one +barren point in Australia, and then to discuss the number and range of its +productions. +</p> + +<p class="p2"> +<i>On the sudden appearance of groups of Allied Species in the lowest known +fossiliferous strata</i>.—There is another and allied difficulty, which +is much graver. I allude to the manner in which numbers of species of the same +group, suddenly appear in the lowest known fossiliferous rocks. Most of the +arguments which have convinced me that all the existing species of the same +group have descended from one progenitor, apply with nearly equal force to the +earliest known species. For instance, I cannot doubt that all the Silurian +trilobites have descended from some one crustacean, which must have lived long +before the Silurian age, and which probably differed greatly from any known +animal. Some of the most ancient Silurian animals, as the Nautilus, Lingula, +etc., do not differ much from living species; and it cannot on my theory be +supposed, that these old species were the progenitors of all the species of the +orders to which they belong, for they do not present characters in any degree +intermediate between them. +<a name="Page307"></a> +If, moreover, they had been the progenitors of these orders, they would almost +certainly have been long ago supplanted and exterminated by their numerous and +improved descendants. +</p> + +<p> +Consequently, if my theory be true, it is indisputable that before the lowest +Silurian stratum was deposited, long periods elapsed, as long as, or probably +far longer than, the whole interval from the Silurian age to the present day; +and that during these vast, yet quite unknown, periods of time, the world +swarmed with living creatures. +</p> + +<p> +To the question why we do not find records of these vast primordial periods, I +can give no satisfactory answer. Several of the most eminent geologists, with +Sir R. Murchison at their head, are convinced that we see in the organic +remains of the lowest Silurian stratum the dawn of life on this planet. Other +highly competent judges, as Lyell and the late E. Forbes, dispute this +conclusion. We should not forget that only a small portion of the world is +known with accuracy. M. Barrande has lately added another and lower stage to +the Silurian system, abounding with new and peculiar species. Traces of life +have been detected in the Longmynd beds beneath Barrande’s so-called +primordial zone. The presence of phosphatic nodules and bituminous matter in +some of the lowest azoic rocks, probably indicates the former existence of life +at these periods. But the difficulty of understanding the absence of vast piles +of fossiliferous strata, which on my theory no doubt were somewhere accumulated +before the Silurian epoch, is very great. If these most ancient beds had been +wholly worn away by denudation, or obliterated by metamorphic action, we ought +to find only small remnants of the formations next succeeding them in age, and +these ought to be very generally in +<a name="Page308"></a> +a metamorphosed condition. But the descriptions which we now possess of the +Silurian deposits over immense territories in Russia and in North America, do +not support the view, that the older a formation is, the more it has suffered +the extremity of denudation and metamorphism. +</p> + +<p> +The case at present must remain inexplicable; and may be truly urged as a valid +argument against the views here entertained. To show that it may hereafter +receive some explanation, I will give the following hypothesis. From the nature +of the organic remains, which do not appear to have inhabited profound depths, +in the several formations of Europe and of the United States; and from the +amount of sediment, miles in thickness, of which the formations are composed, +we may infer that from first to last large islands or tracts of land, whence +the sediment was derived, occurred in the neighbourhood of the existing +continents of Europe and North America. But we do not know what was the state +of things in the intervals between the successive formations; whether Europe +and the United States during these intervals existed as dry land, or as a +submarine surface near land, on which sediment was not deposited, or again as +the bed of an open and unfathomable sea. +</p> + +<p> +Looking to the existing oceans, which are thrice as extensive as the land, we +see them studded with many islands; but not one oceanic island is as yet known +to afford even a remnant of any palæozoic or secondary formation. Hence we may +perhaps infer, that during the palæozoic and secondary periods, neither +continents nor continental islands existed where our oceans now extend; for had +they existed there, palæozoic and secondary formations would in all +probability have been accumulated from sediment derived from their wear and +<a name="Page309"></a> +tear; and would have been at least partially upheaved by the oscillations of +level, which we may fairly conclude must have intervened during these +enormously long periods. If then we may infer anything from these facts, we may +infer that where our oceans now extend, oceans have extended from the remotest +period of which we have any record; and on the other hand, that where +continents now exist, large tracts of land have existed, subjected no doubt to +great oscillations of level, since the earliest silurian period. The coloured +map appended to my volume on Coral Reefs, led me to conclude that the great +oceans are still mainly areas of subsidence, the great archipelagoes still +areas of oscillations of level, and the continents areas of elevation. But have +we any right to assume that things have thus remained from eternity? Our +continents seem to have been formed by a preponderance, during many +oscillations of level, of the force of elevation; but may not the areas of +preponderant movement have changed in the lapse of ages? At a period +immeasurably antecedent to the silurian epoch, continents may have existed +where oceans are now spread out; and clear and open oceans may have existed +where our continents now stand. Nor should we be justified in assuming that if, +for instance, the bed of the Pacific Ocean were now converted into a continent, +we should there find formations older than the silurian strata, supposing such +to have been formerly deposited; for it might well happen that strata which had +subsided some miles nearer to the centre of the earth, and which had been +pressed on by an enormous weight of superincumbent water, might have undergone +far more metamorphic action than strata which have always remained nearer to +the surface. The immense areas in some parts of the world, for instance in +South America, of bare metamorphic rocks, which +<a name="Page310"></a> +must have been heated under great pressure, have always seemed to me to require +some special explanation; and we may perhaps believe that we see in these large +areas, the many formations long anterior to the silurian epoch in a completely +metamorphosed condition. +</p> + +<p class="p2"> +The several difficulties here discussed, namely our not finding in the +successive formations infinitely numerous transitional links between the many +species which now exist or have existed; the sudden manner in which whole +groups of species appear in our European formations; the almost entire absence, +as at present known, of fossiliferous formations beneath the Silurian strata, +are all undoubtedly of the gravest nature. We see this in the plainest manner +by the fact that all the most eminent palæontologists, namely Cuvier, Owen, +Agassiz, Barrande, Falconer, E. Forbes, etc., and all our greatest geologists, +as Lyell, Murchison, Sedgwick, etc., have unanimously, often vehemently, +maintained the immutability of species. But I have reason to believe that one +great authority, Sir Charles Lyell, from further reflexion entertains grave +doubts on this subject. I feel how rash it is to differ from these great +authorities, to whom, with others, we owe all our knowledge. Those who think +the natural geological record in any degree perfect, and who do not attach much +weight to the facts and arguments of other kinds given in this volume, will +undoubtedly at once reject my theory. For my part, following out Lyell’s +metaphor, I look at the natural geological record, as a history of the world +imperfectly kept, and written in a changing dialect; of this history we possess +the last volume alone, relating only to two or three countries. Of this volume, +only here and there a short chapter has +<a name="Page311"></a> +been preserved; and of each page, only here and there a few lines. Each word of +the slowly-changing language, in which the history is supposed to be written, +being more or less different in the interrupted succession of chapters, may +represent the apparently abruptly changed forms of life, entombed in our +consecutive, but widely separated formations. On this view, the difficulties +above discussed are greatly diminished, or even disappear. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page312"></a><a name="chap10"></a>CHAPTER X.<br /> +ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.</h2> + +<p class="letter"> +On the slow and successive appearance of new species. On their different rates +of change. Species once lost do not reappear. Groups of species follow the same +general rules in their appearance and disappearance as do single species. On +Extinction. On simultaneous changes in the forms of life throughout the world. +On the affinities of extinct species to each other and to living species. On +the state of development of ancient forms. On the succession of the same types +within the same areas. Summary of preceding and present chapters. +</p> + +<p> +Let us now see whether the several facts and rules relating to the geological +succession of organic beings, better accord with the common view of the +immutability of species, or with that of their slow and gradual modification, +through descent and natural selection. +</p> + +<p> +New species have appeared very slowly, one after another, both on the land and +in the waters. Lyell has shown that it is hardly possible to resist the +evidence on this head in the case of the several tertiary stages; and every +year tends to fill up the blanks between them, and to make the percentage +system of lost and new forms more gradual. In some of the most recent beds, +though undoubtedly of high antiquity if measured by years, only one or two +species are lost forms, and only one or two are new forms, having here appeared +for the first time, either locally, or, as far as we know, on the face of the +earth. If we may trust the observations of Philippi in Sicily, the successive +changes in the marine inhabitants of that island have been many and most +gradual. The secondary formations are more broken; but, as Bronn has remarked, +neither the appearance +<a name="Page313"></a> +nor disappearance of their many now extinct species has been simultaneous in +each separate formation. +</p> + +<p> +Species of different genera and classes have not changed at the same rate, or +in the same degree. In the oldest tertiary beds a few living shells may still +be found in the midst of a multitude of extinct forms. Falconer has given a +striking instance of a similar fact, in an existing crocodile associated with +many strange and lost mammals and reptiles in the sub-Himalayan deposits. The +Silurian Lingula differs but little from the living species of this genus; +whereas most of the other Silurian Molluscs and all the Crustaceans have +changed greatly. The productions of the land seem to change at a quicker rate +than those of the sea, of which a striking instance has lately been observed in +Switzerland. There is some reason to believe that organisms, considered high in +the scale of nature, change more quickly than those that are low: though there +are exceptions to this rule. The amount of organic change, as Pictet has +remarked, does not strictly correspond with the succession of our geological +formations; so that between each two consecutive formations, the forms of life +have seldom changed in exactly the same degree. Yet if we compare any but the +most closely related formations, all the species will be found to have +undergone some change. When a species has once disappeared from the face of the +earth, we have reason to believe that the same identical form never reappears. +The strongest apparent exception to this latter rule, is that of the so-called +“colonies” of M. Barrande, which intrude for a period in the midst +of an older formation, and then allow the pre-existing fauna to reappear; but +Lyell’s explanation, namely, that it is a case of temporary migration +from a distinct geographical province, seems to me satisfactory. +</p> + +<p> +<a name="Page314"></a> +These several facts accord well with my theory. I believe in no fixed law of +development, causing all the inhabitants of a country to change abruptly, or +simultaneously, or to an equal degree. The process of modification must be +extremely slow. The variability of each species is quite independent of that of +all others. Whether such variability be taken advantage of by natural +selection, and whether the variations be accumulated to a greater or lesser +amount, thus causing a greater or lesser amount of modification in the varying +species, depends on many complex contingencies,—on the variability being +of a beneficial nature, on the power of intercrossing, on the rate of breeding, +on the slowly changing physical conditions of the country, and more especially +on the nature of the other inhabitants with which the varying species comes +into competition. Hence it is by no means surprising that one species should +retain the same identical form much longer than others; or, if changing, that +it should change less. We see the same fact in geographical distribution; for +instance, in the land-shells and coleopterous insects of Madeira having come to +differ considerably from their nearest allies on the continent of Europe, +whereas the marine shells and birds have remained unaltered. We can perhaps +understand the apparently quicker rate of change in terrestrial and in more +highly organised productions compared with marine and lower productions, by the +more complex relations of the higher beings to their organic and inorganic +conditions of life, as explained in a former chapter. When many of the +inhabitants of a country have become modified and improved, we can understand, +on the principle of competition, and on that of the many all-important +relations of organism to organism, that any form which does not become in some +degree modified and improved, +<a name="Page315"></a> +will be liable to be exterminated. Hence we can see why all the species in the +same region do at last, if we look to wide enough intervals of time, become +modified; for those which do not change will become extinct. +</p> + +<p> +In members of the same class the average amount of change, during long and +equal periods of time, may, perhaps, be nearly the same; but as the +accumulation of long-enduring fossiliferous formations depends on great masses +of sediment having been deposited on areas whilst subsiding, our formations +have been almost necessarily accumulated at wide and irregularly intermittent +intervals; consequently the amount of organic change exhibited by the fossils +embedded in consecutive formations is not equal. Each formation, on this view, +does not mark a new and complete act of creation, but only an occasional scene, +taken almost at hazard, in a slowly changing drama. +</p> + +<p> +We can clearly understand why a species when once lost should never reappear, +even if the very same conditions of life, organic and inorganic, should recur. +For though the offspring of one species might be adapted (and no doubt this has +occurred in innumerable instances) to fill the exact place of another species +in the economy of nature, and thus supplant it; yet the two forms—the old +and the new—would not be identically the same; for both would almost +certainly inherit different characters from their distinct progenitors. For +instance, it is just possible, if our fantail-pigeons were all destroyed, that +fanciers, by striving during long ages for the same object, might make a new +breed hardly distinguishable from our present fantail; but if the parent +rock-pigeon were also destroyed, and in nature we have every reason to believe +that the parent-form will generally be supplanted and +<a name="Page316"></a> +exterminated by its improved offspring, it is quite incredible that a fantail, +identical with the existing breed, could be raised from any other species of +pigeon, or even from the other well-established races of the domestic pigeon, +for the newly-formed fantail would be almost sure to inherit from its new +progenitor some slight characteristic differences. +</p> + +<p> +Groups of species, that is, genera and families, follow the same general rules +in their appearance and disappearance as do single species, changing more or +less quickly, and in a greater or lesser degree. A group does not reappear +after it has once disappeared; or its existence, as long as it lasts, is +continuous. I am aware that there are some apparent exceptions to this rule, +but the exceptions are surprisingly few, so few, that E. Forbes, Pictet, and +Woodward (though all strongly opposed to such views as I maintain) admit its +truth; and the rule strictly accords with my theory. For as all the species of +the same group have descended from some one species, it is clear that as long +as any species of the group have appeared in the long succession of ages, so +long must its members have continuously existed, in order to have generated +either new and modified or the same old and unmodified forms. Species of the +genus Lingula, for instance, must have continuously existed by an unbroken +succession of generations, from the lowest Silurian stratum to the present day. +</p> + +<p> +We have seen in the last chapter that the species of a group sometimes falsely +appear to have come in abruptly; and I have attempted to give an explanation of +this fact, which if true would have been fatal to my views. But such cases are +certainly exceptional; the general rule being a gradual increase in number, +till the group reaches its maximum, and then, sooner or later, it gradually +decreases. If the +<a name="Page317"></a> +number of the species of a genus, or the number of the genera of a family, be +represented by a vertical line of varying thickness, crossing the successive +geological formations in which the species are found, the line will sometimes +falsely appear to begin at its lower end, not in a sharp point, but abruptly; +it then gradually thickens upwards, sometimes keeping for a space of equal +thickness, and ultimately thins out in the upper beds, marking the decrease and +final extinction of the species. This gradual increase in number of the species +of a group is strictly conformable with my theory; as the species of the same +genus, and the genera of the same family, can increase only slowly and +progressively; for the process of modification and the production of a number +of allied forms must be slow and gradual,—one species giving rise first +to two or three varieties, these being slowly converted into species, which in +their turn produce by equally slow steps other species, and so on, like the +branching of a great tree from a single stem, till the group becomes large. +</p> + +<p class="p2"> +<i>On Extinction</i>.—We have as yet spoken only incidentally of the +disappearance of species and of groups of species. On the theory of natural +selection the extinction of old forms and the production of new and improved +forms are intimately connected together. The old notion of all the inhabitants +of the earth having been swept away at successive periods by catastrophes, is +very generally given up, even by those geologists, as Elie de Beaumont, +Murchison, Barrande, etc., whose general views would naturally lead them to +this conclusion. On the contrary, we have every reason to believe, from the +study of the tertiary formations, that species and groups of species gradually +disappear, one after another, first from one spot, then from another, and +<a name="Page318"></a> +finally from the world. Both single species and whole groups of species last +for very unequal periods; some groups, as we have seen, having endured from the +earliest known dawn of life to the present day; some having disappeared before +the close of the palæozoic period. No fixed law seems to determine the length +of time during which any single species or any single genus endures. There is +reason to believe that the complete extinction of the species of a group is +generally a slower process than their production: if the appearance and +disappearance of a group of species be represented, as before, by a vertical +line of varying thickness, the line is found to taper more gradually at its +upper end, which marks the progress of extermination, than at its lower end, +which marks the first appearance and increase in numbers of the species. In +some cases, however, the extermination of whole groups of beings, as of +ammonites towards the close of the secondary period, has been wonderfully +sudden. +</p> + +<p> +The whole subject of the extinction of species has been involved in the most +gratuitous mystery. Some authors have even supposed that as the individual has +a definite length of life, so have species a definite duration. No one I think +can have marvelled more at the extinction of species, than I have done. When I +found in La Plata the tooth of a horse embedded with the remains of Mastodon, +Megatherium, Toxodon, and other extinct monsters, which all co-existed with +still living shells at a very late geological period, I was filled with +astonishment; for seeing that the horse, since its introduction by the +Spaniards into South America, has run wild over the whole country and has +increased in numbers at an unparalleled rate, I asked myself what could so +recently have exterminated the former horse under conditions of life apparently +so favourable. But +<a name="Page319"></a> +how utterly groundless was my astonishment! Professor Owen soon perceived that +the tooth, though so like that of the existing horse, belonged to an extinct +species. Had this horse been still living, but in some degree rare, no +naturalist would have felt the least surprise at its rarity; for rarity is the +attribute of a vast number of species of all classes, in all countries. If we +ask ourselves why this or that species is rare, we answer that something is +unfavourable in its conditions of life; but what that something is, we can +hardly ever tell. On the supposition of the fossil horse still existing as a +rare species, we might have felt certain from the analogy of all other mammals, +even of the slow-breeding elephant, and from the history of the naturalisation +of the domestic horse in South America, that under more favourable conditions +it would in a very few years have stocked the whole continent. But we could not +have told what the unfavourable conditions were which checked its increase, +whether some one or several contingencies, and at what period of the +horse’s life, and in what degree, they severally acted. If the conditions +had gone on, however slowly, becoming less and less favourable, we assuredly +should not have perceived the fact, yet the fossil horse would certainly have +become rarer and rarer, and finally extinct;—its place being seized on by +some more successful competitor. +</p> + +<p> +It is most difficult always to remember that the increase of every living being +is constantly being checked by unperceived injurious agencies; and that these +same unperceived agencies are amply sufficient to cause rarity, and finally +extinction. We see in many cases in the more recent tertiary formations, that +rarity precedes extinction; and we know that this has been the progress of +events with those animals which have +<a name="Page320"></a> +been exterminated, either locally or wholly, through man’s agency. I may +repeat what I published in 1845, namely, that to admit that species generally +become rare before they become extinct—to feel no surprise at the rarity +of a species, and yet to marvel greatly when it ceases to exist, is much the +same as to admit that sickness in the individual is the forerunner of +death—to feel no surprise at sickness, but when the sick man dies, to +wonder and to suspect that he died by some unknown deed of violence. +</p> + +<p> +The theory of natural selection is grounded on the belief that each new +variety, and ultimately each new species, is produced and maintained by having +some advantage over those with which it comes into competition; and the +consequent extinction of less-favoured forms almost inevitably follows. It is +the same with our domestic productions: when a new and slightly improved +variety has been raised, it at first supplants the less improved varieties in +the same neighbourhood; when much improved it is transported far and near, like +our short-horn cattle, and takes the place of other breeds in other countries. +Thus the appearance of new forms and the disappearance of old forms, both +natural and artificial, are bound together. In certain flourishing groups, the +number of new specific forms which have been produced within a given time is +probably greater than that of the old forms which have been exterminated; but +we know that the number of species has not gone on indefinitely increasing, at +least during the later geological periods, so that looking to later times we +may believe that the production of new forms has caused the extinction of about +the same number of old forms. +</p> + +<p> +The competition will generally be most severe, as formerly explained and +illustrated by examples, between the forms which are most like each other in +all respects. +<a name="Page321"></a> +Hence the improved and modified descendants of a species will generally cause +the extermination of the parent-species; and if many new forms have been +developed from any one species, the nearest allies of that species, <i>i.e.</i> +the species of the same genus, will be the most liable to extermination. Thus, +as I believe, a number of new species descended from one species, that is a new +genus, comes to supplant an old genus, belonging to the same family. But it +must often have happened that a new species belonging to some one group will +have seized on the place occupied by a species belonging to a distinct group, +and thus caused its extermination; and if many allied forms be developed from +the successful intruder, many will have to yield their places; and it will +generally be allied forms, which will suffer from some inherited inferiority in +common. But whether it be species belonging to the same or to a distinct class, +which yield their places to other species which have been modified and +improved, a few of the sufferers may often long be preserved, from being fitted +to some peculiar line of life, or from inhabiting some distant and isolated +station, where they have escaped severe competition. For instance, a single +species of Trigonia, a great genus of shells in the secondary formations, +survives in the Australian seas; and a few members of the great and almost +extinct group of Ganoid fishes still inhabit our fresh waters. Therefore the +utter extinction of a group is generally, as we have seen, a slower process +than its production. +</p> + +<p> +With respect to the apparently sudden extermination of whole families or +orders, as of Trilobites at the close of the palæozoic period and of Ammonites +at the close of the secondary period, we must remember what has been already +said on the probable wide intervals of time +<a name="Page322"></a> +between our consecutive formations; and in these intervals there may have been +much slow extermination. Moreover, when by sudden immigration or by unusually +rapid development, many species of a new group have taken possession of a new +area, they will have exterminated in a correspondingly rapid manner many of the +old inhabitants; and the forms which thus yield their places will commonly be +allied, for they will partake of some inferiority in common. +</p> + +<p> +Thus, as it seems to me, the manner in which single species and whole groups of +species become extinct, accords well with the theory of natural selection. We +need not marvel at extinction; if we must marvel, let it be at our presumption +in imagining for a moment that we understand the many complex contingencies, on +which the existence of each species depends. If we forget for an instant, that +each species tends to increase inordinately, and that some check is always in +action, yet seldom perceived by us, the whole economy of nature will be utterly +obscured. Whenever we can precisely say why this species is more abundant in +individuals than that; why this species and not another can be naturalised in a +given country; then, and not till then, we may justly feel surprise why we +cannot account for the extinction of this particular species or group of +species. +</p> + +<p class="p2"> +<i>On the Forms of Life changing almost simultaneously throughout the +World</i>.—Scarcely any palæontological discovery is more striking than +the fact, that the forms of life change almost simultaneously throughout the +world. Thus our European Chalk formation can be recognised in many distant +parts of the world, under the most different climates, where not a fragment of +the mineral chalk itself can be found; namely, in North +<a name="Page323"></a> +America, in equatorial South America, in Tierra del Fuego, at the Cape of Good +Hope, and in the peninsula of India. For at these distant points, the organic +remains in certain beds present an unmistakeable degree of resemblance to those +of the Chalk. It is not that the same species are met with; for in some cases +not one species is identically the same, but they belong to the same families, +genera, and sections of genera, and sometimes are similarly characterised in +such trifling points as mere superficial sculpture. Moreover other forms, which +are not found in the Chalk of Europe, but which occur in the formations either +above or below, are similarly absent at these distant points of the world. In +the several successive palæozoic formations of Russia, Western Europe and North +America, a similar parallelism in the forms of life has been observed by +several authors: so it is, according to Lyell, with the several European and +North American tertiary deposits. Even if the few fossil species which are +common to the Old and New Worlds be kept wholly out of view, the general +parallelism in the successive forms of life, in the stages of the widely +separated palæozoic and tertiary periods, would still be manifest, and the +several formations could be easily correlated. +</p> + +<p> +These observations, however, relate to the marine inhabitants of distant parts +of the world: we have not sufficient data to judge whether the productions of +the land and of fresh water change at distant points in the same parallel +manner. We may doubt whether they have thus changed: if the Megatherium, +Mylodon, Macrauchenia, and Toxodon had been brought to Europe from La Plata, +without any information in regard to their geological position, no one would +have suspected that they had coexisted with still living sea-shells; but as +these anomalous monsters coexisted with the Mastodon +<a name="Page324"></a> +and Horse, it might at least have been inferred that they had lived during one +of the latter tertiary stages. +</p> + +<p> +When the marine forms of life are spoken of as having changed simultaneously +throughout the world, it must not be supposed that this expression relates to +the same thousandth or hundred-thousandth year, or even that it has a very +strict geological sense; for if all the marine animals which live at the +present day in Europe, and all those that lived in Europe during the +pleistocene period (an enormously remote period as measured by years, including +the whole glacial epoch), were to be compared with those now living in South +America or in Australia, the most skilful naturalist would hardly be able to +say whether the existing or the pleistocene inhabitants of Europe resembled +most closely those of the southern hemisphere. So, again, several highly +competent observers believe that the existing productions of the United States +are more closely related to those which lived in Europe during certain later +tertiary stages, than to those which now live here; and if this be so, it is +evident that fossiliferous beds deposited at the present day on the shores of +North America would hereafter be liable to be classed with somewhat older +European beds. Nevertheless, looking to a remotely future epoch, there can, I +think, be little doubt that all the more modern <i>marine</i> formations, +namely, the upper pliocene, the pleistocene and strictly modern beds, of +Europe, North and South America, and Australia, from containing fossil remains +in some degree allied, and from not including those forms which are only found +in the older underlying deposits, would be correctly ranked as simultaneous in +a geological sense. +</p> + +<p> +The fact of the forms of life changing simultaneously, in the above large +sense, at distant parts of the world, has greatly struck those admirable +observers, MM. +<a name="Page325"></a> +de Verneuil and d’Archiac. After referring to the parallelism of the +palæozoic forms of life in various parts of Europe, they add, “If struck +by this strange sequence, we turn our attention to North America, and there +discover a series of analogous phenomena, it will appear certain that all these +modifications of species, their extinction, and the introduction of new ones, +cannot be owing to mere changes in marine currents or other causes more or less +local and temporary, but depend on general laws which govern the whole animal +kingdom.” M. Barrande has made forcible remarks to precisely the same +effect. It is, indeed, quite futile to look to changes of currents, climate, or +other physical conditions, as the cause of these great mutations in the forms +of life throughout the world, under the most different climates. We must, as +Barrande has remarked, look to some special law. We shall see this more clearly +when we treat of the present distribution of organic beings, and find how +slight is the relation between the physical conditions of various countries, +and the nature of their inhabitants. +</p> + +<p> +This great fact of the parallel succession of the forms of life throughout the +world, is explicable on the theory of natural selection. New species are formed +by new varieties arising, which have some advantage over older forms; and those +forms, which are already dominant, or have some advantage over the other forms +in their own country, would naturally oftenest give rise to new varieties or +incipient species; for these latter must be victorious in a still higher degree +in order to be preserved and to survive. We have distinct evidence on this +head, in the plants which are dominant, that is, which are commonest in their +own homes, and are most widely diffused, having produced the greatest number of +new varieties. It is also natural that the dominant, +<a name="Page326"></a> +varying, and far-spreading species, which already have invaded to a certain +extent the territories of other species, should be those which would have the +best chance of spreading still further, and of giving rise in new countries to +new varieties and species. The process of diffusion may often be very slow, +being dependent on climatal and geographical changes, or on strange accidents, +but in the long run the dominant forms will generally succeed in spreading. The +diffusion would, it is probable, be slower with the terrestrial inhabitants of +distinct continents than with the marine inhabitants of the continuous sea. We +might therefore expect to find, as we apparently do find, a less strict degree +of parallel succession in the productions of the land than of the sea. +</p> + +<p> +Dominant species spreading from any region might encounter still more dominant +species, and then their triumphant course, or even their existence, would +cease. We know not at all precisely what are all the conditions most favourable +for the multiplication of new and dominant species; but we can, I think, +clearly see that a number of individuals, from giving a better chance of the +appearance of favourable variations, and that severe competition with many +already existing forms, would be highly favourable, as would be the power of +spreading into new territories. A certain amount of isolation, recurring at +long intervals of time, would probably be also favourable, as before explained. +One quarter of the world may have been most favourable for the production of +new and dominant species on the land, and another for those in the waters of +the sea. If two great regions had been for a long period favourably +circumstanced in an equal degree, whenever their inhabitants met, the battle +would be prolonged and severe; and some from one birthplace and some from the +other might be victorious. But in the course of time, the +<a name="Page327"></a> +forms dominant in the highest degree, wherever produced, would tend everywhere +to prevail. As they prevailed, they would cause the extinction of other and +inferior forms; and as these inferior forms would be allied in groups by +inheritance, whole groups would tend slowly to disappear; though here and there +a single member might long be enabled to survive. +</p> + +<p> +Thus, as it seems to me, the parallel, and, taken in a large sense, +simultaneous, succession of the same forms of life throughout the world, +accords well with the principle of new species having been formed by dominant +species spreading widely and varying; the new species thus produced being +themselves dominant owing to inheritance, and to having already had some +advantage over their parents or over other species; these again spreading, +varying, and producing new species. The forms which are beaten and which yield +their places to the new and victorious forms, will generally be allied in +groups, from inheriting some inferiority in common; and therefore as new and +improved groups spread throughout the world, old groups will disappear from the +world; and the succession of forms in both ways will everywhere tend to +correspond. +</p> + +<p> +There is one other remark connected with this subject worth making. I have +given my reasons for believing that all our greater fossiliferous formations +were deposited during periods of subsidence; and that blank intervals of vast +duration occurred during the periods when the bed of the sea was either +stationary or rising, and likewise when sediment was not thrown down quickly +enough to embed and preserve organic remains. During these long and blank +intervals I suppose that the inhabitants of each region underwent a +considerable amount of modification and extinction, and that there was much +migration from +<a name="Page328"></a> +other parts of the world. As we have reason to believe that large areas are +affected by the same movement, it is probable that strictly contemporaneous +formations have often been accumulated over very wide spaces in the same +quarter of the world; but we are far from having any right to conclude that +this has invariably been the case, and that large areas have invariably been +affected by the same movements. When two formations have been deposited in two +regions during nearly, but not exactly the same period, we should find in both, +from the causes explained in the foregoing paragraphs, the same general +succession in the forms of life; but the species would not exactly correspond; +for there will have been a little more time in the one region than in the other +for modification, extinction, and immigration. +</p> + +<p> +I suspect that cases of this nature have occurred in Europe. Mr. Prestwich, in +his admirable Memoirs on the eocene deposits of England and France, is able to +draw a close general parallelism between the successive stages in the two +countries; but when he compares certain stages in England with those in France, +although he finds in both a curious accordance in the numbers of the species +belonging to the same genera, yet the species themselves differ in a manner +very difficult to account for, considering the proximity of the two +areas,—unless, indeed, it be assumed that an isthmus separated two seas +inhabited by distinct, but contemporaneous, faunas. Lyell has made similar +observations on some of the later tertiary formations. Barrande, also, shows +that there is a striking general parallelism in the successive Silurian +deposits of Bohemia and Scandinavia; nevertheless he finds a surprising amount +of difference in the species. If the several formations in these regions have +not been deposited during the same exact +<a name="Page329"></a> +periods,—a formation in one region often corresponding with a blank +interval in the other,—and if in both regions the species have gone on +slowly changing during the accumulation of the several formations and during +the long intervals of time between them; in this case, the several formations +in the two regions could be arranged in the same order, in accordance with the +general succession of the form of life, and the order would falsely appear to +be strictly parallel; nevertheless the species would not all be the same in the +apparently corresponding stages in the two regions. +</p> + +<p class="p2"> +<i>On the Affinities of extinct Species to each other, and to living +forms</i>.—Let us now look to the mutual affinities of extinct and living +species. They all fall into one grand natural system; and this fact is at once +explained on the principle of descent. The more ancient any form is, the more, +as a general rule, it differs from living forms. But, as Buckland long ago +remarked, all fossils can be classed either in still existing groups, or +between them. That the extinct forms of life help to fill up the wide intervals +between existing genera, families, and orders, cannot be disputed. For if we +confine our attention either to the living or to the extinct alone, the series +is far less perfect than if we combine both into one general system. With +respect to the Vertebrata, whole pages could be filled with striking +illustrations from our great palæontologist, Owen, showing how extinct animals +fall in between existing groups. Cuvier ranked the Ruminants and Pachyderms, as +the two most distinct orders of mammals; but Owen has discovered so many fossil +links, that he has had to alter the whole classification of these two orders; +and has placed certain pachyderms in the same sub-order with ruminants: for +example, he dissolves by fine gradations the apparently +<a name="Page330"></a> +wide difference between the pig and the camel. In regard to the Invertebrata, +Barrande, and a higher authority could not be named, asserts that he is every +day taught that palæozoic animals, though belonging to the same orders, +families, or genera with those living at the present day, were not at this +early epoch limited in such distinct groups as they now are. +</p> + +<p> +Some writers have objected to any extinct species or group of species being +considered as intermediate between living species or groups. If by this term it +is meant that an extinct form is directly intermediate in all its characters +between two living forms, the objection is probably valid. But I apprehend that +in a perfectly natural classification many fossil species would have to stand +between living species, and some extinct genera between living genera, even +between genera belonging to distinct families. The most common case, especially +with respect to very distinct groups, such as fish and reptiles, seems to be, +that supposing them to be distinguished at the present day from each other by a +dozen characters, the ancient members of the same two groups would be +distinguished by a somewhat lesser number of characters, so that the two +groups, though formerly quite distinct, at that period made some small approach +to each other. +</p> + +<p> +It is a common belief that the more ancient a form is, by so much the more it +tends to connect by some of its characters groups now widely separated from +each other. This remark no doubt must be restricted to those groups which have +undergone much change in the course of geological ages; and it would be +difficult to prove the truth of the proposition, for every now and then even a +living animal, as the Lepidosiren, is discovered having affinities directed +towards very distinct groups. Yet if we compare the older Reptiles and +<a name="Page331"></a> +Batrachians, the older Fish, the older Cephalopods, and the eocene Mammals, +with the more recent members of the same classes, we must admit that there is +some truth in the remark. +</p> + +<p> +Let us see how far these several facts and inferences accord with the theory of +descent with modification. As the subject is somewhat complex, I must request +the reader to turn to the diagram in the fourth chapter. We may suppose that +the numbered letters represent genera, and the dotted lines diverging from them +the species in each genus. The diagram is much too simple, too few genera and +too few species being given, but this is unimportant for us. The horizontal +lines may represent successive geological formations, and all the forms beneath +the uppermost line may be considered as extinct. The three existing genera, +<i>a</i><sup>14</sup>, <i>q</i><sup>14</sup>, <i>p</i><sup>14</sup>, will form +a small family; <i>b</i><sup>14</sup> and <i>f</i><sup>14</sup> a closely +allied family or sub-family; and <i>o</i><sup>14</sup>, <i>e</i><sup>14</sup>, +<i>m</i><sup>14</sup>, a third family. These three families, together with the +many extinct genera on the several lines of descent diverging from the +parent-form A, will form an order; for all will have inherited something in +common from their ancient and common progenitor. On the principle of the +continued tendency to divergence of character, which was formerly illustrated +by this diagram, the more recent any form is, the more it will generally differ +from its ancient progenitor. Hence we can understand the rule that the most +ancient fossils differ most from existing forms. We must not, however, assume +that divergence of character is a necessary contingency; it depends solely on +the descendants from a species being thus enabled to seize on many and +different places in the economy of nature. Therefore it is quite possible, as +we have seen in the case of some Silurian forms, that a species might go on +being slightly +<a name="Page332"></a> +modified in relation to its slightly altered conditions of life, and yet retain +throughout a vast period the same general characteristics. This is represented +in the diagram by the letter <small>F</small><sup>14</sup>. +</p> + +<p> +All the many forms, extinct and recent, descended from A, make, as before +remarked, one order; and this order, from the continued effects of extinction +and divergence of character, has become divided into several sub-families and +families, some of which are supposed to have perished at different periods, and +some to have endured to the present day. +</p> + +<p> +By looking at the diagram we can see that if many of the extinct forms, +supposed to be embedded in the successive formations, were discovered at +several points low down in the series, the three existing families on the +uppermost line would be rendered less distinct from each other. If, for +instance, the genera <i>a</i><sup>1</sup>, <i>a</i><sup>5</sup>, +<i>a</i><sup>10</sup>, <i>f</i><sup>8</sup>, <i>m</i><sup>3</sup>, +<i>m</i><sup>6</sup>, <i>m</i><sup>9</sup> were disinterred, these three +families would be so closely linked together that they probably would have to +be united into one great family, in nearly the same manner as has occurred with +ruminants and pachyderms. Yet he who objected to call the extinct genera, which +thus linked the living genera of three families together, intermediate in +character, would be justified, as they are intermediate, not directly, but only +by a long and circuitous course through many widely different forms. If many +extinct forms were to be discovered above one of the middle horizontal lines or +geological formations—for instance, above Number VI.—but none from +beneath this line, then only the two families on the left hand (namely, +<i>a</i><sup>14</sup>, etc., and <i>b</i><sup>14</sup>, etc.) would have to be +united into one family; and the two other families (namely, +<i>a</i><sup>14</sup> to <i>f</i><sup>14</sup> now including five genera, and +<i>o</i><sup>14</sup> to <i>m</i><sup>14</sup>) would yet remain distinct. +These two families, however, would be less distinct from each other than they +were before the +<a name="Page333"></a> +discovery of the fossils. If, for instance, we suppose the existing genera of +the two families to differ from each other by a dozen characters, in this case +the genera, at the early period marked VI., would differ by a lesser number of +characters; for at this early stage of descent they have not diverged in +character from the common progenitor of the order, nearly so much as they +subsequently diverged. Thus it comes that ancient and extinct genera are often +in some slight degree intermediate in character between their modified +descendants, or between their collateral relations. +</p> + +<p> +In nature the case will be far more complicated than is represented in the +diagram; for the groups will have been more numerous, they will have endured +for extremely unequal lengths of time, and will have been modified in various +degrees. As we possess only the last volume of the geological record, and that +in a very broken condition, we have no right to expect, except in very rare +cases, to fill up wide intervals in the natural system, and thus unite distinct +families or orders. All that we have a right to expect, is that those groups, +which have within known geological periods undergone much modification, should +in the older formations make some slight approach to each other; so that the +older members should differ less from each other in some of their characters +than do the existing members of the same groups; and this by the concurrent +evidence of our best palæontologists seems frequently to be the case. +</p> + +<p> +Thus, on the theory of descent with modification, the main facts with respect +to the mutual affinities of the extinct forms of life to each other and to +living forms, seem to me explained in a satisfactory manner. And they are +wholly inexplicable on any other view. +</p> + +<p> +On this same theory, it is evident that the fauna of any great period in the +earth’s history will be intermediate +<a name="Page334"></a> +in general character between that which preceded and that which succeeded it. +Thus, the species which lived at the sixth great stage of descent in the +diagram are the modified offspring of those which lived at the fifth stage, and +are the parents of those which became still more modified at the seventh stage; +hence they could hardly fail to be nearly intermediate in character between the +forms of life above and below. We must, however, allow for the entire +extinction of some preceding forms, and for the coming in of quite new forms by +immigration, and for a large amount of modification, during the long and blank +intervals between the successive formations. Subject to these allowances, the +fauna of each geological period undoubtedly is intermediate in character, +between the preceding and succeeding faunas. I need give only one instance, +namely, the manner in which the fossils of the Devonian system, when this +system was first discovered, were at once recognised by palæontologists as +intermediate in character between those of the overlying carboniferous, and +underlying Silurian system. But each fauna is not necessarily exactly +intermediate, as unequal intervals of time have elapsed between consecutive +formations. +</p> + +<p> +It is no real objection to the truth of the statement, that the fauna of each +period as a whole is nearly intermediate in character between the preceding and +succeeding faunas, that certain genera offer exceptions to the rule. For +instance, mastodons and elephants, when arranged by Dr. Falconer in two series, +first according to their mutual affinities and then according to their periods +of existence, do not accord in arrangement. The species extreme in character +are not the oldest, or the most recent; nor are those which are intermediate in +character, intermediate in age. But +<a name="Page335"></a> +supposing for an instant, in this and other such cases, that the record of the +first appearance and disappearance of the species was perfect, we have no +reason to believe that forms successively produced necessarily endure for +corresponding lengths of time: a very ancient form might occasionally last much +longer than a form elsewhere subsequently produced, especially in the case of +terrestrial productions inhabiting separated districts. To compare small things +with great: if the principal living and extinct races of the domestic pigeon +were arranged as well as they could be in serial affinity, this arrangement +would not closely accord with the order in time of their production, and still +less with the order of their disappearance; for the parent rock-pigeon now +lives; and many varieties between the rock-pigeon and the carrier have become +extinct; and carriers which are extreme in the important character of length of +beak originated earlier than short-beaked tumblers, which are at the opposite +end of the series in this same respect. +</p> + +<p> +Closely connected with the statement, that the organic remains from an +intermediate formation are in some degree intermediate in character, is the +fact, insisted on by all palæontologists, that fossils from two consecutive +formations are far more closely related to each other, than are the fossils +from two remote formations. Pictet gives as a well-known instance, the general +resemblance of the organic remains from the several stages of the chalk +formation, though the species are distinct in each stage. This fact alone, from +its generality, seems to have shaken Professor Pictet in his firm belief in the +immutability of species. He who is acquainted with the distribution of existing +species over the globe, will not attempt to account for the close resemblance +of the distinct species in closely consecutive +<a name="Page336"></a> +formations, by the physical conditions of the ancient areas having remained +nearly the same. Let it be remembered that the forms of life, at least those +inhabiting the sea, have changed almost simultaneously throughout the world, +and therefore under the most different climates and conditions. Consider the +prodigious vicissitudes of climate during the pleistocene period, which +includes the whole glacial period, and note how little the specific forms of +the inhabitants of the sea have been affected. +</p> + +<p> +On the theory of descent, the full meaning of the fact of fossil remains from +closely consecutive formations, though ranked as distinct species, being +closely related, is obvious. As the accumulation of each formation has often +been interrupted, and as long blank intervals have intervened between +successive formations, we ought not to expect to find, as I attempted to show +in the last chapter, in any one or two formations all the intermediate +varieties between the species which appeared at the commencement and close of +these periods; but we ought to find after intervals, very long as measured by +years, but only moderately long as measured geologically, closely allied forms, +or, as they have been called by some authors, representative species; and these +we assuredly do find. We find, in short, such evidence of the slow and scarcely +sensible mutation of specific forms, as we have a just right to expect to find. +</p> + +<p class="p2"> +<i>On the state of Development of Ancient Forms</i>.—There has been much +discussion whether recent forms are more highly developed than ancient. I will +not here enter on this subject, for naturalists have not as yet defined to each +other’s satisfaction what is meant by high and low forms. But in one +particular sense the +<a name="Page337"></a> +more recent forms must, on my theory, be higher than the more ancient; for each +new species is formed by having had some advantage in the struggle for life +over other and preceding forms. If under a nearly similar climate, the eocene +inhabitants of one quarter of the world were put into competition with the +existing inhabitants of the same or some other quarter, the eocene fauna or +flora would certainly be beaten and exterminated; as would a secondary fauna by +an eocene, and a palæozoic fauna by a secondary fauna. I do not doubt that this +process of improvement has affected in a marked and sensible manner the +organisation of the more recent and victorious forms of life, in comparison +with the ancient and beaten forms; but I can see no way of testing this sort of +progress. Crustaceans, for instance, not the highest in their own class, may +have beaten the highest molluscs. From the extraordinary manner in which +European productions have recently spread over New Zealand, and have seized on +places which must have been previously occupied, we may believe, if all the +animals and plants of Great Britain were set free in New Zealand, that in the +course of time a multitude of British forms would become thoroughly naturalized +there, and would exterminate many of the natives. On the other hand, from what +we see now occurring in New Zealand, and from hardly a single inhabitant of the +southern hemisphere having become wild in any part of Europe, we may doubt, if +all the productions of New Zealand were set free in Great Britain, whether any +considerable number would be enabled to seize on places now occupied by our +native plants and animals. Under this point of view, the productions of Great +Britain may be said to be higher than those of New Zealand. Yet the most +skilful naturalist from an examination of the species +<a name="Page338"></a> +of the two countries could not have foreseen this result. +</p> + +<p> +Agassiz insists that ancient animals resemble to a certain extent the embryos +of recent animals of the same classes; or that the geological succession of +extinct forms is in some degree parallel to the embryological development of +recent forms. I must follow Pictet and Huxley in thinking that the truth of +this doctrine is very far from proved. Yet I fully expect to see it hereafter +confirmed, at least in regard to subordinate groups, which have branched off +from each other within comparatively recent times. For this doctrine of Agassiz +accords well with the theory of natural selection. In a future chapter I shall +attempt to show that the adult differs from its embryo, owing to variations +supervening at a not early age, and being inherited at a corresponding age. +This process, whilst it leaves the embryo almost unaltered, continually adds, +in the course of successive generations, more and more difference to the adult. +</p> + +<p> +Thus the embryo comes to be left as a sort of picture, preserved by nature, of +the ancient and less modified condition of each animal. This view may be true, +and yet it may never be capable of full proof. Seeing, for instance, that the +oldest known mammals, reptiles, and fish strictly belong to their own proper +classes, though some of these old forms are in a slight degree less distinct +from each other than are the typical members of the same groups at the present +day, it would be vain to look for animals having the common embryological +character of the Vertebrata, until beds far beneath the lowest Silurian strata +are discovered—a discovery of which the chance is very small. +</p> + +<p class="p2"> +<i>On the Succession of the same Types within the same +<a name="Page339"></a> +areas, during the later tertiary periods</i>.—Mr. Clift many years ago +showed that the fossil mammals from the Australian caves were closely allied to +the living marsupials of that continent. In South America, a similar +relationship is manifest, even to an uneducated eye, in the gigantic pieces of +armour like those of the armadillo, found in several parts of La Plata; and +Professor Owen has shown in the most striking manner that most of the fossil +mammals, buried there in such numbers, are related to South American types. +This relationship is even more clearly seen in the wonderful collection of +fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so much +impressed with these facts that I strongly insisted, in 1839 and 1845, on this +“law of the succession of types,”—on “this wonderful +relationship in the same continent between the dead and the living.” +Professor Owen has subsequently extended the same generalisation to the mammals +of the Old World. We see the same law in this author’s restorations of +the extinct and gigantic birds of New Zealand. We see it also in the birds of +the caves of Brazil. Mr. Woodward has shown that the same law holds good with +sea-shells, but from the wide distribution of most genera of molluscs, it is +not well displayed by them. Other cases could be added, as the relation between +the extinct and living land-shells of Madeira; and between the extinct and +living brackish-water shells of the Aralo-Caspian Sea. +</p> + +<p> +Now what does this remarkable law of the succession of the same types within +the same areas mean? He would be a bold man, who after comparing the present +climate of Australia and of parts of South America under the same latitude, +would attempt to account, on the one hand, by dissimilar physical conditions +for the dissimilarity of the inhabitants of these two continents, +<a name="Page340"></a> +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 Edentata 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 present character of the southern half of the continent; and the +southern half was formerly more closely allied, than it is at present, to the +northern half. In a similar manner we know from Falconer and Cautley’s +discoveries, that northern India was formerly more closely related in its +mammals to Africa than it is at the present time. Analogous facts could be +given in relation to the distribution of marine animals. +</p> + +<p> +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 geographical +changes, permitting much inter-migration, the feebler will yield to the more +dominant forms, and there will be nothing immutable in the laws of past and +present distribution. +</p> + +<p> +<a name="Page341"></a> +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 animals have become wholly extinct, and have +left no progeny. But in the 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 forgotten 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 formation there be six other allied or +representative genera with the same number of species, then we may conclude +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 the 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, still fewer genera and species will have +left modified blood-descendants. +</p> + +<p class="p2"> +<i>Summary of the preceding and present Chapters</i>.—I have attempted to +show that the geological record is extremely imperfect; that only a small +portion of the globe has been geologically explored with care; that only +<a name="Page342"></a> +certain classes of organic beings have been largely preserved in a fossil +state; that the number both of specimens and of species, preserved in our +museums, is absolutely as nothing compared with the incalculable number of +generations which must have passed away even during a single formation; that, +owing to subsidence being necessary for the accumulation of fossiliferous +deposits thick enough to resist future degradation, enormous intervals of time +have elapsed between the successive formations; that there has probably been +more extinction during the periods of subsidence, and more variation during the +periods of elevation, and during the latter the record will have been least +perfectly kept; that each single formation has not been continuously deposited; +that the duration of each formation is, perhaps, short compared with the +average duration of specific forms; that migration has played an important part +in the first appearance of new forms in any one area and formation; that widely +ranging species are those which have varied most, and have oftenest given rise +to new species; and that varieties have at first often been local. All these +causes taken conjointly, must have tended to make the geological record +extremely imperfect, and will to a large extent explain why we do not find +interminable varieties, connecting together all the extinct and existing forms +of life by the finest graduated steps. +</p> + +<p> +He who rejects these views on the nature of the geological record, will rightly +reject my whole theory. For he may ask in vain where are the numberless +transitional links which must formerly have connected the closely allied or +representative species, found in the several stages of the same great +formation. He may disbelieve in the enormous intervals of time which have +elapsed between our consecutive formations; he +<a name="Page343"></a> +may overlook how important a part migration must have played, when the +formations of any one great region alone, as that of Europe, are considered; he +may urge the apparent, but often falsely apparent, sudden coming in of whole +groups of species. He may ask where are the remains of those infinitely +numerous organisms which must have existed long before the first bed of the +Silurian system was deposited: I can answer this latter question only +hypothetically, by saying that as far as we can see, where our oceans now +extend they have for an enormous period extended, and where our oscillating +continents now stand they have stood ever since the Silurian epoch; but that +long before that period, the world may have presented a wholly different +aspect; and that the older continents, formed of formations older than any +known to us, may now all be in a metamorphosed condition, or may lie buried +under the ocean. +</p> + +<p> +Passing from these difficulties, all the other great leading facts in +palæontology seem to me simply to follow on the theory of descent with +modification through natural selection. We can thus understand how it is that +new species come in slowly and successively; how species of different classes +do not necessarily change together, or at the same rate, or in the same degree; +yet in the long run that all undergo modification to some extent. The +extinction of old forms is the almost inevitable consequence of the production +of new forms. We can understand why when a species has once disappeared it +never reappears. Groups of species increase in numbers slowly, and endure for +unequal periods of time; for the process of modification is necessarily slow, +and depends on many complex contingencies. The dominant species of the larger +dominant groups tend to leave many modified +<a name="Page344"></a> +descendants, and thus new sub-groups and groups are formed. As these are +formed, the species of the less vigorous groups, from their inferiority +inherited from a common progenitor, tend to become extinct together, and to +leave no modified offspring on the face of the earth. But the utter extinction +of a whole group of species may often be a very slow process, from the survival +of a few descendants, lingering in protected and isolated situations. When a +group has once wholly disappeared, it does not reappear; for the link of +generation has been broken. +</p> + +<p> +We can understand how the spreading of the dominant forms of life, which are +those that oftenest vary, will in the long run tend to people the world with +allied, but modified, descendants; and these will generally succeed in taking +the places of those groups of species which are their inferiors in the struggle +for existence. Hence, after long intervals of time, the productions of the +world will appear to have changed simultaneously. +</p> + +<p> +We can understand how it is that all the forms of life, ancient and recent, +make together one grand system; for all are connected by generation. We can +understand, from the continued tendency to divergence of character, why the +more ancient a form is, the more it generally differs from those now living. +Why ancient and extinct forms often tend to fill up gaps between existing +forms, sometimes blending two groups previously classed as distinct into one; +but more commonly only bringing them a little closer together. The more ancient +a form is, the more often, apparently, it displays characters in some degree +intermediate between groups now distinct; for the more ancient a form is, the +more nearly it will be related to, and consequently resemble, the common +progenitor of groups, since become +<a name="Page345"></a> +widely divergent. Extinct forms are seldom directly intermediate between +existing forms; but are intermediate only by a long and circuitous course +through many extinct and very different forms. We can clearly see why the +organic remains of closely consecutive formations are more closely allied to +each other, than are those of remote formations; for the forms are more closely +linked together by generation: we can clearly see why the remains of an +intermediate formation are intermediate in character. +</p> + +<p> +The inhabitants of each successive period in the world’s history have +beaten their predecessors in the race for life, and are, in so far, higher in +the scale of nature; and this may account for that vague yet ill-defined +sentiment, felt by many palæontologists, that organisation on the whole has +progressed. If it should hereafter be proved that ancient animals resemble to a +certain extent the embryos of more recent animals of the same class, the fact +will be intelligible. The succession of the same types of structure within the +same areas during the later geological periods ceases to be mysterious, and is +simply explained by inheritance. +</p> + +<p> +If then the geological record be as imperfect as I believe it to be, and it may +at least be asserted that the record cannot be proved to be much more perfect, +the main objections to the theory of natural selection are greatly diminished +or disappear. On the other hand, all the chief laws of palæontology plainly +proclaim, as it seems to me, that species have been produced by ordinary +generation: old forms having been supplanted by new and improved forms of life, +produced by the laws of variation still acting round us, and preserved by +Natural Selection. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page346"></a><a name="chap11"></a>CHAPTER XI.<br /> +GEOGRAPHICAL DISTRIBUTION.</h2> + +<p class="letter"> +Present distribution cannot be accounted for by differences in physical +conditions. Importance of barriers. Affinity of the productions of the same +continent. Centres of creation. Means of dispersal, by changes of climate and +of the level of the land, and by occasional means. Dispersal during the Glacial +period co-extensive with the world. +</p> + +<p> +In considering the distribution of organic beings over the face of the globe, +the first great fact which strikes us is, that neither the similarity nor the +dissimilarity of the inhabitants of various regions can be accounted for by +their climatal and other physical conditions. Of late, almost every author who +has studied the subject has come to this conclusion. The case of America alone +would almost suffice to prove its truth: for if we exclude the northern parts +where the circumpolar land is almost continuous, all authors agree that one of +the most fundamental divisions in geographical distribution is that between the +New and Old Worlds; yet if we travel over the vast American continent, from the +central parts of the United States to its extreme southern point, we meet with +the most diversified conditions; the most humid districts, arid deserts, lofty +mountains, grassy plains, forests, marshes, lakes, and great rivers, under +almost every temperature. There is hardly a climate or condition in the Old +World which cannot be paralleled in the New—at least as closely as the +same species generally require; for it is a most rare case to find a group of +organisms confined to any small spot, having conditions peculiar in only a +slight +<a name="Page347"></a> +degree; for instance, small areas in the Old World could be pointed out hotter +than any in the New World, yet these are not inhabited by a peculiar fauna or +flora. Notwithstanding this parallelism in the conditions of the Old and New +Worlds, how widely different are their living productions! +</p> + +<p> +In the southern hemisphere, if we compare large tracts of land in Australia, +South Africa, and western South America, between latitudes 25° and 35°, we +shall find parts extremely similar in all their conditions, yet it would not be +possible to point out three faunas and floras more utterly dissimilar. Or again +we may compare the productions of South America south of lat. 35° with those +north of 25°, which consequently inhabit a considerably different climate, and +they will be found incomparably more closely related to each other, than they +are to the productions of Australia or Africa under nearly the same climate. +Analogous facts could be given with respect to the inhabitants of the sea. +</p> + +<p> +A second great fact which strikes us in our general review is, that barriers of +any kind, or obstacles to free migration, are related in a close and important +manner to the differences between the productions of various regions. We see +this in the great difference of nearly all the terrestrial productions of the +New and Old Worlds, excepting in the northern parts, where the land almost +joins, and where, under a slightly different climate, there might have been +free migration for the northern temperate forms, as there now is for the +strictly arctic productions. We see the same fact in the great difference +between the inhabitants of Australia, Africa, and South America under the same +latitude: for these countries are almost as much isolated from each other as is +possible. On each continent, also, we see the same fact; for on the opposite +sides of +<a name="Page348"></a> +lofty and continuous mountain-ranges, and of great deserts, and +sometimes even of large rivers, we find different productions; though as +mountain chains, deserts, etc., are not as impassable, or likely to have +endured so long as the oceans separating continents, the differences are very +inferior in degree to those characteristic of distinct continents. +</p> + +<p> +Turning to the sea, we find the same law. No two marine faunas are more +distinct, with hardly a fish, shell, or crab in common, than those of the +eastern and western shores of South and Central America; yet these great faunas +are separated only by the narrow, but impassable, isthmus of Panama. Westward +of the shores of America, a wide space of open ocean extends, with not an +island as a halting-place for emigrants; here we have a barrier of another +kind, and as soon as this is passed we meet in the eastern islands of the +Pacific, with another and totally distinct fauna. So that here three marine +faunas range far northward and southward, in parallel lines not far from each +other, under corresponding climates; but from being separated from each other +by impassable barriers, either of land or open sea, they are wholly distinct. +On the other hand, proceeding still further westward from the eastern islands +of the tropical parts of the Pacific, we encounter no impassable barriers, and +we have innumerable islands as halting-places, until after travelling over a +hemisphere we come to the shores of Africa; and over this vast space we meet +with no well-defined and distinct marine faunas. Although hardly one shell, +crab or fish is common to the above-named three approximate faunas of Eastern +and Western America and the eastern Pacific islands, yet many fish range from +the Pacific into the Indian Ocean, and many shells are common to the eastern +islands of the Pacific +<a name="Page349"></a> +and the eastern shores of Africa, on almost exactly +opposite meridians of longitude. +</p> + +<p> +A third great fact, partly included in the foregoing statements, is the +affinity of the productions of the same continent or sea, though the species +themselves are distinct at different points and stations. It is a law of the +widest generality, and every continent offers innumerable instances. +Nevertheless the naturalist in travelling, for instance, from north to south +never fails to be struck by the manner in which successive groups of beings, +specifically distinct, yet clearly related, replace each other. He hears from +closely allied, yet distinct kinds of birds, notes nearly similar, and sees +their nests similarly constructed, but not quite alike, with eggs coloured in +nearly the same manner. The plains near the Straits of Magellan are inhabited +by one species of Rhea (American ostrich), and northward the plains of La Plata +by another species of the same genus; and not by a true ostrich or emeu, like +those found in Africa and Australia under the same latitude. On these same +plains of La Plata, we see the agouti and bizcacha, animals having nearly the +same habits as our hares and rabbits and belonging to the same order of +Rodents, but they plainly display an American type of structure. We ascend the +lofty peaks of the Cordillera and we find an alpine species of bizcacha; we +look to the waters, and we do not find the beaver or musk-rat, but the coypu +and capybara, rodents of the American type. Innumerable other instances could +be given. If we look to the islands off the American shore, however much they +may differ in geological structure, the inhabitants, though they may be all +peculiar species, are essentially American. We may look back to past ages, as +shown in the last chapter, and we find American types then prevalent on +<a name="Page350"></a> +the American continent and in the American seas. We see in these facts some +deep organic bond, prevailing throughout space and time, over the same areas of +land and water, and independent of their physical conditions. The naturalist +must feel little curiosity, who is not led to inquire what this bond is. +</p> + +<p> +This bond, on my theory, is simply inheritance, that cause which alone, as far +as we positively know, produces organisms quite like, or, as we see in the case +of varieties nearly like each other. The dissimilarity of the inhabitants of +different regions may be attributed to modification through natural selection, +and in a quite subordinate degree to the direct influence of different physical +conditions. The degree of dissimilarity will depend on the migration of the +more dominant forms of life from one region into another having been effected +with more or less ease, at periods more or less remote;—on the nature and +number of the former immigrants;—and on their action and reaction, in +their mutual struggles for life;—the relation of organism to organism +being, as I have already often remarked, the most important of all relations. +Thus the high importance of barriers comes into play by checking migration; as +does time for the slow process of modification through natural selection. +Widely-ranging species, abounding in individuals, which have already triumphed +over many competitors in their own widely-extended homes will have the best +chance of seizing on new places, when they spread into new countries. In their +new homes they will be exposed to new conditions, and will frequently undergo +further modification and improvement; and thus they will become still further +victorious, and will produce groups of modified descendants. On this principle +of inheritance with modification, we can understand how it is that sections of +genera, whole genera, +<a name="Page351"></a> +and even families are confined to the same areas, as is so commonly and +notoriously the case. +</p> + +<p> +I believe, as was remarked in the last chapter, in no law of necessary +development. As the variability of each species is an independent property, and +will be taken advantage of by natural selection, only so far as it profits the +individual in its complex struggle for life, so the degree of modification in +different species will be no uniform quantity. If, for instance, a number of +species, which stand in direct competition with each other, migrate in a body +into a new and afterwards isolated country, they will be little liable to +modification; for neither migration nor isolation in themselves can do +anything. These principles come into play only by bringing organisms into new +relations with each other, and in a lesser degree with the surrounding physical +conditions. As we have seen in the last chapter that some forms have retained +nearly the same character from an enormously remote geological period, so +certain species have migrated over vast spaces, and have not become greatly +modified. +</p> + +<p> +On these views, it is obvious, that the several species of the same genus, +though inhabiting the most distant quarters of the world, must originally have +proceeded from the same source, as they have descended from the same +progenitor. In the case of those species, which have undergone during whole +geological periods but little modification, there is not much difficulty in +believing that they may have migrated from the same region; for during the vast +geographical and climatal changes which will have supervened since ancient +times, almost any amount of migration is possible. But in many other cases, in +which we have reason to believe that the species of a genus have been produced +within comparatively recent times, there is great difficulty on this head. It +<a name="Page352"></a> +is also obvious that the individuals of the same species, though now inhabiting +distant and isolated regions, must have proceeded from one spot, where their +parents were first produced: for, as explained in the last chapter, it is +incredible that individuals identically the same should ever have been produced +through natural selection from parents specifically distinct. +</p> + +<p> +We are thus brought to the question which has been largely discussed by +naturalists, namely, whether species have been created at one or more points of +the earth’s surface. Undoubtedly there are very many cases of extreme +difficulty, in understanding how the same species could possibly have migrated +from some one point to the several distant and isolated points, where now +found. Nevertheless the simplicity of the view that each species was first +produced within a single region captivates the mind. He who rejects it, rejects +the <i>vera causa</i> of ordinary generation with subsequent migration, and +calls in the agency of a miracle. It is universally admitted, that in most +cases the area inhabited by a species is continuous; and when a plant or animal +inhabits two points so distant from each other, or with an interval of such a +nature, that the space could not be easily passed over by migration, the fact +is given as something remarkable and exceptional. The capacity of migrating +across the sea is more distinctly limited in terrestrial mammals, than perhaps +in any other organic beings; and, accordingly, we find no inexplicable cases of +the same mammal inhabiting distant points of the world. No geologist will feel +any difficulty in such cases as Great Britain having been formerly united to +Europe, and consequently possessing the same quadrupeds. But if the same +species can be produced at two separate points, why do we not find a single +mammal common to Europe and Australia or South America? The conditions of life +are +<a name="Page353"></a> +nearly the same, so that a multitude of European animals and plants have become +naturalised in America and Australia; and some of the aboriginal plants are +identically the same at these distant points of the northern and southern +hemispheres? The answer, as I believe, is, that mammals have not been able to +migrate, whereas some plants, from their varied means of dispersal, have +migrated across the vast and broken interspace. The great and striking +influence which barriers of every kind have had on distribution, is +intelligible only on the view that the great majority of species have been +produced on one side alone, and have not been able to migrate to the other +side. Some few families, many sub-families, very many genera, and a still +greater number of sections of genera are confined to a single region; and it +has been observed by several naturalists, that the most natural genera, or +those genera in which the species are most closely related to each other, are +generally local, or confined to one area. What a strange anomaly it would be, +if, when coming one step lower in the series, to the individuals of the same +species, a directly opposite rule prevailed; and species were not local, but +had been produced in two or more distinct areas! +</p> + +<p> +Hence it seems to me, as it has to many other naturalists, that the view of +each species having been produced in one area alone, and having subsequently +migrated from that area as far as its powers of migration and subsistence under +past and present conditions permitted, is the most probable. Undoubtedly many +cases occur, in which we cannot explain how the same species could have passed +from one point to the other. But the geographical and climatal changes, which +have certainly occurred within recent geological times, must have interrupted +or rendered discontinuous the formerly continuous range of many species. So +that we are reduced to consider whether the exceptions to +<a name="Page354"></a> +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 would 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 could 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 +freshwater 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 ignorance with respect to former climatal and geographical +changes and various occasional means of transport, the belief that this has +been the universal law, seems to me incomparably the safest. +</p> + +<p> +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, +<a name="Page355"></a> +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 fully 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 concludes, 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 correspondence, that this coincidence he +attributes to generation with modification. +</p> + +<p> +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 name="Page356"></a> +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 +descended from a succession of improved varieties, which will never have +blended with other individuals or varieties, 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 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 racehorses differ slightly from the horses of every other +breed; but they do not owe their difference and superiority to descent from any +single pair, but to continued care in selecting and training many individuals +during many generations. +</p> + +<p> +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. +</p> + +<p class="p2"> +<i>Means of Dispersal</i>.—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 influential: a narrow isthmus now separates two +marine faunas; submerge it, or let it formerly have been submerged, and the two +faunas will now blend or may formerly 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. +<a name="Page357"></a> +No geologist will dispute that great mutations of level have occurred within +the period of existing organisms. Edward Forbes insisted that all the islands +in the Atlantic must recently have been connected with Europe or Africa, and +Europe 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 intervening oceanic +islands. I freely admit the former existence 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 atolls 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 distribution, 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 +<a name="Page358"></a> +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 such facts seem to me opposed to the 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 +mountain-ranges 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. +</p> + +<p> +I must now say a few words on what are called accidental 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 be said to be almost wholly unknown. Until I tried, with +Mr. Berkeley’s aid, a few experiments, it was not even known how far +seeds could resist the injurious action of sea-water. To my surprise I found +that out of 87 kinds, 64 germinated after an immersion of 28 days, and a few +survived an immersion of 137 days. +<a name="Page359"></a> +For convenience sake I chiefly tried small seeds, without 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, etc., 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 germinated: +the ripe seeds of Helosciadium sank in two days, when dried they floated for +above 90 days, and afterwards germinated. Altogether out of the 94 dried +plants, 18 floated for above 28 days, and some of the 18 floated for a very +much longer period. So that as 64/87 seeds germinated after an immersion of 28 +days; and as 18/94 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 14/100 plants of any country might be floated by sea-currents during +28 days, and would retain their power of germination. In Johnston’s +Physical Atlas, the average rate of the several Atlantic currents is 33 miles +per diem (some currents running at the rate of 60 miles +<a name="Page360"></a> +per diem); on this average, the seeds of 14/100 plants belonging to one country +might be floated across 924 miles of sea to another country; and when stranded, +if blown to a favourable spot by an inland gale, they would germinate. +</p> + +<p> +Subsequently to my experiments, M. Martens tried similar ones, but in a much +better manner, for he placed the seeds in a box in the actual sea, so that they +were alternately wet and exposed to the air like really floating plants. He +tried 98 seeds, mostly different from mine; but he chose many large fruits and +likewise seeds from plants which live near the sea; and this would have +favoured the average length of their flotation and of their resistance to the +injurious action of the salt-water. On the other hand he did not previously dry +the plants or branches with the fruit; and this, as we have seen, would have +caused some of them to have floated much longer. The result was that 18/98 of +his seeds floated for 42 days, and were then capable of germination. But I do +not doubt that plants exposed to the waves would float for a less time than +those protected from violent movement as in our experiments. Therefore it would +perhaps be safer to assume that the seeds of about 10/100 plants of a flora, +after having been dried, could be floated across a space of sea 900 miles in +width, and would then germinate. The fact of the larger fruits often floating +longer than the small, is interesting; as plants with large seeds or fruit +could hardly be transported by any other means; and Alph. de Candolle has shown +that such plants generally have restricted ranges. +</p> + +<p> +But seeds may be occasionally transported in another manner. Drift timber is +thrown up on most islands, even on those in the midst of the widest oceans; and +the natives of the coral-islands in the Pacific, procure +<a name="Page361"></a> +stones for their tools, solely from the roots of drifted trees, these stones +being a valuable royal tax. I find on examination, that when irregularly shaped +stones are embedded in the roots of trees, small parcels of earth are very +frequently enclosed in their interstices and behind them,—so perfectly +that not a particle could be washed away in the longest transport: out of one +small portion of earth thus <i>completely</i> enclosed by wood in an oak about +50 years old, three dicotyledonous plants germinated: I am certain of the +accuracy of this observation. Again, I can show that the carcasses of birds, +when floating on the sea, sometimes escape being immediately devoured; and +seeds of many kinds in the crops of floating birds long retain their vitality: +peas and vetches, for instance, are killed by even a few days’ immersion +in sea-water; but some taken out of the crop of a pigeon, which had floated on +artificial salt-water for 30 days, to my surprise nearly all germinated. +</p> + +<p> +Living birds can hardly fail to be highly effective agents in the +transportation of seeds. I could give many facts showing how frequently birds +of many kinds are blown by gales to vast distances across the ocean. We may I +think safely assume that under such circumstances their rate of flight would +often be 35 miles an hour; and some authors have given a far higher estimate. I +have never seen an instance of nutritious seeds passing through the intestines +of a bird; but hard seeds of fruit will pass uninjured through even the +digestive organs of a turkey. In the course of two months, I picked up in my +garden 12 kinds of seeds, out of the excrement of small birds, and these seemed +perfect, and some of them, which I tried, germinated. But the following fact is +more important: the crops of birds do not secrete gastric juice, and do not in +the +<a name="Page362"></a> +least injure, as I know by trial, the germination of seeds; now after a bird +has found and devoured a large supply of food, it is positively asserted that +all the grains do not pass into the gizzard for 12 or even 18 hours. A bird in +this interval might easily be blown to the distance of 500 miles, and hawks are +known to look out for tired birds, and the contents of their torn crops might +thus readily get scattered. Mr. Brent informs me that a friend of his had to +give up flying carrier-pigeons from France to England, as the hawks on the +English coast destroyed so many on their arrival. Some hawks and owls bolt +their prey whole, and after an interval of from twelve to twenty hours, +disgorge pellets, which, as I know from experiments made in the Zoological +Gardens, include seeds capable of germination. Some seeds of the oat, wheat, +millet, canary, hemp, clover, and beet germinated after having been from twelve +to twenty-one hours in the stomachs of different birds of prey; and two seeds +of beet grew after having been thus retained for two days and fourteen hours. +Freshwater fish, I find, eat seeds of many land and water plants: fish are +frequently devoured by birds, and thus the seeds might be transported from +place to place. I forced many kinds of seeds into the stomachs of dead fish, +and then gave their bodies to fishing-eagles, storks, and pelicans; these birds +after an interval of many hours, either rejected the seeds in pellets or passed +them in their excrement; and several of these seeds retained their power of +germination. Certain seeds, however, were always killed by this process. +</p> + +<p> +Although the beaks and feet of birds are generally quite clean, I can show that +earth sometimes adheres to them: in one instance I removed twenty-two grains of +dry argillaceous earth from one foot of a partridge, and in this earth there +was a pebble quite as large as +<a name="Page363"></a> +the seed of a vetch. Thus seeds might occasionally be transported to great +distances; for many facts could be given showing that soil almost everywhere is +charged with seeds. Reflect for a moment on the millions of quails which +annually cross the Mediterranean; and can we doubt that the earth adhering to +their feet would sometimes include a few minute seeds? But I shall presently +have to recur to this subject. +</p> + +<p> +As icebergs are known to be sometimes loaded with earth and stones, and have +even carried brushwood, bones, and the nest of a land-bird, I can hardly doubt +that they must occasionally have transported seeds from one part to another of +the arctic and antarctic regions, as suggested by Lyell; and during the Glacial +period from one part of the now temperate regions to another. In the Azores, +from the large number of the species of plants common to Europe, in comparison +with the plants of other oceanic islands nearer to the mainland, and (as +remarked by Mr. H. C. Watson) from the somewhat northern character of the flora +in comparison with the latitude, I suspected that these islands had been partly +stocked by ice-borne seeds, during the Glacial epoch. At my request Sir C. +Lyell wrote to M. Hartung to inquire whether he had observed erratic boulders +on these islands, and he answered that he had found large fragments of granite +and other rocks, which do not occur in the archipelago. Hence we may safely +infer that icebergs formerly landed their rocky burthens on the shores of these +mid-ocean islands, and it is at least possible that they may have brought +thither the seeds of northern plants. +</p> + +<p> +Considering that the several above means of transport, and that several other +means, which without doubt remain to be discovered, have been in action year +after year, for centuries and tens of thousands of +<a name="Page364"></a> +years, it would I think be a marvellous fact if many plants had not thus become +widely transported. These means of transport are sometimes called accidental, +but this is not strictly correct: the currents of the sea are not accidental, +nor is the direction of prevalent gales of wind. It should be observed that +scarcely any means of transport would carry seeds for very great distances; for +seeds do not retain their vitality when exposed for a great length of time to +the action of seawater; nor could they be long carried in the crops or +intestines of birds. These means, however, would suffice for occasional +transport across tracts of sea some hundred miles in breadth, or from island to +island, or from a continent to a neighbouring island, but not from one distant +continent to another. The floras of distant continents would not by such means +become mingled in any great degree; but would remain as distinct as we now see +them to be. The currents, from their course, would never bring seeds from North +America to Britain, though they might and do bring seeds from the West Indies +to our western shores, where, if not killed by so long an immersion in +salt-water, they could not endure our climate. Almost every year, one or two +land-birds are blown across the whole Atlantic Ocean, from North America to the +western shores of Ireland and England; but seeds could be transported by these +wanderers only by one means, namely, in dirt sticking to their feet, which is +in itself a rare accident. Even in this case, how small would the chance be of +a seed falling on favourable soil, and coming to maturity! But it would be a +great error to argue that because a well-stocked island, like Great Britain, +has not, as far as is known (and it would be very difficult to prove this), +received within the last few centuries, through occasional means +<a name="Page365"></a> +of transport, immigrants from Europe or any other continent, that a +poorly-stocked island, though standing more remote from the mainland, would not +receive colonists by similar means. I do not doubt that out of twenty seeds or +animals transported to an island, even if far less well-stocked than Britain, +scarcely more than one would be so well fitted to its new home, as to become +naturalised. But this, as it seems to me, is no valid argument against what +would be effected by occasional means of transport, during the long lapse of +geological time, whilst an island was being upheaved and formed, and before it +had become fully stocked with inhabitants. On almost bare land, with few or no +destructive insects or birds living there, nearly every seed, which chanced to +arrive, would be sure to germinate and survive. +</p> + +<p class="p2"> +<i>Dispersal during the Glacial period</i>.—The identity of many plants +and animals, on mountain-summits, separated from each other by hundreds of +miles of lowlands, where the Alpine species could not possibly exist, is one of +the most striking cases known of the same species living at distant points, +without the apparent possibility of their having migrated from one to the +other. It is indeed a remarkable fact to see so many of the same plants living +on the snowy regions of the Alps or Pyrenees, and in the extreme northern parts +of Europe; but it is far more remarkable, that the plants on the White +Mountains, in the United States of America, are all the same with those of +Labrador, and nearly all the same, as we hear from Asa Gray, with those on the +loftiest mountains of Europe. Even as long ago as 1747, such facts led Gmelin +to conclude that the same species must have been independently created at +several distinct points; and we might have remained +<a name="Page366"></a> +in this same belief, had not Agassiz and others called vivid attention to the +Glacial period, which, as we shall immediately see, affords a simple +explanation of these facts. We have evidence of almost every conceivable kind, +organic and inorganic, that within a very recent geological period, central +Europe and North America suffered under an Arctic climate. The ruins of a house +burnt by fire do not tell their tale more plainly, than do the mountains of +Scotland and Wales, with their scored flanks, polished surfaces, and perched +boulders, of the icy streams with which their valleys were lately filled. So +greatly has the climate of Europe changed, that in Northern Italy, gigantic +moraines, left by old glaciers, are now clothed by the vine and maize. +Throughout a large part of the United States, erratic boulders, and rocks +scored by drifted icebergs and coast-ice, plainly reveal a former cold period. +</p> + +<p> +The former influence of the glacial climate on the distribution of the +inhabitants of Europe, as explained with remarkable clearness by Edward Forbes, +is substantially as follows. But we shall follow the changes more readily, by +supposing a new glacial period to come slowly on, and then pass away, as +formerly occurred. As the cold came on, and as each more southern zone became +fitted for arctic beings and ill-fitted for their former more temperate +inhabitants, the latter would be supplanted and arctic productions would take +their places. The inhabitants of the more temperate regions would at the same +time travel southward, unless they were stopped by barriers, in which case they +would perish. The mountains would become covered with snow and ice, and their +former Alpine inhabitants would descend to the plains. By the time that the +cold had reached its maximum, we should have a uniform arctic fauna and flora, +covering the central parts of Europe, as far +<a name="Page367"></a> +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 southward, 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. +</p> + +<p> +As the warmth returned, the arctic forms would retreat 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 mountain-summits (having been +exterminated on all lesser heights) and in the arctic regions of both +hemispheres. +</p> + +<p> +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, +<a name="Page368"></a> +and those of the Pyrenees, as remarked by Ramond, are more especially allied to +the plants of northern Scandinavia; 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 +mountain-summits, we may almost conclude without other evidence, that a colder +climate permitted their former migration across the low intervening tracts, +since become too warm for their existence. +</p> + +<p> +If the climate, since the Glacial period, has ever been in any degree warmer +than at present (as some geologists in the United States believe to have been +the case, chiefly from the distribution of the fossil Gnathodon), 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 warmer period, since the Glacial period. +</p> + +<p> +The arctic forms, during their long southern migration 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; consequently +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 different; +<a name="Page369"></a> +for it is not likely that all the same arctic species 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 mountain-ranges, 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. +</p> + +<p> +In illustrating what, as I believe, actually took place during the Glacial +period, I assumed that at its commencement the arctic productions were as +uniform round the polar regions as they are at the present day. But the +foregoing 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 +degree of uniformity of the sub-arctic and northern temperate 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 inhabitants +<a name="Page370"></a> +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 +warmer than at the present day. Hence we may suppose 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 this 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. +</p> + +<p> +Believing, from reasons before alluded to, that our continents have long +remained in nearly the same relative 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 slowly to migrate southwards as the climate +became less warm, long before the commencement +<a name="Page371"></a> +of the Glacial period. We now see, as I believe, their descendants, mostly in a +modified condition, in the central parts of Europe and the United States. On +this view we can understand the relationship, 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 present 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. +</p> + +<p> +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 productions 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 everything 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 +<a name="Page372"></a> +species (though Asa Gray has lately shown that more plants are identical than +was formerly supposed), but we find in every great class many forms, which some +naturalists rank as geographical races, and others as distinct species; and a +host of closely allied or representative forms which are ranked by all +naturalists as specifically distinct. +</p> + +<p> +As on the land, so in the waters of the sea, a slow southern migration of a +marine fauna, which 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 presence of +many existing and tertiary representative forms on the eastern and western +shores of temperate North America; and the still more striking case of many +closely allied crustaceans (as described in Dana’s admirable work), of +some fish and other marine animals, in the Mediterranean and in the seas of +Japan,—areas now separated by a continent and by nearly a hemisphere of +equatorial ocean. +</p> + +<p> +These cases of relationship, without identity, of the inhabitants of seas now +disjoined, and likewise of the past and present inhabitants of the temperate +lands of North America and Europe, are inexplicable on the theory of creation. +We cannot say that they have been created alike, in correspondence with the +nearly similar physical conditions of the areas; for if we compare, 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. +</p> + +<p> +But we must return to our more immediate subject, the Glacial period. I am +convinced that Forbes’s view +<a name="Page373"></a> +may be largely extended. In Europe we have 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 mountain +vegetation, that Siberia was similarly affected. Along the Himalaya, at points +900 miles apart, glaciers have 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 plants, found on widely separated mountains in this +island, tell the same story. If one account which has been published can be +trusted, we have direct evidence of glacial action in the south-eastern corner +of Australia. +</p> + +<p> +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 +deg; erratic boulders have, also, been noticed on the Rocky 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. +</p> + +<p> +We do not know that the Glacial epoch was strictly simultaneous at these +several far distant points on opposite sides of the world. But we have good +evidence in almost every case, that the epoch was included within +<a name="Page374"></a> +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 simultaneous 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 western +sides of North America, in the Cordillera under the equator and under the +warmer 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 whole world was at this period simultaneously cooler. But it +would suffice for my purpose, if the temperature was at the same time lower +along certain broad belts of longitude. +</p> + +<p> +On this view of the whole world, or at least of broad longitudinal belts, +having been simultaneously colder from pole to pole, much light can be thrown +on the present distribution of identical and allied species. In America, Dr. +Hooker has shown 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 two 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 belonging +<a name="Page375"></a> +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. +</p> + +<p> +On the southern mountains of Australia, Dr. F. Müller 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 +admirable ‘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 +<a name="Page376"></a> +identically the same; but they are much oftener specifically distinct, though +related to each other in a most remarkable manner. +</p> + +<p> +This brief abstract applies to plants alone: some strictly analogous facts +could be given on the distribution of terrestrial animals. In marine +productions, similar cases occur; as an example, I may quote a remark by the +highest authority, Professor 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 reappearance on the shores of New Zealand, +Tasmania, etc., of northern forms of fish. Dr. Hooker informs me that +twenty-five species of Algæ are common to New Zealand and to Europe, but have +not been found in the intermediate tropical seas. +</p> + +<p> +It should be observed that the northern species and forms found in the southern +parts of the southern hemisphere, and on the mountain-ranges of the +intertropical regions, are not arctic, but belong to the northern temperate +zones. As Mr. H. C. Watson has recently remarked, “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. +</p> + +<p> +Now let us see what light can be thrown on the foregoing 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 simultaneously much +<a name="Page377"></a> +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 within 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 will 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 concerned. The tropical +plants probably suffered much extinction; how much no one can say; perhaps +formerly the tropics supported 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 +certain extent. On the other hand, the temperate productions, after migrating +nearer to the equator, though they will have been placed under somewhat new +conditions, 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 +<a name="Page378"></a> +by a dry 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 +climate. On the other hand, the most humid and hottest districts will have +afforded an asylum to the tropical natives. The mountain-ranges north-west of +the Himalaya, and the long line of the Cordillera, seem to have afforded two +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, which must have +lain on the line of march. But I do not doubt that some temperate productions +entered and crossed even the <i>lowlands</i> of the tropics at the period when +the cold was most intense,—when arctic forms had migrated some +twenty-five degrees of latitude from their native country and covered the land +at the foot of the Pyrenees. At this period of extreme 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 were +clothed with a mingled tropical and temperate vegetation, like that now growing +with strange luxuriance at the base of the Himalaya, as graphically described +by Hooker. +</p> + +<p> +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 regions, and some +even crossed the equator. As the warmth returned, these temperate forms would +naturally ascend the higher mountains, being exterminated on the lowlands; +those which had not reached the equator, would re-migrate northward or +southward towards their former +<a name="Page379"></a> +homes; but the forms, chiefly northern, which had crossed the equator, would +travel still further from their homes into the 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 different 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 with +many new forms of life; and it is probable that selected modifications in their +structure, habits, and constitutions 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. +</p> + +<p> +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 own homes in greater +numbers, and having consequently been advanced through natural selection and +competition to a higher stage of perfection or dominating power, than the +southern forms. And thus, when they became commingled 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, +<a name="Page380"></a> +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 imported into Europe during the last two or three centuries from La +Plata, and during the last thirty or forty years from Australia. Something of +the same kind must have occurred on the intertropical mountains: no doubt +before the Glacial period they were stocked with endemic Alpine forms; but +these have almost everywhere largely yielded to the more dominant forms, +generated in the larger areas and more efficient workshops of the north. In +many islands the native productions are nearly equalled or even outnumbered by +the naturalised; and if the natives have not been actually 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 productions 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, naturalised by man’s agency. +</p> + +<p> +I am far from supposing that all difficulties are removed 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 +intertropical regions. Very many 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; +<a name="Page381"></a> +why certain species have been modified and have given rise to new groups of +forms, and others have remained unaltered. We cannot hope to explain such +facts, until we can say why one species and not another becomes 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 within their own homes. +</p> + +<p> +I have said that many difficulties remain to be solved: some of the most +remarkable are stated with admirable clearness by Dr. Hooker in his botanical +works 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 +towards 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 commencement of the Glacial period for their migration, and for +their subsequent modification to the necessary degree. The facts seem to me to +indicate that peculiar and very distinct species have migrated in radiating +lines from some common centre; and I am inclined to look in the southern, as in +the northern hemisphere, to a former and warmer period, before the commencement +of the Glacial period, when the antarctic lands, 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 +<a name="Page382"></a> +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 believe, the southern shores of America, +Australia, New Zealand have become slightly tinted by the same peculiar forms +of vegetable life. +</p> + +<p> +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 +distribution. 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 life can be explained. 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 +mountain-summits, in a line gently rising from the arctic lowlands 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 mountain-fastnesses of +almost every land, which serve as a record, full of interest to us, of the +former inhabitants of the surrounding lowlands. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page383"></a><a name="chap12"></a>CHAPTER XII.<br /> +GEOGRAPHICAL DISTRIBUTION—<i>continued</i>.</h2> + +<p class="letter"> +Distribution of fresh-water productions. On the inhabitants of oceanic islands. +Absence of Batrachians and of terrestrial Mammals. On the relation of the +inhabitants of islands to those of the nearest mainland. On colonisation from +the nearest source with subsequent modification. Summary of the last and +present chapters. +</p> + +<p> +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 apparently 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 throughout the world. I well remember, when +first collecting in the fresh waters of Brazil, feeling much surprise at the +similarity of the fresh-water insects, shells, etc., and at the dissimilarity +of the surrounding terrestrial beings, compared with those of Britain. +</p> + +<p> +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. We can here +consider only a few cases. In regard to +<a name="Page384"></a> +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 transport 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 very recent geological period, and when the surface was peopled by +existing land and fresh-water shells. The wide difference of the fish on +opposite sides of continuous mountain-ranges, which from an early period must +have parted river-systems and completely prevented 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 explained: 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 consequently 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 marine +member of a fresh-water group might travel far along the shores of the sea, and +subsequently +<a name="Page385"></a> +become modified and adapted to the fresh waters of a distant land. +</p> + +<p> +Some species of fresh-water shells have a very wide range, and allied species, +which, on my theory, are descended 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, which I have observed—and no doubt many others +remain to be observed—throw some light on this subject. When 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 represent those of a bird sleeping in a natural pond, in an +aquarium, where many ova of fresh-water shells were hatching; and I found that +numbers of the extremely minute and just hatched shells crawled on the feet, +and clung to them so firmly that when taken out of the water they could not be +jarred off, though at a somewhat more advanced age they would voluntarily drop +off. These just hatched molluscs, though aquatic in their nature, survived on +the duck’s feet, in damp air, from twelve to twenty hours; and in this +length of time a duck or heron might fly at least six or seven hundred miles, +and would be sure to alight on a pool or rivulet, if blown across sea to an +oceanic island or to any other distant point. Sir Charles Lyell also +<a name="Page386"></a> +informs me that a Dyticus has been caught with an Ancylus (a fresh-water shell +like a limpet) firmly adhering to it; and a water-beetle of the same family, a +Colymbetes, once flew on board the ‘Beagle,’ when forty-five miles +distant from the nearest land: how much farther it might have flown with a +favouring gale no one can tell. +</p> + +<p> +With respect to plants, it has long been known what enormous ranges many +fresh-water and even marsh-species have, both over continents and to the most +remote oceanic islands. This is strikingly shown, as remarked by Alph. de +Candolle, in large groups of terrestrial plants, which have only a very few +aquatic members; for these latter seem immediately to acquire, as if in +consequence, a very wide range. I think favourable means of dispersal explain +this fact. I have before mentioned that earth occasionally, though rarely, +adheres in some quantity to the feet and beaks of birds. Wading birds, which +frequent the muddy edges of ponds, if suddenly flushed, would be the most +likely to have muddy feet. Birds of this order I can show are the greatest +wanderers, and are occasionally found on the most remote and barren islands in +the open ocean; they would not be likely to alight on the surface of the sea, +so that the dirt would not be washed off their feet; when making land, they +would be sure to fly to their natural fresh-water haunts. I do not believe that +botanists are aware how charged the mud of ponds is with seeds: I have tried +several little experiments, but will here give only the most striking case: I +took in February three table-spoonfuls of mud from three different points, +beneath water, on the edge of a little pond; this mud when dry weighed only 6 +3/4 ounces; I kept it covered up in my study for six months, pulling up and +counting each plant as it grew; the plants were +<a name="Page387"></a> +of many kinds, and were altogether 537 in number; and yet the viscid mud was +all contained in a breakfast cup! Considering these facts, I think it would be +an inexplicable circumstance if water-birds did not transport the seeds of +fresh-water plants to vast distances, and if consequently the range of these +plants was not very great. The same agency may have come into play with the +eggs of some of the smaller fresh-water animals. +</p> + +<p> +Other and unknown agencies probably have also played a part. I have stated that +fresh-water fish eat some kinds of seeds, though they reject many other kinds +after having swallowed them; even small fish swallow seeds of moderate size, as +of the yellow water-lily and Potamogeton. Herons and other birds, century after +century, have gone on daily devouring fish; they then take flight and go to +other waters, or are blown across the sea; and we have seen that seeds retain +their power of germination, when rejected in pellets or in excrement, many +hours afterwards. When I saw the great size of the seeds of that fine +water-lily, the Nelumbium, and remembered Alph. de Candolle’s remarks on +this plant, I thought that its distribution must remain quite inexplicable; but +Audubon states that he found the seeds of the great southern water-lily +(probably, according to Dr. Hooker, the Nelumbium luteum) in a heron’s +stomach; although I do not know the fact, yet analogy makes me believe that a +heron flying to another pond and getting a hearty meal of fish, would probably +reject from its stomach a pellet containing the seeds of the Nelumbium +undigested; or the seeds might be dropped by the bird whilst feeding its young, +in the same way as fish are known sometimes to be dropped. +</p> + +<p> +In considering these several means of distribution, +<a name="Page388"></a> +it should be remembered that when a pond or stream is first formed, for +instance, on a rising islet, it will be unoccupied; and a single seed or egg +will have a good chance of succeeding. Although there will always be a struggle +for life between the individuals of the species, however few, already occupying +any pond, yet as the number of kinds is small, compared with those on the land, +the competition will probably be less severe between aquatic than between +terrestrial species; consequently an intruder from the waters of a foreign +country, would have a better chance of seizing on a place, than in the case of +terrestrial colonists. We should, also, remember that some, perhaps many, +fresh-water productions are low in the scale of nature, and that we have reason +to believe that such low beings change or become modified less quickly than the +high; and this will give longer time than the average for the migration of the +same aquatic species. We should not forget the probability of many species +having formerly ranged as continuously as fresh-water productions ever can +range, over immense areas, and having subsequently become extinct in +intermediate regions. But the wide distribution of fresh-water plants and of +the lower animals, whether retaining the same identical form or in some degree +modified, I believe mainly depends on the wide dispersal of their seeds and +eggs by animals, more especially by fresh-water birds, which have large powers +of flight, and naturally travel from one to another and often distant piece of +water. Nature, like a careful gardener, thus takes her seeds from a bed of a +particular nature, and drops them in another equally well fitted for them. +</p> + +<p class="p2"> +<i>On the Inhabitants of Oceanic Islands</i>.—We now come to the last of +the three classes of facts, which I +<a name="Page389"></a> +have selected as presenting the greatest amount of difficulty, on the view that +all the individuals both of the same and of allied species have descended from +a single parent; and therefore have all proceeded from a common birthplace, +notwithstanding that in the course of time they have come to inhabit distant +points of the globe. I have already stated that I cannot honestly admit +Forbes’s view on continental extensions, which, if legitimately followed +out, would lead to the belief that within the recent period all existing +islands have been nearly or quite joined to some continent. This view would +remove many difficulties, but it would not, I think, explain all the facts in +regard to insular productions. In the following remarks I shall not confine +myself to the mere question of dispersal; but shall consider some other facts, +which bear on the truth of the two theories of independent creation and of +descent with modification. +</p> + +<p> +The species of all kinds which inhabit oceanic islands are few in number +compared with those on equal continental areas: Alph. de Candolle admits this +for plants, and Wollaston for insects. If we look to the large size and varied +stations of New Zealand, extending over 780 miles of latitude, and compare its +flowering plants, only 750 in number, with those on an equal area at the Cape +of Good Hope or in Australia, we must, I think, admit that something quite +independently of any difference in physical conditions has caused so great a +difference in number. Even the uniform county of Cambridge has 847 plants, and +the little island of Anglesea 764, but a few ferns and a few introduced plants +are included in these numbers, and the comparison in some other respects is not +quite fair. We have evidence that the barren island of Ascension aboriginally +possessed under half-a-dozen flowering +<a name="Page390"></a> +plants; yet many have become naturalised on it, as they have on New Zealand and +on every other oceanic island which can be named. In St. Helena there is reason +to believe that the naturalised plants and animals have nearly or quite +exterminated many native productions. He who admits the doctrine of the +creation of each separate species, will have to admit, that a sufficient number +of the best adapted plants and animals have not been created on oceanic +islands; for man has unintentionally stocked them from various sources far more +fully and perfectly than has nature. +</p> + +<p> +Although in oceanic islands the number of kinds of inhabitants is scanty, the +proportion of endemic species (<i>i.e.</i> those found nowhere else in the +world) is often extremely large. If we compare, for instance, the number of the +endemic land-shells in Madeira, or of the endemic birds in the Galapagos +Archipelago, with the number found on any continent, and then compare the area +of the islands with that of the continent, we shall see that this is true. This +fact might have been expected on my theory, for, as already explained, species +occasionally arriving after long intervals in a new and isolated district, and +having to compete with new associates, will be eminently liable to +modification, and will often produce groups of modified descendants. But it by +no means follows, that, because in an island nearly all the species of one +class are peculiar, those of another class, or of another section of the same +class, are peculiar; and this difference seems to depend on the species which +do not become modified having immigrated with facility and in a body, so that +their mutual relations have not been much disturbed. Thus in the Galapagos +Islands nearly every land-bird, but only two out of the eleven marine birds, +are peculiar; and it is obvious that +<a name="Page391"></a> +marine birds could arrive at these islands more easily than land-birds. +Bermuda, on the other hand, which lies at about the same distance from North +America as the Galapagos Islands do from South America, and which has a very +peculiar soil, does not possess one endemic land bird; and we know from Mr. J. +M. Jones’s admirable account of Bermuda, that very many North American +birds, during their great annual migrations, visit either periodically or +occasionally this island. Madeira does not possess one peculiar bird, and many +European and African birds are almost every year blown there, as I am informed +by Mr. E. V. Harcourt. So that these two islands of Bermuda and Madeira have +been stocked by birds, which for long ages have struggled together in their +former homes, and have become mutually adapted to each other; and when settled +in their new homes, each kind will have been kept by the others to their proper +places and habits, and will consequently have been little liable to +modification. Madeira, again, is inhabited by a wonderful number of peculiar +land-shells, whereas not one species of sea-shell is confined to its shores: +now, though we do not know how seashells are dispersed, yet we can see that +their eggs or larvæ, perhaps attached to seaweed or floating timber, or to the +feet of wading-birds, might be transported far more easily than land-shells, +across three or four hundred miles of open sea. The different orders of insects +in Madeira apparently present analogous facts. +</p> + +<p> +Oceanic islands are sometimes deficient in certain classes, and their places +are apparently occupied by the other inhabitants; in the Galapagos Islands +reptiles, and in New Zealand gigantic wingless birds, take the place of +mammals. In the plants of the Galapagos Islands, Dr. Hooker has shown that the +proportional numbers of the different orders are very different from +<a name="Page392"></a> +what they are elsewhere. Such cases are generally accounted for by the physical +conditions of the islands; but this explanation seems to me not a little +doubtful. Facility of immigration, I believe, has been at least as important as +the nature of the conditions. +</p> + +<p> +Many remarkable little facts could be given with respect to the inhabitants of +remote islands. For instance, in certain islands not tenanted by mammals, some +of the endemic plants have beautifully hooked seeds; yet few relations are more +striking than the adaptation of hooked seeds for transportal by the wool and +fur of quadrupeds. This case presents no difficulty on my view, for a hooked +seed might be transported to an island by some other means; and the plant then +becoming slightly modified, but still retaining its hooked seeds, would form an +endemic species, having as useless an appendage as any rudimentary +organ,—for instance, as the shrivelled wings under the soldered elytra of +many insular beetles. Again, islands often possess trees or bushes belonging to +orders which elsewhere include only herbaceous species; now trees, as Alph. de +Candolle has shown, generally have, whatever the cause may be, confined ranges. +Hence trees would be little likely to reach distant oceanic islands; and an +herbaceous plant, though it would have no chance of successfully competing in +stature with a fully developed tree, when established on an island and having +to compete with herbaceous plants alone, might readily gain an advantage by +growing taller and taller and overtopping the other plants. If so, natural +selection would often tend to add to the stature of herbaceous plants when +growing on an island, to whatever order they belonged, and thus convert them +first into bushes and ultimately into trees. +</p> + +<p> +With respect to the absence of whole orders on +<a name="Page393"></a> +oceanic islands, Bory St. Vincent long ago remarked that Batrachians (frogs, +toads, newts) have never been found on any of the many islands with which the +great oceans are studded. I have taken pains to verify this assertion, and I +have found it strictly true. I have, however, been assured that a frog exists +on the mountains of the great island of New Zealand; but I suspect that this +exception (if the information be correct) may be explained through glacial +agency. This general absence of frogs, toads, and newts on so many oceanic +islands cannot be accounted for by their physical conditions; indeed it seems +that islands are peculiarly well fitted for these animals; for frogs have been +introduced into Madeira, the Azores, and Mauritius, and have multiplied so as +to become a nuisance. But as these animals and their spawn are known to be +immediately killed by sea-water, on my view we can see that there would be +great difficulty in their transportal across the sea, and therefore why they do +not exist on any oceanic island. But why, on the theory of creation, they +should not have been created there, it would be very difficult to explain. +</p> + +<p> +Mammals offer another and similar case. I have carefully searched the oldest +voyages, but have not finished my search; as yet I have not found a single +instance, free from doubt, of a terrestrial mammal (excluding domesticated +animals kept by the natives) inhabiting an island situated above 300 miles from +a continent or great continental island; and many islands situated at a much +less distance are equally barren. The Falkland Islands, which are inhabited by +a wolf-like fox, come nearest to an exception; but this group cannot be +considered as oceanic, as it lies on a bank connected with the mainland; +moreover, icebergs formerly brought boulders to its western shores, and they +may +<a name="Page394"></a> +have formerly transported foxes, as so frequently now happens in the arctic +regions. Yet it cannot be said that small islands will not support small +mammals, for they occur in many parts of the world on very small islands, if +close to a continent; and hardly an island can be named on which our smaller +quadrupeds have not become naturalised and greatly multiplied. It cannot be +said, on the ordinary view of creation, that there has not been time for the +creation of mammals; many volcanic islands are sufficiently ancient, as shown +by the stupendous degradation which they have suffered and by their tertiary +strata: there has also been time for the production of endemic species +belonging to other classes; and on continents it is thought that mammals appear +and disappear at a quicker rate than other and lower animals. Though +terrestrial mammals do not occur on oceanic islands, ærial mammals do occur on +almost every island. New Zealand possesses two bats found nowhere else in the +world: Norfolk Island, the Viti Archipelago, the Bonin Islands, the Caroline +and Marianne Archipelagoes, and Mauritius, all possess their peculiar bats. +Why, it may be asked, has the supposed creative force produced bats and no +other mammals on remote islands? On my view this question can easily be +answered; for no terrestrial mammal can be transported across a wide space of +sea, but bats can fly across. Bats have been seen wandering by day far over the +Atlantic Ocean; and two North American species either regularly or occasionally +visit Bermuda, at the distance of 600 miles from the mainland. I hear from Mr. +Tomes, who has specially studied this family, that many of the same species +have enormous ranges, and are found on continents and on far distant islands. +Hence we have only to suppose that such wandering species have been modified +<a name="Page395"></a> +through natural selection in their new homes in relation to their new position, +and we can understand the presence of endemic bats on islands, with the absence +of all terrestrial mammals. +</p> + +<p> +Besides the absence of terrestrial mammals in relation to the remoteness of +islands from continents, there is also a relation, to a certain extent +independent of distance, between the depth of the sea separating an island from +the neighbouring mainland, and the presence in both of the same mammiferous +species or of allied species in a more or less modified condition. Mr. Windsor +Earl has made some striking observations on this head in regard to the great +Malay Archipelago, which is traversed near Celebes by a space of deep ocean; +and this space separates two widely distinct mammalian faunas. On either side +the islands are situated on moderately deep submarine banks, and they are +inhabited by closely allied or identical quadrupeds. No doubt some few +anomalies occur in this great archipelago, and there is much difficulty in +forming a judgment in some cases owing to the probable naturalisation of +certain mammals through man’s agency; but we shall soon have much light +thrown on the natural history of this archipelago by the admirable zeal and +researches of Mr. Wallace. I have not as yet had time to follow up this subject +in all other quarters of the world; but as far as I have gone, the relation +generally holds good. We see Britain separated by a shallow channel from +Europe, and the mammals are the same on both sides; we meet with analogous +facts on many islands separated by similar channels from Australia. The West +Indian Islands stand on a deeply submerged bank, nearly 1000 fathoms in depth, +and here we find American forms, but the species and even the genera are +distinct. As the amount of modification in all cases depends to +<a name="Page396"></a> +a certain degree on the lapse of time, and as during changes of level it is +obvious that islands separated by shallow channels are more likely to have been +continuously united within a recent period to the mainland than islands +separated by deeper channels, we can understand the frequent relation between +the depth of the sea and the degree of affinity of the mammalian inhabitants of +islands with those of a neighbouring continent,—an inexplicable relation +on the view of independent acts of creation. +</p> + +<p> +All the foregoing remarks on the inhabitants of oceanic islands,—namely, +the scarcity of kinds—the richness in endemic forms in particular classes +or sections of classes,—the absence of whole groups, as of batrachians, +and of terrestrial mammals notwithstanding the presence of ærial +bats,—the singular proportions of certain orders of +plants,—herbaceous forms having been developed into trees, +etc.,—seem to me to accord better with the view of occasional means of +transport having been largely efficient in the long course of time, than with +the view of all our oceanic islands having been formerly connected by +continuous land with the nearest continent; for on this latter view the +migration would probably have been more complete; and if modification be +admitted, all the forms of life would have been more equally modified, in +accordance with the paramount importance of the relation of organism to +organism. +</p> + +<p> +I do not deny that there are many and grave difficulties in understanding how +several of the inhabitants of the more remote islands, whether still retaining +the same specific form or modified since their arrival, could have reached +their present homes. But the probability of many islands having existed as +halting-places, of which not a wreck now remains, must not be overlooked. +<a name="Page397"></a> +I will here give a single instance of one of the cases of difficulty. Almost +all oceanic islands, even the most isolated and smallest, are inhabited by +land-shells, generally by endemic species, but sometimes by species found +elsewhere. Dr. Aug. A. Gould has given several interesting cases in regard to +the land-shells of the islands of the Pacific. Now it is notorious that +land-shells are very easily killed by salt; their eggs, at least such as I have +tried, sink in sea-water and are killed by it. Yet there must be, on my view, +some unknown, but highly efficient means for their transportal. Would the +just-hatched young occasionally crawl on and adhere to the feet of birds +roosting on the ground, and thus get transported? It occurred to me that +land-shells, when hybernating and having a membranous diaphragm over the mouth +of the shell, might be floated in chinks of drifted timber across moderately +wide arms of the sea. And I found that several species did in this state +withstand uninjured an immersion in sea-water during seven days: one of these +shells was the Helix pomatia, and after it had again hybernated I put it in +sea-water for twenty days, and it perfectly recovered. As this species has a +thick calcareous operculum, I removed it, and when it had formed a new +membranous one, I immersed it for fourteen days in sea-water, and it recovered +and crawled away: but more experiments are wanted on this head. +</p> + +<p> +The most striking and important fact for us in regard to the inhabitants of +islands, is their affinity to those of the nearest mainland, without being +actually the same species. Numerous instances could be given of this fact. I +will give only one, that of the Galapagos Archipelago, situated under the +equator, between 500 and 600 miles from the shores of South America. Here +<a name="Page398"></a> +almost every product of the land and water bears the unmistakeable stamp of the +American continent. There are twenty-six land birds, and twenty-five of these +are ranked by Mr. Gould as distinct species, supposed to have been created +here; yet the close affinity of most of these birds to American species in +every character, in their habits, gestures, and tones of voice, was manifest. +So it is with the other animals, and with nearly all the plants, as shown by +Dr. Hooker in his admirable memoir on the Flora of this archipelago. The +naturalist, looking at the inhabitants of these volcanic islands in the +Pacific, distant several hundred miles from the continent, yet feels that he is +standing on American land. Why should this be so? why should the species which +are supposed to have been created in the Galapagos Archipelago, and nowhere +else, bear so plain a stamp of affinity to those created in America? There is +nothing in the conditions of life, in the geological nature of the islands, in +their height or climate, or in the proportions in which the several classes are +associated together, which resembles closely the conditions of the South +American coast: in fact there is a considerable dissimilarity in all these +respects. On the other hand, there is a considerable degree of resemblance in +the volcanic nature of the soil, in climate, height, and size of the islands, +between the Galapagos and Cape de Verde Archipelagos: but what an entire and +absolute difference in their inhabitants! The inhabitants of the Cape de Verde +Islands are related to those of Africa, like those of the Galapagos to America. +I believe this grand fact can receive no sort of explanation on the ordinary +view of independent creation; whereas on the view here maintained, it is +obvious that the Galapagos Islands would be likely to receive colonists, +whether by occasional means of transport or +<a name="Page399"></a> +by formerly continuous land, from America; and the Cape de Verde Islands from +Africa; and that such colonists would be liable to modification;—the +principle of inheritance still betraying their original birthplace. +</p> + +<p> +Many analogous facts could be given: indeed it is an almost universal rule that +the endemic productions of islands are related to those of the nearest +continent, or of other near islands. The exceptions are few, and most of them +can be explained. Thus the plants of Kerguelen Land, though standing nearer to +Africa than to America, are related, and that very closely, as we know from Dr. +Hooker’s account, to those of America: but on the view that this island +has been mainly stocked by seeds brought with earth and stones on icebergs, +drifted by the prevailing currents, this anomaly disappears. New Zealand in its +endemic plants is much more closely related to Australia, the nearest mainland, +than to any other region: and this is what might have been expected; but it is +also plainly related to South America, which, although the next nearest +continent, is so enormously remote, that the fact becomes an anomaly. But this +difficulty almost disappears on the view that both New Zealand, South America, +and other southern lands were long ago partially stocked from a nearly +intermediate though distant point, namely from the antarctic islands, when they +were clothed with vegetation, before the commencement of the Glacial period. +The affinity, which, though feeble, I am assured by Dr. Hooker is real, between +the flora of the south-western corner of Australia and of the Cape of Good +Hope, is a far more remarkable case, and is at present inexplicable: but this +affinity is confined to the plants, and will, I do not doubt, be some day +explained. +</p> + +<p> +The law which causes the inhabitants of an archipelago, +<a name="Page400"></a> +though specifically distinct, to be closely allied to those of the nearest +continent, we sometimes see displayed on a small scale, yet in a most +interesting manner, within the limits of the same archipelago. Thus the several +islands of the Galapagos Archipelago are tenanted, as I have elsewhere shown, +in a quite marvellous manner, by very closely related species; so that the +inhabitants of each separate island, though mostly distinct, are related in an +incomparably closer degree to each other than to the inhabitants of any other +part of the world. And this is just what might have been expected on my view, +for the islands are situated so near each other that they would almost +certainly receive immigrants from the same original source, or from each other. +But this dissimilarity between the endemic inhabitants of the islands may be +used as an argument against my views; for it may be asked, how has it happened +in the several islands situated within sight of each other, having the same +geological nature, the same height, climate, etc., that many of the immigrants +should have been differently modified, though only in a small degree. This long +appeared to me a great difficulty: but it arises in chief part from the +deeply-seated error of considering the physical conditions of a country as the +most important for its inhabitants; whereas it cannot, I think, be disputed +that the nature of the other inhabitants, with which each has to compete, is at +least as important, and generally a far more important element of success. Now +if we look to those inhabitants of the Galapagos Archipelago which are found in +other parts of the world (laying on one side for the moment the endemic +species, which cannot be here fairly included, as we are considering how they +have come to be modified since their arrival), we find a considerable amount +<a name="Page401"></a> +of difference in the several islands. This difference might indeed have been +expected on the view of the islands having been stocked by occasional means of +transport—a seed, for instance, of one plant having been brought to one +island, and that of another plant to another island. Hence when in former times +an immigrant settled on any one or more of the islands, or when it subsequently +spread from one island to another, it would undoubtedly be exposed to different +conditions of life in the different islands, for it would have to compete with +different sets of organisms: a plant, for instance, would find the best-fitted +ground more perfectly occupied by distinct plants in one island than in +another, and it would be exposed to the attacks of somewhat different enemies. +If then it varied, natural selection would probably favour different varieties +in the different islands. Some species, however, might spread and yet retain +the same character throughout the group, just as we see on continents some +species spreading widely and remaining the same. +</p> + +<p> +The really surprising fact in this case of the Galapagos Archipelago, and in a +lesser degree in some analogous instances, is that the new species formed in +the separate islands have not quickly spread to the other islands. But the +islands, though in sight of each other, are separated by deep arms of the sea, +in most cases wider than the British Channel, and there is no reason to suppose +that they have at any former period been continuously united. The currents of +the sea are rapid and sweep across the archipelago, and gales of wind are +extraordinarily rare; so that the islands are far more effectually separated +from each other than they appear to be on a map. Nevertheless a good many +species, both those found in other parts of the world and those confined to the +archipelago, are common to +<a name="Page402"></a> +the several islands, and we may infer from certain facts that these have +probably spread from some one island to the others. But we often take, I think, +an erroneous view of the probability of closely allied species invading each +other’s territory, when put into free intercommunication. Undoubtedly if +one species has any advantage whatever over another, it will in a very brief +time wholly or in part supplant it; but if both are equally well fitted for +their own places in nature, both probably will hold their own places and keep +separate for almost any length of time. Being familiar with the fact that many +species, naturalised through man’s agency, have spread with astonishing +rapidity over new countries, we are apt to infer that most species would thus +spread; but we should remember that the forms which become naturalised in new +countries are not generally closely allied to the aboriginal inhabitants, but +are very distinct species, belonging in a large proportion of cases, as shown +by Alph. de Candolle, to distinct genera. In the Galapagos Archipelago, many +even of the birds, though so well adapted for flying from island to island, are +distinct on each; thus there are three closely-allied species of +mocking-thrush, each confined to its own island. Now let us suppose the +mocking-thrush of Chatham Island to be blown to Charles Island, which has its +own mocking-thrush: why should it succeed in establishing itself there? We may +safely infer that Charles Island is well stocked with its own species, for +annually more eggs are laid there than can possibly be reared; and we may infer +that the mocking-thrush peculiar to Charles Island is at least as well fitted +for its home as is the species peculiar to Chatham Island. Sir C. Lyell and Mr. +Wollaston have communicated to me a remarkable fact bearing on this subject; +namely, that Madeira and the adjoining islet of +<a name="Page403"></a> +Porto Santo possess many distinct but representative land-shells, some of which +live in crevices of stone; and although large quantities of stone are annually +transported from Porto Santo to Madeira, yet this latter island has not become +colonised by the Porto Santo species: nevertheless both islands have been +colonised by some European land-shells, which no doubt had some advantage over +the indigenous species. From these considerations I think we need not greatly +marvel at the endemic and representative species, which inhabit the several +islands of the Galapagos Archipelago, not having universally spread from island +to island. In many other instances, as in the several districts of the same +continent, pre-occupation has probably played an important part in checking the +commingling of species under the same conditions of life. Thus, the south-east +and south-west corners of Australia have nearly the same physical conditions, +and are united by continuous land, yet they are inhabited by a vast number of +distinct mammals, birds, and plants. +</p> + +<p> +The principle which determines the general character of the fauna and flora of +oceanic islands, namely, that the inhabitants, when not identically the same, +yet are plainly related to the inhabitants of that region whence colonists +could most readily have been derived,—the colonists having been +subsequently modified and better fitted to their new homes,—is of the +widest application throughout nature. We see this on every mountain, in every +lake and marsh. For Alpine species, excepting in so far as the same forms, +chiefly of plants, have spread widely throughout the world during the recent +Glacial epoch, are related to those of the surrounding lowlands;—thus we +have in South America, Alpine humming-birds, Alpine rodents, Alpine plants, +etc., all of strictly American forms, and it is obvious +<a name="Page404"></a> +that a mountain, as it became slowly upheaved, would naturally be colonised +from the surrounding lowlands. So it is with the inhabitants of lakes and +marshes, excepting in so far as great facility of transport has given the same +general forms to the whole world. We see this same principle in the blind +animals inhabiting the caves of America and of Europe. Other analogous facts +could be given. And it will, I believe, be universally found to be true, that +wherever in two regions, let them be ever so distant, many closely allied or +representative species occur, there will likewise be found some identical +species, showing, in accordance with the foregoing view, that at some former +period there has been intercommunication or migration between the two regions. +And wherever many closely-allied species occur, there will be found many forms +which some naturalists rank as distinct species, and some as varieties; these +doubtful forms showing us the steps in the process of modification. +</p> + +<p> +This relation between the power and extent of migration of a species, either at +the present time or at some former period under different physical conditions, +and the existence at remote points of the world of other species allied to it, +is shown in another and more general way. Mr. Gould remarked to me long ago, +that in those genera of birds which range over the world, many of the species +have very wide ranges. I can hardly doubt that this rule is generally true, +though it would be difficult to prove it. Amongst mammals, we see it strikingly +displayed in Bats, and in a lesser degree in the Felidæ and Canidæ. We see it, +if we compare the distribution of butterflies and beetles. So it is with most +fresh-water productions, in which so many genera range over the world, and many +individual species have enormous ranges. It is not meant that in world-ranging +<a name="Page405"></a> +genera all the species have a wide range, or even that they have on an +<i>average</i> a wide range; but only that some of the species range very +widely; for the facility with which widely-ranging species vary and give rise +to new forms will largely determine their average range. For instance, two +varieties of the same species inhabit America and Europe, and the species thus +has an immense range; but, if the variation had been a little greater, the two +varieties would have been ranked as distinct species, and the common range +would have been greatly reduced. Still less is it meant, that a species which +apparently has the capacity of crossing barriers and ranging widely, as in the +case of certain powerfully-winged birds, will necessarily range widely; for we +should never forget that to range widely implies not only the power of crossing +barriers, but the more important power of being victorious in distant lands in +the struggle for life with foreign associates. But on the view of all the +species of a genus having descended from a single parent, though now +distributed to the most remote points of the world, we ought to find, and I +believe as a general rule we do find, that some at least of the species range +very widely; for it is necessary that the unmodified parent should range +widely, undergoing modification during its diffusion, and should place itself +under diverse conditions favourable for the conversion of its offspring, +firstly into new varieties and ultimately into new species. +</p> + +<p> +In considering the wide distribution of certain genera, we should bear in mind +that some are extremely ancient, and must have branched off from a common +parent at a remote epoch; so that in such cases there will have been ample time +for great climatal and geographical changes and for accidents of transport; and +consequently for the migration of some of the species into all +<a name="Page406"></a> +quarters of the world, where they may have become slightly modified in relation +to their new conditions. There is, also, some reason to believe from geological +evidence that organisms low in the scale within each great class, generally +change at a slower rate than the higher forms; and consequently the lower forms +will have had a better chance of ranging widely and of still retaining the same +specific character. This fact, together with the seeds and eggs of many low +forms being very minute and better fitted for distant transportation, probably +accounts for a law which has long been observed, and which has lately been +admirably discussed by Alph. de Candolle in regard to plants, namely, that the +lower any group of organisms is, the more widely it is apt to range. +</p> + +<p> +The relations just discussed,—namely, low and slowly-changing organisms +ranging more widely than the high,—some of the species of widely-ranging +genera themselves ranging widely,—such facts, as alpine, lacustrine, and +marsh productions being related (with the exceptions before specified) to those +on the surrounding low lands and dry lands, though these stations are so +different—the very close relation of the distinct species which inhabit +the islets of the same archipelago,—and especially the striking relation +of the inhabitants of each whole archipelago or island to those of the nearest +mainland,—are, I think, utterly inexplicable on the ordinary view of the +independent creation of each species, but are explicable on the view of +colonisation from the nearest and readiest source, together with the subsequent +modification and better adaptation of the colonists to their new homes. +</p> + +<p class="p2"> +<i>Summary of last and present Chapters</i>.—In these chapters I have +endeavoured to show, that if we make due allowance for our ignorance of the +full effects of all +<a name="Page407"></a> +the changes of climate and of the level of the land, which have certainly +occurred within the recent period, and of other similar changes which may have +occurred within the same period; if we remember how profoundly ignorant we are +with respect to the many and curious means of occasional transport,—a +subject which has hardly ever been properly experimentised on; if we bear in +mind how often a species may have ranged continuously over a wide area, and +then have become extinct in the intermediate tracts, I think the difficulties +in believing that all the individuals of the same species, wherever located, +have descended from the same parents, are not insuperable. And we are led to +this conclusion, which has been arrived at by many naturalists under the +designation of single centres of creation, by some general considerations, more +especially from the importance of barriers and from the analogical distribution +of sub-genera, genera, and families. +</p> + +<p> +With respect to the distinct species of the same genus, which on my theory must +have spread from one parent-source; if we make the same allowances as before +for our ignorance, and remember that some forms of life change most slowly, +enormous periods of time being thus granted for their migration, I do not think +that the difficulties are insuperable; though they often are in this case, and +in that of the individuals of the same species, extremely grave. +</p> + +<p> +As exemplifying the effects of climatal changes on distribution, I have +attempted to show how important has been the influence of the modern Glacial +period, which I am fully convinced simultaneously affected the whole world, or +at least great meridional belts. As showing how diversified are the means of +occasional transport, I have discussed at some little length the means of +dispersal of fresh-water productions. +</p> + +<p> +<a name="Page408"></a> +If the difficulties be not insuperable in admitting that in the long course of +time the individuals of the same species, and likewise of allied species, have +proceeded from some one source; then I think all the grand leading facts of +geographical distribution are explicable on the theory of migration (generally +of the more dominant forms of life), together with subsequent modification and +the multiplication of new forms. We can thus understand the high importance of +barriers, whether of land or water, which separate our several zoological and +botanical provinces. We can thus understand the localisation of sub-genera, +genera, and families; and how it is that under different latitudes, for +instance in South America, the inhabitants of the plains and mountains, of the +forests, marshes, and deserts, are in so mysterious a manner linked together by +affinity, and are likewise linked to the extinct beings which formerly +inhabited the same continent. Bearing in mind that the mutual relations of +organism to organism are of the highest importance, we can see why two areas +having nearly the same physical conditions should often be inhabited by very +different forms of life; for according to the length of time which has elapsed +since new inhabitants entered one region; according to the nature of the +communication which allowed certain forms and not others to enter, either in +greater or lesser numbers; according or not, as those which entered happened to +come in more or less direct competition with each other and with the +aborigines; and according as the immigrants were capable of varying more or +less rapidly, there would ensue in different regions, independently of their +physical conditions, infinitely diversified conditions of life,—there +would be an almost endless amount of organic action and reaction,—and we +should find, as we do find, some groups of beings greatly, and some only +slightly modified,—some developed +<a name="Page409"></a> +in great force, some existing in scanty numbers—in the different great +geographical provinces of the world. +</p> + +<p> +On these same principles, we can understand, as I have endeavoured to show, why +oceanic islands should have few inhabitants, but of these a great number should +be endemic or peculiar; and why, in relation to the means of migration, one +group of beings, even within the same class, should have all its species +endemic, and another group should have all its species common to other quarters +of the world. We can see why whole groups of organisms, as batrachians and +terrestrial mammals, should be absent from oceanic islands, whilst the most +isolated islands possess their own peculiar species of ærial mammals or bats. +We can see why there should be some relation between the presence of mammals, +in a more or less modified condition, and the depth of the sea between an +island and the mainland. We can clearly see why all the inhabitants of an +archipelago, though specifically distinct on the several islets, should be +closely related to each other, and likewise be related, but less closely, to +those of the nearest continent or other source whence immigrants were probably +derived. We can see why in two areas, however distant from each other, there +should be a correlation, in the presence of identical species, of varieties, of +doubtful species, and of distinct but representative species. +</p> + +<p> +As the late Edward Forbes often insisted, there is a striking parallelism in +the laws of life throughout time and space: the laws governing the succession +of forms in past times being nearly the same with those governing at the +present time the differences in different areas. We see this in many facts. The +endurance of each species and group of species is continuous in time; for the +exceptions to the rule are so few, that they may +<a name="Page410"></a> +fairly be attributed to our not having as yet discovered in an intermediate +deposit the forms which are therein absent, but which occur above and below: so +in space, it certainly is the general rule that the area inhabited by a single +species, or by a group of species, is continuous; and the exceptions, which are +not rare, may, as I have attempted to show, be accounted for by migration at +some former period under different conditions or by occasional means of +transport, and by the species having become extinct in the intermediate tracts. +Both in time and space, species and groups of species have their points of +maximum development. Groups of species, belonging either to a certain period of +time, or to a certain area, are often characterised by trifling characters in +common, as of sculpture or colour. In looking to the long succession of ages, +as in now looking to distant provinces throughout the world, we find that some +organisms differ little, whilst others belonging to a different class, or to a +different order, or even only to a different family of the same order, differ +greatly. In both time and space the lower members of each class generally +change less than the higher; but there are in both cases marked exceptions to +the rule. On my theory these several relations throughout time and space are +intelligible; for whether we look to the forms of life which have changed +during successive ages within the same quarter of the world, or to those which +have changed after having migrated into distant quarters, in both cases the +forms within each class have been connected by the same bond of ordinary +generation; and the more nearly any two forms are related in blood, the nearer +they will generally stand to each other in time and space; in both cases the +laws of variation have been the same, and modifications have been accumulated +by the same power of natural selection. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page411"></a><a name="chap13"></a>CHAPTER XIII.<br /> +MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY: +EMBRYOLOGY: RUDIMENTARY ORGANS. +</h2> + +<p class="letter"> +C<small>LASSIFICATION</small>, groups subordinate to groups. Natural system. +Rules and difficulties in classification, explained on the theory of descent +with modification. Classification of varieties. Descent always used in +classification. Analogical or adaptive characters. Affinities, general, complex +and radiating. Extinction separates and defines groups. +M<small>ORPHOLOGY</small>, between members of the same class, between parts of +the same individual. E<small>MBRYOLOGY</small>, laws of, explained by +variations not supervening at an early age, and being inherited at a +corresponding age. R<small>UDIMENTARY ORGANS</small>; their origin explained. +Summary. +</p> + +<p> +From the first dawn of life, all organic beings are found to resemble each +other in descending degrees, so that they can be classed in groups under +groups. This classification is evidently not arbitrary like the grouping of the +stars in constellations. The existence of groups would have been of simple +signification, if one group had been exclusively fitted to inhabit the land, +and another the water; one to feed on flesh, another on vegetable matter, and +so on; but the case is widely different in nature; for it is notorious how +commonly members of even the same subgroup have different habits. In our second +and fourth chapters, on Variation and on Natural Selection, I have attempted to +show that it is the widely ranging, the much diffused and common, that is the +dominant species belonging to the larger genera, which vary most. The +varieties, or incipient species, thus produced ultimately become converted, as +I believe, into new and distinct species; and these, on the principle of +inheritance, tend to produce other new and dominant +<a name="Page412"></a> +species. Consequently the groups which are now large, and which generally +include many dominant species, tend to go on increasing indefinitely in size. I +further attempted to show that from the varying descendants of each species +trying to occupy as many and as different places as possible in the economy of +nature, there is a constant tendency in their characters to diverge. This +conclusion was supported by looking at the great diversity of the forms of life +which, in any small area, come into the closest competition, and by looking to +certain facts in naturalisation. +</p> + +<p> +I attempted also to show that there is a constant tendency in the forms which +are increasing in number and diverging in character, to supplant and +exterminate the less divergent, the less improved, and preceding forms. I +request the reader to turn to the diagram illustrating the action, as formerly +explained, of these several principles; and he will see that the inevitable +result is that the modified descendants proceeding from one progenitor become +broken up into groups subordinate to groups. In the diagram each letter on the +uppermost line may represent a genus including several species; and all the +genera on this line form together one class, for all have descended from one +ancient but unseen parent, and, consequently, have inherited something in +common. But the three genera on the left hand have, on this same principle, +much in common, and form a sub-family, distinct from that including the next +two genera on the right hand, which diverged from a common parent at the fifth +stage of descent. These five genera have also much, though less, in common; and +they form a family distinct from that including the three genera still further +to the right hand, which diverged at a still earlier period. And all these +genera, descended from (A), form an order distinct from the +<a name="Page413"></a> +genera descended from (I). So that we here have many species descended from a +single progenitor grouped into genera; and the genera are included in, or +subordinate to, sub-families, families, and orders, all united into one class. +Thus, the grand fact in natural history of the subordination of group under +group, which, from its familiarity, does not always sufficiently strike us, is +in my judgment fully explained. +</p> + +<p> +Naturalists try to arrange the species, genera, and families in each class, on +what is called the Natural System. But what is meant by this system? Some +authors look at it merely as a scheme for arranging together those living +objects which are most alike, and for separating those which are most unlike; +or as an artificial means for enunciating, as briefly as possible, general +propositions,—that is, by one sentence to give the characters common, for +instance, to all mammals, by another those common to all carnivora, by another +those common to the dog-genus, and then by adding a single sentence, a full +description is given of each kind of dog. The ingenuity and utility of this +system are indisputable. But many naturalists think that something more is +meant by the Natural System; they believe that it reveals the plan of the +Creator; but unless it be specified whether order in time or space, or what +else is meant by the plan of the Creator, it seems to me that nothing is thus +added to our knowledge. Such expressions as that famous one of Linnæus, and +which we often meet with in a more or less concealed form, that the characters +do not make the genus, but that the genus gives the characters, seem to imply +that something more is included in our classification, than mere resemblance. I +believe that something more is included; and that propinquity of +descent,—the only known cause of the similarity of organic +beings,—is the bond, hidden as it is by various degrees of modification, +<a name="Page414"></a> +which is partially revealed to us by our classifications. +</p> + +<p> +Let us now consider the rules followed in classification, and the difficulties +which are encountered on the view that classification either gives some unknown +plan of creation, or is simply a scheme for enunciating general propositions +and of placing together the forms most like each other. It might have been +thought (and was in ancient times thought) that those parts of the structure +which determined the habits of life, and the general place of each being in the +economy of nature, would be of very high importance in classification. Nothing +can be more false. No one regards the external similarity of a mouse to a +shrew, of a dugong to a whale, of a whale to a fish, as of any importance. +These resemblances, though so intimately connected with the whole life of the +being, are ranked as merely “adaptive or analogical characters;” +but to the consideration of these resemblances we shall have to recur. It may +even be given as a general rule, that the less any part of the organisation is +concerned with special habits, the more important it becomes for +classification. As an instance: Owen, in speaking of the dugong, says, +“The generative organs being those which are most remotely related to the +habits and food of an animal, I have always regarded as affording very clear +indications of its true affinities. We are least likely in the modifications of +these organs to mistake a merely adaptive for an essential character.” So +with plants, how remarkable it is that the organs of vegetation, on which their +whole life depends, are of little signification, excepting in the first main +divisions; whereas the organs of reproduction, with their product the seed, are +of paramount importance! +</p> + +<p> +We must not, therefore, in classifying, trust to resemblances in parts of the +organisation, however important +<a name="Page415"></a> +they may be for the welfare of the being in relation to the outer world. +Perhaps from this cause it has partly arisen, that almost all naturalists lay +the greatest stress on resemblances in organs of high vital or physiological +importance. No doubt this view of the classificatory importance of organs which +are important is generally, but by no means always, true. But their importance +for classification, I believe, depends on their greater constancy throughout +large groups of species; and this constancy depends on such organs having +generally been subjected to less change in the adaptation of the species to +their conditions of life. That the mere physiological importance of an organ +does not determine its classificatory value, is almost shown by the one fact, +that in allied groups, in which the same organ, as we have every reason to +suppose, has nearly the same physiological value, its classificatory value is +widely different. No naturalist can have worked at any group without being +struck with this fact; and it has been most fully acknowledged in the writings +of almost every author. It will suffice to quote the highest authority, Robert +Brown, who in speaking of certain organs in the Proteaceæ, says their generic +importance, “like that of all their parts, not only in this but, as I +apprehend, in every natural family, is very unequal, and in some cases seems to +be entirely lost.” Again in another work he says, the genera of the +Connaraceæ “differ in having one or more ovaria, in the existence or +absence of albumen, in the imbricate or valvular æstivation. Any one of these +characters singly is frequently of more than generic importance, though here +even when all taken together they appear insufficient to separate Cnestis from +Connarus.” To give an example amongst insects, in one great division of +the Hymenoptera, the antennæ, as Westwood has remarked, are most constant in +structure; +<a name="Page416"></a> +in another division they differ much, and the differences are of quite +subordinate value in classification; yet no one probably will say that the +antennæ in these two divisions of the same order are of unequal physiological +importance. Any number of instances could be given of the varying importance +for classification of the same important organ within the same group of beings. +</p> + +<p> +Again, no one will say that rudimentary or atrophied organs are of high +physiological or vital importance; yet, undoubtedly, organs in this condition +are often of high value in classification. No one will dispute that the +rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary +bones of the leg, are highly serviceable in exhibiting the close affinity +between Ruminants and Pachyderms. Robert Brown has strongly insisted on the +fact that the rudimentary florets are of the highest importance in the +classification of the Grasses. +</p> + +<p> +Numerous instances could be given of characters derived from parts which must +be considered of very trifling physiological importance, but which are +universally admitted as highly serviceable in the definition of whole groups. +For instance, whether or not there is an open passage from the nostrils to the +mouth, the only character, according to Owen, which absolutely distinguishes +fishes and reptiles—the inflection of the angle of the jaws in +Marsupials—the manner in which the wings of insects are folded—mere +colour in certain Algæ—mere pubescence on parts of the flower in +grasses—the nature of the dermal covering, as hair or feathers, in the +Vertebrata. If the Ornithorhynchus had been covered with feathers instead of +hair, this external and trifling character would, I think, have been considered +by naturalists as important an aid in determining the degree of affinity of +this strange creature to +<a name="Page417"></a> +birds and reptiles, as an approach in structure in any one internal and +important organ. +</p> + +<p> +The importance, for classification, of trifling characters, mainly depends on +their being correlated with several other characters of more or less +importance. The value indeed of an aggregate of characters is very evident in +natural history. Hence, as has often been remarked, a species may depart from +its allies in several characters, both of high physiological importance and of +almost universal prevalence, and yet leave us in no doubt where it should be +ranked. Hence, also, it has been found, that a classification founded on any +single character, however important that may be, has always failed; for no part +of the organisation is universally constant. The importance of an aggregate of +characters, even when none are important, alone explains, I think, that saying +of Linnæus, that the characters do not give the genus, but the genus gives the +characters; for this saying seems founded on an appreciation of many trifling +points of resemblance, too slight to be defined. Certain plants, belonging to +the Malpighiaceæ, bear perfect and degraded flowers; in the latter, as A. de +Jussieu has remarked, “the greater number of the characters proper to the +species, to the genus, to the family, to the class, disappear, and thus laugh +at our classification.” But when Aspicarpa produced in France, during +several years, only degraded flowers, departing so wonderfully in a number of +the most important points of structure from the proper type of the order, yet +M. Richard sagaciously saw, as Jussieu observes, that this genus should still +be retained amongst the Malpighiaceæ. This case seems to me well to illustrate +the spirit with which our classifications are sometimes necessarily founded. +</p> + +<p> +Practically when naturalists are at work, they do +<a name="Page418"></a> +not trouble themselves about the physiological value of the characters which +they use in defining a group, or in allocating any particular species. If they +find a character nearly uniform, and common to a great number of forms, and not +common to others, they use it as one of high value; if common to some lesser +number, they use it as of subordinate value. This principle has been broadly +confessed by some naturalists to be the true one; and by none more clearly than +by that excellent botanist, Aug. St. Hilaire. If certain characters are always +found correlated with others, though no apparent bond of connexion can be +discovered between them, especial value is set on them. As in most groups of +animals, important organs, such as those for propelling the blood, or for +ærating it, or those for propagating the race, are found nearly uniform, they +are considered as highly serviceable in classification; but in some groups of +animals all these, the most important vital organs, are found to offer +characters of quite subordinate value. +</p> + +<p> +We can see why characters derived from the embryo should be of equal importance +with those derived from the adult, for our classifications of course include +all ages of each species. But it is by no means obvious, on the ordinary view, +why the structure of the embryo should be more important for this purpose than +that of the adult, which alone plays its full part in the economy of nature. +Yet it has been strongly urged by those great naturalists, Milne Edwards and +Agassiz, that embryonic characters are the most important of any in the +classification of animals; and this doctrine has very generally been admitted +as true. The same fact holds good with flowering plants, of which the two main +divisions have been founded on characters derived from the embryo,—on the +number and position of the embryonic +<a name="Page419"></a> +leaves or cotyledons, and on the mode of development of the plumule and +radicle. In our discussion on embryology, we shall see why such characters are +so valuable, on the view of classification tacitly including the idea of +descent. +</p> + +<p> +Our classifications are often plainly influenced by chains of affinities. +Nothing can be easier than to define a number of characters common to all +birds; but in the case of crustaceans, such definition has hitherto been found +impossible. There are crustaceans at the opposite ends of the series, which +have hardly a character in common; yet the species at both ends, from being +plainly allied to others, and these to others, and so onwards, can be +recognised as unequivocally belonging to this, and to no other class of the +Articulata. +</p> + +<p> +Geographical distribution has often been used, though perhaps not quite +logically, in classification, more especially in very large groups of closely +allied forms. Temminck insists on the utility or even necessity of this +practice in certain groups of birds; and it has been followed by several +entomologists and botanists. +</p> + +<p> +Finally, with respect to the comparative value of the various groups of +species, such as orders, sub-orders, families, sub-families, and genera, they +seem to be, at least at present, almost arbitrary. Several of the best +botanists, such as Mr. Bentham and others, have strongly insisted on their +arbitrary value. Instances could be given amongst plants and insects, of a +group of forms, first ranked by practised naturalists as only a genus, and then +raised to the rank of a sub-family or family; and this has been done, not +because further research has detected important structural differences, at +first overlooked, but because numerous allied species, with slightly different +grades of difference, have been subsequently discovered. +</p> + +<p> +<a name="Page420"></a> +All the foregoing rules and aids and difficulties in classification are +explained, if I do not greatly deceive myself, on the view that the natural +system is founded on descent with modification; that the characters which +naturalists consider as showing true affinity between any two or more species, +are those which have been inherited from a common parent, and, in so far, all +true classification is genealogical; that community of descent is the hidden +bond which naturalists have been unconsciously seeking, and not some unknown +plan of creation, or the enunciation of general propositions, and the mere +putting together and separating objects more or less alike. +</p> + +<p> +But I must explain my meaning more fully. I believe that the <i>arrangement</i> +of the groups within each class, in due subordination and relation to the other +groups, must be strictly genealogical in order to be natural; but that the +<i>amount</i> of difference in the several branches or groups, though allied in +the same degree in blood to their common progenitor, may differ greatly, being +due to the different degrees of modification which they have undergone; and +this is expressed by the forms being ranked under different genera, families, +sections, or orders. The reader will best understand what is meant, if he will +take the trouble of referring to the diagram in the fourth chapter. We will +suppose the letters A to L to represent allied genera, which lived during the +Silurian epoch, and these have descended from a species which existed at an +unknown anterior period. Species of three of these genera (A, F, and I) have +transmitted modified descendants to the present day, represented by the fifteen +genera (<i>a</i><sup>14</sup> to <i>z</i><sup>14</sup>) on the uppermost +horizontal line. Now all these modified descendants from a single species, are +represented as related in blood or descent to the same +<a name="Page421"></a> +degree; they may metaphorically be called cousins to the same millionth degree; +yet they differ widely and in different degrees from each other. The forms +descended from A, now broken up into two or three families, constitute a +distinct order from those descended from I, also broken up into two families. +Nor can the existing species, descended from A, be ranked in the same genus +with the parent A; or those from I, with the parent I. But the existing genus +F<sup>14</sup> may be supposed to have been but slightly modified; and it will +then rank with the parent-genus F; just as some few still living organic beings +belong to Silurian genera. So that the amount or value of the differences +between organic beings all related to each other in the same degree in blood, +has come to be widely different. Nevertheless their genealogical +<i>arrangement</i> remains strictly true, not only at the present time, but at +each successive period of descent. All the modified descendants from A will +have inherited something in common from their common parent, as will all the +descendants from I; so will it be with each subordinate branch of descendants, +at each successive period. If, however, we choose to suppose that any of the +descendants of A or of I have been so much modified as to have more or less +completely lost traces of their parentage, in this case, their places in a +natural classification will have been more or less completely lost,—as +sometimes seems to have occurred with existing organisms. All the descendants +of the genus F, along its whole line of descent, are supposed to have been but +little modified, and they yet form a single genus. But this genus, though much +isolated, will still occupy its proper intermediate position; for F originally +was intermediate in character between A and I, and the several genera descended +from these two genera will +<a name="Page422"></a> +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 arrangement, 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. +</p> + +<p> +It may be worth while to illustrate this view of classification, by taking the +case of languages. If we possessed 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 civilisation 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 difference 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 +<a name="Page423"></a> +it would connect together all languages, extinct and modern, by the closest +affinities, and would give the filiation and origin of each tongue. +</p> + +<p> +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 our +domestic productions, several 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 +followed 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 cautioned, for instance, not to class two varieties of the +pine-apple together, merely because their fruit, though 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 +purpose with cattle, because they are less variable than the shape or colour of +the body, etc.; whereas with sheep the horns are much less serviceable, because +less constant. 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 sure, 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 +<a name="Page424"></a> +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. +</p> + +<p> +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 characters, 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 separating 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 includes monsters; he includes +varieties, not solely because 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), which 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. +<a name="Page425"></a> +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 <i>argumentum ad hominem</i>, 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. +</p> + +<p> +As descent has universally been used in classing together the individuals of +the same species, though the males and females and larvæ are sometimes +extremely different; and as it has been used in classing varieties which have +undergone a certain, and sometimes a considerable 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 +complete? 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 this view are as good as, or even sometimes +better than, other parts of the organisation. We +<a name="Page426"></a> +care not how trifling a character may be—let it be the mere inflection of +the angle of the jaw, the manner in which an insect’s wing is folded, +whether the skin be covered by hair or feathers—if it prevail throughout +many and different species, especially those having very different habits of +life, it assumes high value; for we can account for its presence in so many +forms with such different habits, only by its inheritance from a common parent. +We may err in this respect in regard to single points of structure, but when +several characters, let them be ever so trifling, occur together throughout a +large group of beings having different habits, we may feel almost sure, on the +theory of descent, that these characters have been inherited from a common +ancestor. And we know that such correlated or aggregated characters have +especial value in classification. +</p> + +<p> +We can understand why a species or a group of species may depart, in several of +its most important characteristics, from its allies, and yet be safely classed +with them. This may be safely done, and is often done, as long as a sufficient +number of characters, let them be ever so unimportant, betrays the hidden bond +of community of descent. Let two forms have not a single character in common, +yet if these extreme forms are connected together by a chain of intermediate +groups, we may at once infer their community of descent, and we put them all +into the same class. As we find organs of high physiological +importance—those which serve to preserve life under the most diverse +conditions of existence—are generally the most constant, we attach +especial value to them; but if these same organs, in another group or section +of a group, are found to differ much, we at once value them less in our +classification. We shall hereafter, I think, clearly see why embryological +characters are of such high classificatory importance. +<a name="Page427"></a> +Geographical distribution may sometimes be brought usefully into play in +classing large and widely-distributed genera, because all the species of the +same genus, inhabiting any distinct and isolated region, have in all +probability descended from the same parents. +</p> + +<p> +We can understand, on these views, the very important distinction between real +affinities and analogical or adaptive resemblances. Lamarck first called +attention to this distinction, and he has been ably followed by Macleay and +others. The resemblance, in the shape of the body and in the fin-like anterior +limbs, between the dugong, which is a pachydermatous animal, and the whale, and +between both these mammals and fishes, is analogical. Amongst insects there are +innumerable instances: thus Linnæus, misled by external appearances, actually +classed an homopterous insect as a moth. We see something of the same kind even +in our domestic varieties, as in the thickened stems of the common and swedish +turnip. The resemblance of the greyhound and racehorse is hardly more fanciful +than the analogies which have been drawn by some authors between very distinct +animals. On my view of characters being of real importance for classification, +only in so far as they reveal descent, we can clearly understand why analogical +or adaptive character, although of the utmost importance to the welfare of the +being, are almost valueless to the systematist. For animals, belonging to two +most distinct lines of descent, may readily become adapted to similar +conditions, and thus assume a close external resemblance; but such resemblances +will not reveal—will rather tend to conceal their blood-relationship to +their proper lines of descent. We can also understand the apparent paradox, +that the very same characters are analogical when one class or order is +compared with another, but give true affinities when the members of +<a name="Page428"></a> +the same class or order are compared one with another: thus the shape of the +body and fin-like limbs are only analogical when whales are compared with +fishes, being adaptations in both classes for swimming through the water; but +the shape of the body and fin-like limbs serve as characters exhibiting true +affinity between the several members of the whale family; for these cetaceans +agree in so many characters, great and small, that we cannot doubt that they +have inherited their general shape of body and structure of limbs from a common +ancestor. So it is with fishes. +</p> + +<p> +As members of distinct classes have often been adapted by successive slight +modifications to live under nearly similar circumstances,—to inhabit for +instance the three elements of land, air, and water,—we can perhaps +understand how it is that a numerical parallelism has sometimes been observed +between the sub-groups in distinct classes. A naturalist, struck by a +parallelism of this nature in any one class, by arbitrarily raising or sinking +the value of the groups in other classes (and all our experience shows that +this valuation has hitherto been arbitrary), could easily extend the +parallelism over a wide range; and thus the septenary, quinary, quaternary, and +ternary classifications have probably arisen. +</p> + +<p> +As the modified descendants of dominant species, belonging to the larger +genera, tend to inherit the advantages, which made the groups to which they +belong large and their parents dominant, they are almost sure to spread widely, +and to seize on more and more places in the economy of nature. The larger and +more dominant groups thus tend to go on increasing in size; and they +consequently supplant many smaller and feebler groups. Thus we can account for +the fact that all organisms, recent and extinct, are included under a few great +<a name="Page429"></a> +orders, under still fewer classes, and all in one great natural system. As +showing how few the higher groups are in number, and how widely spread they are +throughout the world, the fact is striking, that the discovery of Australia has +not added a single insect belonging to a new order; and that in the vegetable +kingdom, as I learn from Dr. Hooker, it has added only two or three orders of +small size. +</p> + +<p> +In the chapter on geological succession I attempted to show, on the principle +of each group having generally diverged much in character during the +long-continued process of modification, how it is that the more ancient forms +of life often present characters in some slight degree intermediate between +existing groups. A few old and intermediate parent-forms having occasionally +transmitted to the present day descendants but little modified, will give to us +our so-called osculant or aberrant groups. The more aberrant any form is, the +greater must be the number of connecting forms which on my theory have been +exterminated and utterly lost. And we have some evidence of aberrant forms +having suffered severely from extinction, for they are generally represented by +extremely few species; and such species as do occur are generally very distinct +from each other, which again implies extinction. The genera Ornithorhynchus and +Lepidosiren, for example, would not have been less aberrant had each been +represented by a dozen species instead of by a single one; but such richness in +species, as I find after some investigation, does not commonly fall to the lot +of aberrant genera. We can, I think, account for this fact only by looking at +aberrant forms as failing groups conquered by more successful competitors, with +a few members preserved by some unusual coincidence of favourable +circumstances. +</p> + +<p> +Mr. Waterhouse has remarked that, when a member +<a name="Page430"></a> +belonging to one group of animals exhibits an affinity to a quite distinct +group, this affinity in most cases is general and not special: thus, according +to Mr. Waterhouse, of all Rodents, the bizcacha is most nearly related to +Marsupials; but in the points in which it approaches this order, its relations +are general, and not to any one marsupial species more than to another. As the +points of affinity of the bizcacha to Marsupials are believed to be real and +not merely adaptive, they are due on my theory to inheritance in common. +Therefore we must suppose either that all Rodents, including the bizcacha, +branched off from some very ancient Marsupial, which will have had a character +in some degree intermediate with respect to all existing Marsupials; or that +both Rodents and Marsupials branched off from a common progenitor, and that +both groups have since undergone much modification in divergent directions. On +either view we may suppose that the bizcacha has retained, by inheritance, more +of the character of its ancient progenitor than have other Rodents; and +therefore it will not be specially related to any one existing Marsupial, but +indirectly to all or nearly all Marsupials, from having partially retained the +character of their common progenitor, or of an early member of the group. On +the other hand, of all Marsupials, as Mr. Waterhouse has remarked, the +phascolomys resembles most nearly, not any one species, but the general order +of Rodents. In this case, however, it may be strongly suspected that the +resemblance is only analogical, owing to the phascolomys having become adapted +to habits like those of a Rodent. The elder De Candolle has made nearly similar +observations on the general nature of the affinities of distinct orders of +plants. +</p> + +<p> +On the principle of the multiplication and gradual divergence in character of +the species descended from +<a name="Page431"></a> +a common parent, together with their retention by inheritance of some +characters in common, we can understand the excessively complex and radiating +affinities by which all the members of the same family or higher group are +connected together. For the common parent of a whole family of species, now +broken up by extinction into distinct groups and sub-groups, will have +transmitted some of its characters, modified in various ways and degrees, to +all; and the several species will consequently be related to each other by +circuitous lines of affinity of various lengths (as may be seen in the diagram +so often referred to), mounting up through many predecessors. As it is +difficult to show the blood-relationship between the numerous kindred of any +ancient and noble family, even by the aid of a genealogical tree, and almost +impossible to do this without this aid, we can understand the extraordinary +difficulty which naturalists have experienced in describing, without the aid of +a diagram, the various affinities which they perceive between the many living +and extinct members of the same great natural class. +</p> + +<p> +Extinction, as we have seen in the fourth chapter, has played an important part +in defining and widening the intervals between the several groups in each +class. We may thus account even for the distinctness of whole classes from each +other—for instance, of birds from all other vertebrate animals—by +the belief that many ancient forms of life have been utterly lost, through +which the early progenitors of birds were formerly connected with the early +progenitors of the other vertebrate classes. There has been less entire +extinction of the forms of life which once connected fishes with batrachians. +There has been still less in some other classes, as in that of the Crustacea, +for here the most wonderfully diverse forms are still tied +<a name="Page432"></a> +together by a long, but broken, chain of affinities. Extinction has only +separated groups: it has by no means made them; for if every form which has +ever lived on this earth were suddenly to reappear, though it would be quite +impossible to give definitions by which each group could be distinguished from +other groups, as all would blend together by steps as fine as those between the +finest existing varieties, nevertheless a natural classification, or at least a +natural arrangement, would be possible. We shall see this by turning to the +diagram: the letters, A to L, may represent eleven Silurian genera, some of +which have produced large groups of modified descendants. Every intermediate +link between these eleven genera and their primordial parent, and every +intermediate link in each branch and sub-branch of their descendants, may be +supposed to be still alive; and the links to be as fine as those between the +finest varieties. In this case it would be quite impossible to give any +definition by which the several members of the several groups could be +distinguished from their more immediate parents; or these parents from their +ancient and unknown progenitor. Yet the natural arrangement in the diagram +would still hold good; and, on the principle of inheritance, all the forms +descended from A, or from I, would have something in common. In a tree we can +specify this or that branch, though at the actual fork the two unite and blend +together. We could not, as I have said, define the several groups; but we could +pick out types, or forms, representing most of the characters of each group, +whether large or small, and thus give a general idea of the value of the +differences between them. This is what we should be driven to, if we were ever +to succeed in collecting all the forms in any class which have lived throughout +all time and space. We shall certainly never succeed in making +<a name="Page433"></a> +so perfect a collection: nevertheless, in certain classes, we are tending in +this direction; and Milne Edwards has lately insisted, in an able paper, on the +high importance of looking to types, whether or not we can separate and define +the groups to which such types belong. +</p> + +<p> +Finally, we have seen that natural selection, which results from the struggle +for existence, and which almost inevitably induces extinction and divergence of +character in the many descendants from one dominant parent-species, explains +that great and universal feature in the affinities of all organic beings, +namely, their subordination in group under group. We use the element of descent +in classing the individuals of both sexes and of all ages, although having few +characters in common, under one species; we use descent in classing +acknowledged varieties, however different they may be from their parent; and I +believe this element of descent is the hidden bond of connexion which +naturalists have sought under the term of the Natural System. On this idea of +the natural system being, in so far as it has been perfected, genealogical in +its arrangement, with the grades of difference between the descendants from a +common parent, expressed by the terms genera, families, orders, etc., we can +understand the rules which we are compelled to follow in our classification. We +can understand why we value certain resemblances far more than others; why we +are permitted to use rudimentary and useless organs, or others of trifling +physiological importance; why, in comparing one group with a distinct group, we +summarily reject analogical or adaptive characters, and yet use these same +characters within the limits of the same group. We can clearly see how it is +that all living and extinct forms can be grouped together in one great system; +and how the several members of each class are connected together by the most +complex and radiating +<a name="Page434"></a> +lines of affinities. We shall never, probably, disentangle the inextricable web +of affinities between the members of any one class; but when we have a distinct +object in view, and do not look to some unknown plan of creation, we may hope +to make sure but slow progress. +</p> + +<p class="p2"> +<i>Morphology</i>.—We have seen that the members of the same class, +independently of their habits of life, resemble each other in the general plan +of their organisation. This resemblance is often expressed by the term +“unity of type;” or by saying that the several parts and organs in +the different species of the class are homologous. The whole subject is +included under the general name of Morphology. This is the most interesting +department of natural history, and may be said to be its very soul. What can be +more curious than that the hand of a man, formed for grasping, that of a mole +for digging, the leg of the horse, the paddle of the porpoise, and the wing of +the bat, should all be constructed on the same pattern, and should include the +same bones, in the same relative positions? Geoffroy St. Hilaire has insisted +strongly on the high importance of relative connexion in homologous organs: the +parts may change to almost any extent in form and size, and yet they always +remain connected together in the same order. We never find, for instance, the +bones of the arm and forearm, or of the thigh and leg, transposed. Hence the +same names can be given to the homologous bones in widely different animals. We +see the same great law in the construction of the mouths of insects: what can +be more different than the immensely long spiral proboscis of a sphinx-moth, +the curious folded one of a bee or bug, and the great jaws of a +beetle?—yet all these organs, serving for such different +<a name="Page435"></a> +purposes, are formed by infinitely numerous modifications of an upper lip, +mandibles, and two pairs of maxillæ. Analogous laws govern the construction of +the mouths and limbs of crustaceans. So it is with the flowers of plants. +</p> + +<p> +Nothing can be more hopeless than to attempt to explain this similarity of +pattern in members of the same class, by utility or by the doctrine of final +causes. The hopelessness of the attempt has been expressly admitted by Owen in +his most interesting work on the ‘Nature of Limbs.’ On the ordinary +view of the independent creation of each being, we can only say that so it +is;—that it has so pleased the Creator to construct each animal and +plant. +</p> + +<p> +The explanation is manifest on the theory of the natural selection of +successive slight modifications,—each modification being profitable in +some way to the modified form, but often affecting by correlation of growth +other parts of the organisation. In changes of this nature, there will be +little or no tendency to modify the original pattern, or to transpose parts. +The bones of a limb might be shortened and widened to any extent, and become +gradually enveloped in thick membrane, so as to serve as a fin; or a webbed +foot might have all its bones, or certain bones, lengthened to any extent, and +the membrane connecting them increased to any extent, so as to serve as a wing: +yet in all this great amount of modification there will be no tendency to alter +the framework of bones or the relative connexion of the several parts. If we +suppose that the ancient progenitor, the archetype as it may be called, of all +mammals, had its limbs constructed on the existing general pattern, for +whatever purpose they served, we can at once perceive the plain signification +of the homologous construction of the limbs throughout the whole class. So with +the mouths of insects, we have only to +<a name="Page436"></a> +suppose that their common progenitor had an upper lip, mandibles, and two pair +of maxillæ, 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 conceivable 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 certain 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. +</p> + +<p> +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 physiologists believe +that the bones of the skull are homologous with—that is correspond in +number and in relative connexion with—the elemental parts of a certain +number of vertebræ. 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 +intelligible 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 animals, and in flowers, that organs, +which when mature +<a name="Page437"></a> +become extremely different, are at an early stage of growth exactly alike. +</p> + +<p> +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 +extraordinarily shaped pieces of bone? As Owen has remarked, 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, stamens, and pistils in any +individual flower, though fitted for such widely different purposes, be all +constructed on the same pattern? +</p> + +<p> +On the theory of natural selection, we can satisfactorily answer these +questions. In the vertebrata, we see a series of internal vertebræ bearing +certain processes and appendages; 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; therefore we may readily believe +that the unknown progenitor of the vertebrata possessed many vertebræ; 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 +<a name="Page438"></a> +natural selection, during a long-continued course of modification, should have +seized on a certain number of the primordially similar elements, many times +repeated, and have adapted them to the most diverse purposes. And as the whole +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. +</p> + +<p> +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 homologies; 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 animal and vegetable kingdoms. +</p> + +<p> +Naturalists frequently speak of the skull as formed of metamorphosed vertebræ: +the jaws of crabs as metamorphosed legs; the stamens and pistils of flowers as +metamorphosed leaves; but it would in these cases probably be more correct, as +Professor Huxley has remarked, to speak of both skull and vertebræ, both jaws +and legs, etc.,—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—vertebræ 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 occurred, that +naturalists can hardly avoid employing language having this plain +signification. On my view +<a name="Page439"></a> +these terms may be used literally; and the wonderful fact of the jaws, for +instance, of a crab retaining numerous characters, which they would probably +have retained through inheritance, if they had really been metamorphosed during +a long course of descent from true legs, or from some simple appendage, is +explained. +</p> + +<p class="p2"> +<i>Embryology</i>.—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 animal, +he cannot now tell whether it be that of a mammal, bird, or reptile. The +vermiform larvæ of moths, flies, beetles, etc., resemble each other much more +closely than do the mature insects; but in the case of larvæ, the embryos are +active, and have been adapted for special lines of life. A trace of the law of +embryonic resemblance, 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 distinguished in the whelp of the lion. We +occasionally 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 +leguminosæ. +</p> + +<p> +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 +conditions +<a name="Page440"></a> +of existence. We cannot, for instance, suppose that in the embryos of the +vertebrata the peculiar loop-like course of the arteries near the branchial +slits are related to similar conditions,—in the young mammal which is +nourished in the womb of its mother, in the egg of the bird which is hatched in +a nest, and in the spawn of a frog under water. We have no more reason to +believe in such a relation, than we have to believe that the same bones in the +hand of a man, wing of a bat, and fin of a porpoise, are related to similar +conditions of life. No one will suppose that the stripes on the whelp of a +lion, or the spots on the young blackbird, are of any use to these animals, or +are related to the conditions to which they are exposed. +</p> + +<p> +The case, however, is different when an animal during any part of its embryonic +career is active, and has to provide for itself. The period of activity may +come on earlier or later in life; but whenever it comes on, the adaptation of +the larva to its conditions of life is just as perfect and as beautiful as in +the adult animal. From such special adaptations, the similarity of the larvæ +or active embryos of allied animals is sometimes much obscured; and cases could +be given of the larvæ of two species, or of two groups of species, differing +quite as much, or even more, from each other than do their adult parents. In +most cases, however, the larvæ, though active, still obey more or less closely +the law of common embryonic resemblance. Cirripedes afford a good instance of +this: even the illustrious Cuvier did not perceive that a barnacle was, as it +certainly is, a crustacean; but a glance at the larva shows this to be the case +in an unmistakeable manner. So again the two main divisions of cirripedes, the +pedunculated and sessile, which differ widely in external appearance, have +larvæ in all their several stages barely distinguishable. +</p> + +<p> +<a name="Page441"></a> +The embryo in the course of development generally rises in organisation: I use +this expression, though I am aware that it is hardly possible to define clearly +what is meant by the organisation being higher or lower. But no one probably +will dispute that the butterfly is higher than the caterpillar. In some cases, +however, the mature animal is generally considered as lower in the scale than +the larva, as with certain parasitic crustaceans. To refer once again to +cirripedes: the larvæ in the first stage have three pairs of legs, a very +simple single eye, and a probosciformed mouth, with which they feed largely, +for they increase much in size. In the second stage, answering to the chrysalis +stage of butterflies, they have six pairs of beautifully constructed natatory +legs, a pair of magnificent compound eyes, and extremely complex antennæ; but +they have a closed and imperfect mouth, and cannot feed: their function at this +stage is, to search by their well-developed organs of sense, and to reach by +their active powers of swimming, a proper place on which to become attached and +to undergo their final metamorphosis. When this is completed they are fixed for +life: their legs are now converted into prehensile organs; they again obtain a +well-constructed mouth; but they have no antennæ, and their two eyes are now +reconverted into a minute, single, and very simple eye-spot. In this last and +complete state, cirripedes may be considered as either more highly or more +lowly organised than they were in the larval condition. But in some genera the +larvæ become developed either into hermaphrodites having the ordinary +structure, or into what I have called complemental males: and in the latter, +the development has assuredly been retrograde; for the male is a mere sack, +which lives for a short time, and is destitute of mouth, stomach, or other +organ of importance, excepting for reproduction. +</p> + +<p> +<a name="Page442"></a> +We are so much accustomed to see differences in structure between the embryo +and the adult, and likewise a close similarity in the embryos of widely +different animals within the same class, that we might be led to look at these +facts as necessarily contingent in some manner on growth. But there is no +obvious reason why, for instance, the wing of a bat, or the fin of a porpoise, +should not have been sketched out with all the parts in proper proportion, as +soon as any structure became visible in the embryo. And in some whole groups of +animals and in certain members of other groups, the embryo does not at any +period differ widely from the adult: thus Owen has remarked in regard to +cuttle-fish, “there is no metamorphosis; the cephalopodic character is +manifested long before the parts of the embryo are completed;” and again +in spiders, “there is nothing worthy to be called a metamorphosis.” +The larvæ of insects, whether adapted to the most diverse and active habits, +or quite inactive, being fed by their parents or placed in the midst of proper +nutriment, yet nearly all pass through a similar worm-like stage of +development; but in some few cases, as in that of Aphis, if we look to the +admirable drawings by Professor Huxley of the development of this insect, we +see no trace of the vermiform stage. +</p> + +<p> +How, then, can we explain these several facts in embryology,—namely the +very general, but not universal difference in structure between the embryo and +the adult;—of parts in the same individual embryo, which ultimately +become very unlike and serve for diverse purposes, being at this early period +of growth alike;—of embryos of different species within the same class, +generally, but not universally, resembling each other;—of the structure +of the embryo not being closely related to its conditions of existence, except +when the +<a name="Page443"></a> +embryo becomes at any period of life active and has to provide for +itself;—of the embryo apparently having sometimes a higher organisation +than the mature animal, into which it is developed. I believe that all these +facts can be explained, as follows, on the view of descent with modification. +</p> + +<p> +It is commonly assumed, perhaps from monstrosities often affecting the embryo +at a very early period, that slight variations necessarily appear at an equally +early period. But we have little evidence on this head—indeed the +evidence rather points the other way; for it is notorious that breeders of +cattle, horses, and various fancy animals, cannot positively tell, until some +time after the animal has been born, what its merits or form will ultimately +turn out. We see this plainly in our own children; we cannot always tell +whether the child will be tall or short, or what its precise features will be. +The question is not, at what period of life any variation has been caused, but +at what period it is fully displayed. The cause may have acted, and I believe +generally has acted, even before the embryo is formed; and the variation may be +due to the male and female sexual elements having been affected by the +conditions to which either parent, or their ancestors, have been exposed. +Nevertheless an effect thus caused at a very early period, even before the +formation of the embryo, may appear late in life; as when an hereditary +disease, which appears in old age alone, has been communicated to the offspring +from the reproductive element of one parent. Or again, as when the horns of +cross-bred cattle have been affected by the shape of the horns of either +parent. For the welfare of a very young animal, as long as it remains in its +mother’s womb, or in the egg, or as long as it is nourished and protected +by its parent, it must be quite unimportant whether most of its characters are +fully +<a name="Page444"></a> +acquired a little earlier or later in life. It would not signify, for instance, +to a bird which obtained its food best by having a long beak, whether or not it +assumed a beak of this particular length, as long as it was fed by its parents. +Hence, I conclude, that it is quite possible, that each of the many successive +modifications, by which each species has acquired its present structure, may +have supervened at a not very early period of life; and some direct evidence +from our domestic animals supports this view. But in other cases it is quite +possible that each successive modification, or most of them, may have appeared +at an extremely early period. +</p> + +<p> +I have stated in the first chapter, that there is some evidence to render it +probable, that at whatever age any variation first appears in the parent, it +tends to reappear at a corresponding age in the offspring. Certain variations +can only appear at corresponding ages, for instance, peculiarities in the +caterpillar, cocoon, or imago states of the silk-moth; or, again, in the horns +of almost full-grown cattle. But further than this, variations which, for all +that we can see, might have appeared earlier or later in life, tend to appear +at a corresponding age in the offspring and parent. I am far from meaning that +this is invariably the case; and I could give a good many cases of variations +(taking the word in the largest sense) which have supervened at an earlier age +in the child than in the parent. +</p> + +<p> +These two principles, if their truth be admitted, will, I believe, explain all +the above specified leading facts in embryology. But first let us look at a few +analogous cases in domestic varieties. Some authors who have written on Dogs, +maintain that the greyhound and bulldog, though appearing so different, are +really varieties most closely allied, and have probably descended from +<a name="Page445"></a> +the same wild stock; hence I was curious to see how far their puppies differed +from each other: I was told by breeders that they differed just as much as +their parents, and this, judging by the eye, seemed almost to be the case; but +on actually measuring the old dogs and their six-days old puppies, I found that +the puppies had not nearly acquired their full amount of proportional +difference. So, again, I was told that the foals of cart and race-horses +differed as much as the full-grown animals; and this surprised me greatly, as I +think it probable that the difference between these two breeds has been wholly +caused by selection under domestication; but having had careful measurements +made of the dam and of a three-days old colt of a race and heavy cart-horse, I +find that the colts have by no means acquired their full amount of proportional +difference. +</p> + +<p> +As the evidence appears to me conclusive, that the several domestic breeds of +Pigeon have descended from one wild species, I compared young pigeons of +various breeds, within twelve hours after being hatched; I carefully measured +the proportions (but will not here give details) of the beak, width of mouth, +length of nostril and of eyelid, size of feet and length of leg, in the wild +stock, in pouters, fantails, runts, barbs, dragons, carriers, and tumblers. Now +some of these birds, when mature, differ so extraordinarily in length and form +of beak, that they would, I cannot doubt, be ranked in distinct genera, had +they been natural productions. But when the nestling birds of these several +breeds were placed in a row, though most of them could be distinguished from +each other, yet their proportional differences in the above specified several +points were incomparably less than in the full-grown birds. Some characteristic +points of difference—for instance, that of the width of mouth—could +hardly be detected in the young. +<a name="Page446"></a> +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. +</p> + +<p> +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-grown 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 inherited by the offspring at a +corresponding not early period. But the case of the short-faced tumbler, which +when twelve hours old had acquired its proper proportions, 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 differences +must have been inherited, not at the corresponding, but at an earlier age. +</p> + +<p> +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 successive steps of variation having +supervened at a rather late age, and having been inherited at a corresponding +<a name="Page447"></a> +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 become, 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 corresponding 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 rather late period of life, and +having thus been converted 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 modifying an organ, such influence will 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. Whereas the young will remain unmodified, or be modified in a +lesser degree, by the effects of use and disuse. +</p> + +<p> +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 would +closely +<a name="Page448"></a> +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 respect 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 wants 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 indispensable 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 consequently to be constructed in a slightly different manner, +then, on the principle of inheritance at corresponding ages, the active young +or larvæ might easily be rendered by natural selection different to any +conceivable extent from their parents. Such differences might, also, become +correlated with successive stages of development; so that the larvæ, in the +first stage, might differ greatly from the larvæ in the second stage, as we +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, etc., would be +useless; and in this case the final metamorphosis would be said to be +retrograde. +</p> + +<p> +As all the organic beings, extinct and recent, which +<a name="Page449"></a> +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 connexion 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 +therefore 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 larvæ 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 progenitors, we can clearly see why ancient and +extinct forms of life should resemble the embryos of their +descendants,—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 variations in a long course of +modification having supervened +<a name="Page450"></a> +at a very early 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 extending far enough back in time, may remain for a long period, or for +ever, incapable of demonstration. +</p> + +<p> +Thus, as it seems to me, the leading facts in embryology, which are second in +importance to none in natural history, are explained on the principle of slight +modifications 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. +</p> + +<p class="p2"> +<i>Rudimentary, atrophied, or aborted organs</i>.—Organs or parts in this +strange condition, bearing the stamp of inutility, are extremely common +throughout nature. For instance, rudimentary mammæ 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 rudimentary 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 rudimentary 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 +<a name="Page451"></a> +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! +</p> + +<p> +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. Rudimentary organs sometimes retain +their potentiality, and are merely not developed: this seems to be the case +with the mammæ 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 +sometimes occur as mere rudiments, and sometimes in a well-developed state. In +plants with separated sexes, the male flowers often have a rudiment of a +pistil; and Kölreuter 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 alike in nature. +</p> + +<p> +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 +<a name="Page452"></a> +supported on the style; but in some Compositæ, 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 developed, and +is clothed with hairs as in other compositæ, 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 +buoyancy, but has become converted into a nascent breathing organ or lung. +Other similar instances could be given. +</p> + +<p> +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. Rudimentary +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. +</p> + +<p> +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 +<a name="Page453"></a> +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. +</p> + +<p> +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 +reasoning 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 generally said to have been created +“for the sake of symmetry,” or in order “to complete the +scheme of nature;” but this seems to me no explanation, merely a +restatement of the fact. Would it be thought sufficient to say that because +planets revolve in elliptic courses round the sun, satellites follow the same +course round the planets, for the sake of symmetry, and to complete the scheme +of nature? An eminent physiologist accounts for the presence of rudimentary +organs, by supposing that they serve to excrete matter in excess, or injurious +to the system; but can we suppose that the minute papilla, which often +represents the pistil in male flowers, and which is formed merely of cellular +tissue, can thus act? Can we suppose that the formation of rudimentary teeth +which are subsequently absorbed, can be of any service to the rapidly growing +embryonic calf by the excretion of precious phosphate of lime? When a +man’s fingers have been amputated, imperfect nails sometimes appear on +the stumps: I could as soon believe that these vestiges of nails have appeared, +not from unknown laws +<a name="Page454"></a> +of growth, but in order to excrete horny matter, as that the rudimentary nails +on the fin of the manatee were formed for this purpose. +</p> + +<p> +On my view of descent with modification, the origin of rudimentary organs is +simple. We have plenty of cases of rudimentary organs in our domestic +productions,—as the stump of a tail in tailless breeds,—the vestige +of an ear in earless breeds,—the reappearance of minute dangling horns in +hornless breeds of cattle, more especially, according to Youatt, in young +animals,—and the state of the whole flower in the cauliflower. We often +see rudiments of various parts in monsters. But I doubt whether any of these +cases throw light on the origin of rudimentary organs in a state of nature, +further than by showing that rudiments can be produced; for I doubt whether +species under nature ever undergo abrupt changes. I believe that disuse has +been the main agency; that it has led in successive generations to the gradual +reduction of various organs, until they have become rudimentary,—as in +the case of the eyes of animals inhabiting dark caverns, and of the wings of +birds inhabiting oceanic islands, which have seldom been forced to take flight, +and have ultimately lost the power of flying. Again, an organ useful under +certain conditions, might become injurious under others, as with the wings of +beetles living on small and exposed islands; and in this case natural selection +would continue slowly to reduce the organ, until it was rendered harmless and +rudimentary. +</p> + +<p> +Any change in function, which can be effected by insensibly small steps, is +within the power of natural selection; so that an organ rendered, during +changed habits of life, useless or injurious for one purpose, might easily be +modified and used for another purpose. Or an organ might be retained for one +alone of its +<a name="Page455"></a> +former functions. An organ, when rendered useless, may well be variable, for +its variations cannot be checked by natural selection. At whatever period of +life disuse or selection reduces an organ, and this will generally be when the +being has come to maturity and to its full powers of action, the principle of +inheritance at corresponding ages will reproduce the organ in its reduced state +at the same age, and consequently will seldom affect or reduce it in the +embryo. Thus we can understand the greater relative size of rudimentary organs +in the embryo, and their lesser relative size in the adult. But if each step of +the process of reduction were to be inherited, not at the corresponding age, +but at an extremely early period of life (as we have good reason to believe to +be possible) the rudimentary part would tend to be wholly lost, and we should +have a case of complete abortion. The principle, also, of economy, explained in +a former chapter, by which the materials forming any part or structure, if not +useful to the possessor, will be saved as far as is possible, will probably +often come into play; and this will tend to cause the entire obliteration of a +rudimentary organ. +</p> + +<p> +As the presence of rudimentary organs is thus due to the tendency in every part +of the organisation, which has long existed, to be inherited—we can +understand, on the genealogical view of classification, how it is that +systematists have found rudimentary parts as useful as, or even sometimes more +useful than, parts of high physiological importance. Rudimentary organs may be +compared with the letters in a word, still retained in the spelling, but become +useless in the pronunciation, but which serve as a clue in seeking for its +derivation. On the view of descent with modification, we may conclude that the +existence of organs in a rudimentary, imperfect, and useless condition, or +quite aborted, far +<a name="Page456"></a> +from presenting a strange difficulty, as they assuredly do on the ordinary +doctrine of creation, might even have been anticipated, and can be accounted +for by the laws of inheritance. +</p> + +<p class="p2"> +<i>Summary</i>.—In this chapter I have attempted to show, that the +subordination of group to group in all organisms throughout all time; that the +nature of the relationship, by which all living and extinct beings are united +by complex, radiating, and circuitous lines of affinities into one grand +system; the rules followed and the difficulties encountered by naturalists in +their classifications; the value set upon characters, if constant and +prevalent, whether of high vital importance, or of the most trifling +importance, or, as in rudimentary organs, of no importance; the wide opposition +in value between analogical or adaptive characters, and characters of true +affinity; and other such rules;—all naturally follow on the view of the +common parentage of those forms which are considered by naturalists as allied, +together with their modification through natural selection, with its +contingencies of extinction and divergence of character. In considering this +view of classification, it should be borne in mind that the element of descent +has been universally used in ranking together the sexes, ages, and acknowledged +varieties of the same species, however different they may be in structure. If +we extend the use of this element of descent,—the only certainly known +cause of similarity in organic beings,—we shall understand what is meant +by the natural system: it is genealogical in its attempted arrangement, with +the grades of acquired difference marked by the terms varieties, species, +genera, families, orders, and classes. +</p> + +<p> +On this same view of descent with modification, all the great facts in +Morphology become intelligible,—whether +<a name="Page457"></a> +we look to the same pattern displayed in the homologous organs, to whatever +purpose applied, of the different species of a class; or to the homologous +parts constructed on the same pattern in each individual animal and plant. +</p> + +<p> +On the principle of successive slight variations, not necessarily or generally +supervening at a very early period of life, and being inherited at a +corresponding period, we can understand the great leading facts in Embryology; +namely, the resemblance in an individual embryo of the homologous parts, which +when matured will become widely different from each other in structure and +function; and the resemblance in different species of a class of the homologous +parts or organs, though fitted in the adult members for purposes as different +as possible. Larvæ are active embryos, which have become specially modified in +relation to their habits of life, through the principle of modifications being +inherited at corresponding ages. On this same principle—and bearing in +mind, that when organs are reduced in size, either from disuse or selection, it +will generally be at that period of life when the being has to provide for its +own wants, and bearing in mind how strong is the principle of +inheritance—the occurrence of rudimentary organs and their final +abortion, present to us no inexplicable difficulties; on the contrary, their +presence might have been even anticipated. The importance of embryological +characters and of rudimentary organs in classification is intelligible, on the +view that an arrangement is only so far natural as it is genealogical. +</p> + +<p> +Finally, the several classes of facts which have been considered in this +chapter, seem to me to proclaim so plainly, that the innumerable species, +genera, and families of organic beings, with which this world is +<a name="Page458"></a> +peopled, have all descended, each within its own class or group, from common +parents, and have all been modified in the course of descent, that I should +without hesitation adopt this view, even if it were unsupported by other facts +or arguments. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page459"></a><a name="chap14"></a>CHAPTER XIV.<br /> +RECAPITULATION AND CONCLUSION.</h2> + +<p class="letter"> +Recapitulation of the difficulties on the theory of Natural Selection. +Recapitulation of the general and special circumstances in its favour. Causes +of the general belief in the immutability of species. How far the theory of +natural selection may be extended. Effects of its adoption on the study of +Natural history. Concluding remarks. +</p> + +<p> +As this whole volume is one long argument, it may be convenient to the reader +to have the leading facts and inferences briefly recapitulated. +</p> + +<p> +That many and grave objections may be advanced against the theory of descent +with modification through natural selection, I do not deny. I have endeavoured +to give to them their full force. Nothing at first can appear more difficult to +believe than that the more complex organs and instincts should have been +perfected, not by means superior to, though analogous with, human reason, but +by the accumulation of innumerable slight variations, each good for the +individual possessor. Nevertheless, this difficulty, though appearing to our +imagination insuperably great, cannot be considered real if we admit the +following propositions, namely,—that gradations in the perfection of any +organ or instinct, which we may consider, either do now exist or could have +existed, each good of its kind,—that all organs and instincts are, in +ever so slight a degree, variable,—and, lastly, that there is a struggle +for existence leading to the preservation of each profitable deviation of +structure or instinct. The truth of these propositions cannot, I think, be +disputed. +</p> + +<p> +<a name="Page460"></a> +It is, no doubt, extremely difficult even to conjecture by what gradations many +structures have been perfected, more especially amongst broken and failing +groups of organic beings; but we see so many strange gradations in nature, as +is proclaimed by the canon, “Natura non facit saltum,” that we +ought to be extremely cautious in saying that any organ or instinct, or any +whole being, could not have arrived at its present state by many graduated +steps. There are, it must be admitted, cases of special difficulty on the +theory of natural selection; and one of the most curious of these is the +existence of two or three defined castes of workers or sterile females in the +same community of ants; but I have attempted to show how this difficulty can be +mastered. +</p> + +<p> +With respect to the almost universal sterility of species when first crossed, +which forms so remarkable a contrast with the almost universal fertility of +varieties when crossed, I must refer the reader to the recapitulation of the +facts given at the end of the eighth chapter, which seem to me conclusively to +show that this sterility is no more a special endowment than is the incapacity +of two trees to be grafted together, but that it is incidental on +constitutional differences in the reproductive systems of the intercrossed +species. We see the truth of this conclusion in the vast difference in the +result, when the same two species are crossed reciprocally; that is, when one +species is first used as the father and then as the mother. +</p> + +<p> +The fertility of varieties when intercrossed and of their mongrel offspring +cannot be considered as universal; nor is their very general fertility +surprising when we remember that it is not likely that either their +constitutions or their reproductive systems should have been profoundly +modified. Moreover, most of the +<a name="Page461"></a> +varieties which have been experimentised on +have been produced under domestication; and as domestication apparently tends +to eliminate sterility, we ought not to expect it also to produce sterility. +</p> + +<p> +The sterility of hybrids is a very different case from that of first crosses, +for their reproductive organs are more or less functionally impotent; whereas +in first crosses the organs on both sides are in a perfect condition. As we +continually see that organisms of all kinds are rendered in some degree sterile +from their constitutions having been disturbed by slightly different and new +conditions of life, we need not feel surprise at hybrids being in some degree +sterile, for their constitutions can hardly fail to have been disturbed from +being compounded of two distinct organisations. This parallelism is supported +by another parallel, but directly opposite, class of facts; namely, that the +vigour and fertility of all organic beings are increased by slight changes in +their conditions of life, and that the offspring of slightly modified forms or +varieties acquire from being crossed increased vigour and fertility. So that, +on the one hand, considerable changes in the conditions of life and crosses +between greatly modified forms, lessen fertility; and on the other hand, lesser +changes in the conditions of life and crosses between less modified forms, +increase fertility. +</p> + +<p> +Turning to geographical distribution, the difficulties encountered on the +theory of descent with modification are grave enough. All the individuals of +the same species, and all the species of the same genus, or even higher group, +must have descended from common parents; and therefore, in however distant and +isolated parts of the world they are now found, they must in the course of +successive generations have passed from some one part to the others. We are +often wholly unable +<a name="Page462"></a> +even to conjecture how this could have been effected. Yet, as we have reason to +believe that some species have retained the same specific form for very long +periods, enormously long as measured by years, too much stress ought not to be +laid on the occasional wide diffusion of the same species; for during very long +periods of time there will always be a good chance for wide migration by many +means. A broken or interrupted range may often be accounted for by the +extinction of the species in the intermediate regions. It cannot be denied that +we are as yet very ignorant of the full extent of the various climatal and +geographical changes which have affected the earth during modern periods; and +such changes will obviously have greatly facilitated migration. As an example, +I have attempted to show how potent has been the influence of the Glacial +period on the distribution both of the same and of representative species +throughout the world. We are as yet profoundly ignorant of the many occasional +means of transport. With respect to distinct species of the same genus +inhabiting very distant and isolated regions, as the process of modification +has necessarily been slow, all the means of migration will have been possible +during a very long period; and consequently the difficulty of the wide +diffusion of species of the same genus is in some degree lessened. +</p> + +<p> +As on the theory of natural selection an interminable number of intermediate +forms must have existed, linking together all the species in each group by +gradations as fine as our present varieties, it may be asked, Why do we not see +these linking forms all around us? Why are not all organic beings blended +together in an inextricable chaos? With respect to existing forms, we should +remember that we have no right to expect (excepting in rare cases) to discover +<i>directly</i> connecting +<a name="Page463"></a> +links between them, but only between each and some extinct and supplanted form. +Even on a wide area, which has during a long period remained continuous, and of +which the climate and other conditions of life change insensibly in going from +a district occupied by one species into another district occupied by a closely +allied species, we have no just right to expect often to find intermediate +varieties in the intermediate zone. For we have reason to believe that only a +few species are undergoing change at any one period; and all changes are slowly +effected. I have also shown that the intermediate varieties which will at first +probably exist in the intermediate zones, will be liable to be supplanted by +the allied forms on either hand; and the latter, from existing in greater +numbers, will generally be modified and improved at a quicker rate than the +intermediate varieties, which exist in lesser numbers; so that the intermediate +varieties will, in the long run, be supplanted and exterminated. +</p> + +<p> +On this doctrine of the extermination of an infinitude of connecting links, +between the living and extinct inhabitants of the world, and at each successive +period between the extinct and still older species, why is not every geological +formation charged with such links? Why does not every collection of fossil +remains afford plain evidence of the gradation and mutation of the forms of +life? We meet with no such evidence, and this is the most obvious and forcible +of the many objections which may be urged against my theory. Why, again, do +whole groups of allied species appear, though certainly they often falsely +appear, to have come in suddenly on the several geological stages? Why do we +not find great piles of strata beneath the Silurian system, stored with the +remains of the progenitors of the Silurian groups of fossils? For certainly on +my theory such +<a name="Page464"></a> +strata must somewhere have been deposited at these ancient and utterly unknown +epochs in the world’s history. +</p> + +<p> +I can answer these questions and grave objections only on the supposition that +the geological record is far more imperfect than most geologists believe. It +cannot be objected that there has not been time sufficient for any amount of +organic change; for the lapse of time has been so great as to be utterly +inappreciable by the human intellect. The number of specimens in all our +museums is absolutely as nothing compared with the countless generations of +countless species which certainly have existed. We should not be able to +recognise a species as the parent of any one or more species if we were to +examine them ever so closely, unless we likewise possessed many of the +intermediate links between their past or parent and present states; and these +many links we could hardly ever expect to discover, owing to the imperfection +of the geological record. Numerous existing doubtful forms could be named which +are probably varieties; but who will pretend that in future ages so many fossil +links will be discovered, that naturalists will be able to decide, on the +common view, whether or not these doubtful forms are varieties? As long as most +of the links between any two species are unknown, if any one link or +intermediate variety be discovered, it will simply be classed as another and +distinct species. Only a small portion of the world has been geologically +explored. Only organic beings of certain classes can be preserved in a fossil +condition, at least in any great number. Widely ranging species vary most, and +varieties are often at first local,—both causes rendering the discovery +of intermediate links less likely. Local varieties will not spread into other +and distant regions until they are considerably modified and improved; +<a name="Page465"></a> +and when they do spread, if discovered in a geological formation, they will +appear as if suddenly created there, and will be simply classed as new species. +Most formations have been intermittent in their accumulation; and their +duration, I am inclined to believe, has been shorter than the average duration +of specific forms. Successive formations are separated from each other by +enormous blank intervals of time; for fossiliferous formations, thick enough to +resist future degradation, can be accumulated only where much sediment is +deposited on the subsiding bed of the sea. During the alternate periods of +elevation and of stationary level the record will be blank. During these latter +periods there will probably be more variability in the forms of life; during +periods of subsidence, more extinction. +</p> + +<p> +With respect to the absence of fossiliferous formations beneath the lowest +Silurian strata, I can only recur to the hypothesis given in the ninth chapter. +That the geological record is imperfect all will admit; but that it is +imperfect to the degree which I require, few will be inclined to admit. If we +look to long enough intervals of time, geology plainly declares that all +species have changed; and they have changed in the manner which my theory +requires, for they have changed slowly and in a graduated manner. We clearly +see this in the fossil remains from consecutive formations invariably being +much more closely related to each other, than are the fossils from formations +distant from each other in time. +</p> + +<p> +Such is the sum of the several chief objections and difficulties which may +justly be urged against my theory; and I have now briefly recapitulated the +answers and explanations which can be given to them. I have felt these +difficulties far too heavily during many years to +<a name="Page466"></a> +doubt their weight. But it deserves especial notice that the more important +objections relate to questions on which we are confessedly ignorant; nor do we +know how ignorant we are. We do not know all the possible transitional +gradations between the simplest and the most perfect organs; it cannot be +pretended that we know all the varied means of Distribution during the long +lapse of years, or that we know how imperfect the Geological Record is. Grave +as these several difficulties are, in my judgment they do not overthrow the +theory of descent with modification. +</p> + +<p class="p2"> +Now let us turn to the other side of the argument. Under domestication we see +much variability. This seems to be mainly due to the reproductive system being +eminently susceptible to changes in the conditions of life; so that this +system, when not rendered impotent, fails to reproduce offspring exactly like +the parent-form. Variability is governed by many complex laws,—by +correlation of growth, by use and disuse, and by the direct action of the +physical conditions of life. There is much difficulty in ascertaining how much +modification our domestic productions have undergone; but we may safely infer +that the amount has been large, and that modifications can be inherited for +long periods. As long as the conditions of life remain the same, we have reason +to believe that a modification, which has already been inherited for many +generations, may continue to be inherited for an almost infinite number of +generations. On the other hand we have evidence that variability, when it has +once come into play, does not wholly cease; for new varieties are still +occasionally produced by our most anciently domesticated productions. +</p> + +<p> +Man does not actually produce variability; he only +<a name="Page467"></a> +unintentionally exposes organic beings to new conditions of life, and then +nature acts on the organisation, and causes variability. But man can and does +select the variations given to him by nature, and thus accumulate them in any +desired manner. He thus adapts animals and plants for his own benefit or +pleasure. He may do this methodically, or he may do it unconsciously by +preserving the individuals most useful to him at the time, without any thought +of altering the breed. It is certain that he can largely influence the +character of a breed by selecting, in each successive generation, individual +differences so slight as to be quite inappreciable by an uneducated eye. This +process of selection has been the great agency in the production of the most +distinct and useful domestic breeds. That many of the breeds produced by man +have to a large extent the character of natural species, is shown by the +inextricable doubts whether very many of them are varieties or aboriginal +species. +</p> + +<p> +There is no obvious reason why the principles which have acted so efficiently +under domestication should not have acted under nature. In the preservation of +favoured individuals and races, during the constantly-recurrent Struggle for +Existence, we see the most powerful and ever-acting means of selection. The +struggle for existence inevitably follows from the high geometrical ratio of +increase which is common to all organic beings. This high rate of increase is +proved by calculation, by the effects of a succession of peculiar seasons, and +by the results of naturalisation, as explained in the third chapter. More +individuals are born than can possibly survive. A grain in the balance will +determine which individual shall live and which shall die,—which variety +or species shall increase in number, and which shall decrease, or finally +become extinct. As the individuals +<a name="Page468"></a> +of the same species come in all respects into the closest competition with each +other, the struggle will generally be most severe between them; it will be +almost equally severe between the varieties of the same species, and next in +severity between the species of the same genus. But the struggle will often be +very severe between beings most remote in the scale of nature. The slightest +advantage in one being, at any age or during any season, over those with which +it comes into competition, or better adaptation in however slight a degree to +the surrounding physical conditions, will turn the balance. +</p> + +<p> +With animals having separated sexes there will in most cases be a struggle +between the males for possession of the females. The most vigorous individuals, +or those which have most successfully struggled with their conditions of life, +will generally leave most progeny. But success will often depend on having +special weapons or means of defence, or on the charms of the males; and the +slightest advantage will lead to victory. +</p> + +<p> +As geology plainly proclaims that each land has undergone great physical +changes, we might have expected that organic beings would have varied under +nature, in the same way as they generally have varied under the changed +conditions of domestication. And if there be any variability under nature, it +would be an unaccountable fact if natural selection had not come into play. It +has often been asserted, but the assertion is quite incapable of proof, that +the amount of variation under nature is a strictly limited quantity. Man, +though acting on external characters alone and often capriciously, can produce +within a short period a great result by adding up mere individual differences +in his domestic productions; and every one admits that there are at least +individual differences in species under nature. But, besides such differences, +all naturalists +<a name="Page469"></a> +have admitted the existence of varieties, which they think sufficiently +distinct to be worthy of record in systematic works. No one can draw any clear +distinction between individual differences and slight varieties; or between +more plainly marked varieties and sub-species, and species. Let it be observed +how naturalists differ in the rank which they assign to the many representative +forms in Europe and North America. +</p> + +<p> +If then we have under nature variability and a powerful agent always ready to +act and select, why should we doubt that variations in any way useful to +beings, under their excessively complex relations of life, would be preserved, +accumulated, and inherited? Why, if man can by patience select variations most +useful to himself, should nature fail in selecting variations useful, under +changing conditions of life, to her living products? What limit can be put to +this power, acting during long ages and rigidly scrutinising the whole +constitution, structure, and habits of each creature,—favouring the good +and rejecting the bad? I can see no limit to this power, in slowly and +beautifully adapting each form to the most complex relations of life. The +theory of natural selection, even if we looked no further than this, seems to +me to be in itself probable. I have already recapitulated, as fairly as I +could, the opposed difficulties and objections: now let us turn to the special +facts and arguments in favour of the theory. +</p> + +<p> +On the view that species are only strongly marked and permanent varieties, and +that each species first existed as a variety, we can see why it is that no line +of demarcation can be drawn between species, commonly supposed to have been +produced by special acts of creation, and varieties which are acknowledged to +have been produced by secondary laws. On this same view we can understand how +it is that in each region +<a name="Page470"></a> +where many species of a genus have been produced, and where they now flourish, +these same species should present many varieties; for where the manufactory of +species has been active, we might expect, as a general rule, to find it still +in action; and this is the case if varieties be incipient species. Moreover, +the species of the larger genera, which afford the greater number of varieties +or incipient species, retain to a certain degree the character of varieties; +for they differ from each other by a less amount of difference than do the +species of smaller genera. The closely allied species also of the larger genera +apparently have restricted ranges, and they are clustered in little groups +round other species—in which respects they resemble varieties. These are +strange relations on the view of each species having been independently +created, but are intelligible if all species first existed as varieties. +</p> + +<p> +As each species tends by its geometrical ratio of reproduction to increase +inordinately in number; and as the modified descendants of each species will be +enabled to increase by so much the more as they become more diversified in +habits and structure, so as to be enabled to seize on many and widely different +places in the economy of nature, there will be a constant tendency in natural +selection to preserve the most divergent offspring of any one species. Hence +during a long-continued course of modification, the slight differences, +characteristic of varieties of the same species, tend to be augmented into the +greater differences characteristic of species of the same genus. New and +improved varieties will inevitably supplant and exterminate the older, less +improved and intermediate varieties; and thus species are rendered to a large +extent defined and distinct objects. Dominant species belonging to the larger +groups tend to give birth to new and dominant +<a name="Page471"></a> +forms; so that each large group tends to become still larger, and at the same +time more divergent in character. But as all groups cannot thus succeed in +increasing in size, for the world would not hold them, the more dominant groups +beat the less dominant. This tendency in the large groups to go on increasing +in size and diverging in character, together with the almost inevitable +contingency of much extinction, explains the arrangement of all the forms of +life, in groups subordinate to groups, all within a few great classes, which we +now see everywhere around us, and which has prevailed throughout all time. This +grand fact of the grouping of all organic beings seems to me utterly +inexplicable on the theory of creation. +</p> + +<p> +As natural selection acts solely by accumulating slight, successive, favourable +variations, it can produce no great or sudden modification; it can act only by +very short and slow steps. Hence the canon of “Natura non facit +saltum,” which every fresh addition to our knowledge tends to make more +strictly correct, is on this theory simply intelligible. We can plainly see why +nature is prodigal in variety, though niggard in innovation. But why this +should be a law of nature if each species has been independently created, no +man can explain. +</p> + +<p> +Many other facts are, as it seems to me, explicable on this theory. How strange +it is that a bird, under the form of woodpecker, should have been created to +prey on insects on the ground; that upland geese, which never or rarely swim, +should have been created with webbed feet; that a thrush should have been +created to dive and feed on sub-aquatic insects; and that a petrel should have +been created with habits and structure fitting it for the life of an auk or +grebe! and so on in endless other cases. But on the view of each +<a name="Page472"></a> +species constantly trying to increase in number, with natural selection always +ready to adapt the slowly varying descendants of each to any unoccupied or +ill-occupied place in nature, these facts cease to be strange, or perhaps might +even have been anticipated. +</p> + +<p> +As natural selection acts by competition, it adapts the inhabitants of each +country only in relation to the degree of perfection of their associates; so +that we need feel no surprise at the inhabitants of any one country, although +on the ordinary view supposed to have been specially created and adapted for +that country, being beaten and supplanted by the naturalised productions from +another land. Nor ought we to marvel if all the contrivances in nature be not, +as far as we can judge, absolutely perfect; and if some of them be abhorrent to +our ideas of fitness. We need not marvel at the sting of the bee causing the +bee’s own death; at drones being produced in such vast numbers for one +single act, and being then slaughtered by their sterile sisters; at the +astonishing waste of pollen by our fir-trees; at the instinctive hatred of the +queen bee for her own fertile daughters; at ichneumonidæ feeding within the +live bodies of caterpillars; and at other such cases. The wonder indeed is, on +the theory of natural selection, that more cases of the want of absolute +perfection have not been observed. +</p> + +<p> +The complex and little known laws governing variation are the same, as far as +we can see, with the laws which have governed the production of so-called +specific forms. In both cases physical conditions seem to have produced but +little direct effect; yet when varieties enter any zone, they occasionally +assume some of the characters of the species proper to that zone. In both +varieties and species, use and disuse seem to have produced some effect; for it +is difficult to resist this conclusion +<a name="Page473"></a> +when we look, for instance, at the logger-headed duck, which has wings +incapable of flight, in nearly the same condition as in the domestic duck; or +when we look at the burrowing tucutucu, which is occasionally blind, and then +at certain moles, which are habitually blind and have their eyes covered with +skin; or when we look at the blind animals inhabiting the dark caves of America +and Europe. In both varieties and species correlation of growth seems to have +played a most important part, so that when one part has been modified other +parts are necessarily modified. In both varieties and species reversions to +long-lost characters occur. How inexplicable on the theory of creation is the +occasional appearance of stripes on the shoulder and legs of the several +species of the horse-genus and in their hybrids! How simply is this fact +explained if we believe that these species have descended from a striped +progenitor, in the same manner as the several domestic breeds of pigeon have +descended from the blue and barred rock-pigeon! +</p> + +<p> +On the ordinary view of each species having been independently created, why +should the specific characters, or those by which the species of the same genus +differ from each other, be more variable than the generic characters in which +they all agree? Why, for instance, should the colour of a flower be more likely +to vary in any one species of a genus, if the other species, supposed to have +been created independently, have differently coloured flowers, than if all the +species of the genus have the same coloured flowers? If species are only +well-marked varieties, of which the characters have become in a high degree +permanent, we can understand this fact; for they have already varied since they +branched off from a common progenitor in certain characters, by which they have +come to be specifically distinct from each other; +<a name="Page474"></a> +and therefore these same characters would be more likely still to be variable +than the generic characters which have been inherited without change for an +enormous period. It is inexplicable on the theory of creation why a part +developed in a very unusual manner in any one species of a genus, and +therefore, as we may naturally infer, of great importance to the species, +should be eminently liable to variation; but, on my view, this part has +undergone, since the several species branched off from a common progenitor, an +unusual amount of variability and modification, and therefore we might expect +this part generally to be still variable. But a part may be developed in the +most unusual manner, like the wing of a bat, and yet not be more variable than +any other structure, if the part be common to many subordinate forms, that is, +if it has been inherited for a very long period; for in this case it will have +been rendered constant by long-continued natural selection. +</p> + +<p> +Glancing at instincts, marvellous as some are, they offer no greater difficulty +than does corporeal structure on the theory of the natural selection of +successive, slight, but profitable modifications. We can thus understand why +nature moves by graduated steps in endowing different animals of the same class +with their several instincts. I have attempted to show how much light the +principle of gradation throws on the admirable architectural powers of the +hive-bee. Habit no doubt sometimes comes into play in modifying instincts; but +it certainly is not indispensable, as we see, in the case of neuter insects, +which leave no progeny to inherit the effects of long-continued habit. On the +view of all the species of the same genus having descended from a common +parent, and having inherited much in common, we can understand how it is that +allied species, when placed under considerably different conditions of life, +<a name="Page475"></a> +yet should follow nearly the same instincts; why the thrush of South America, +for instance, lines her nest with mud like our British species. On the view of +instincts having been slowly acquired through natural selection we need not +marvel at some instincts being apparently not perfect and liable to mistakes, +and at many instincts causing other animals to suffer. +</p> + +<p> +If species be only well-marked and permanent varieties, we can at once see why +their crossed offspring should follow the same complex laws in their degrees +and kinds of resemblance to their parents,—in being absorbed into each +other by successive crosses, and in other such points,—as do the crossed +offspring of acknowledged varieties. On the other hand, these would be strange +facts if species have been independently created, and varieties have been +produced by secondary laws. +</p> + +<p> +If we admit that the geological record is imperfect in an extreme degree, then +such facts as the record gives, support the theory of descent with +modification. New species have come on the stage slowly and at successive +intervals; and the amount of change, after equal intervals of time, is widely +different in different groups. The extinction of species and of whole groups of +species, which has played so conspicuous a part in the history of the organic +world, almost inevitably follows on the principle of natural selection; for old +forms will be supplanted by new and improved forms. Neither single species nor +groups of species reappear when the chain of ordinary generation has once been +broken. The gradual diffusion of dominant forms, with the slow modification of +their descendants, causes the forms of life, after long intervals of time, to +appear as if they had changed simultaneously throughout the world. The fact of +the fossil remains of each formation being in some degree intermediate in +character between the +<a name="Page476"></a> +fossils in the formations above and below, is simply explained by their +intermediate position in the chain of descent. The grand fact that all extinct +organic beings belong to the same system with recent beings, falling either +into the same or into intermediate groups, follows from the living and the +extinct being the offspring of common parents. As the groups which have +descended from an ancient progenitor have generally diverged in character, the +progenitor with its early descendants will often be intermediate in character +in comparison with its later descendants; and thus we can see why the more +ancient a fossil is, the oftener it stands in some degree intermediate between +existing and allied groups. Recent forms are generally looked at as being, in +some vague sense, higher than ancient and extinct forms; and they are in so far +higher as the later and more improved forms have conquered the older and less +improved organic beings in the struggle for life. Lastly, the law of the long +endurance of allied forms on the same continent,—of marsupials in +Australia, of edentata in America, and other such cases,—is intelligible, +for within a confined country, the recent and the extinct will naturally be +allied by descent. +</p> + +<p> +Looking to geographical distribution, if we admit that there has been during +the long course of ages much migration from one part of the world to another, +owing to former climatal and geographical changes and to the many occasional +and unknown means of dispersal, then we can understand, on the theory of +descent with modification, most of the great leading facts in Distribution. We +can see why there should be so striking a parallelism in the distribution of +organic beings throughout space, and in their geological succession throughout +time; for in both cases the beings have been connected by the bond of ordinary +generation, and the means of +<a name="Page477"></a> +modification have been the same. We see the full meaning of the wonderful fact, +which must have struck every traveller, namely, that on the same continent, +under the most diverse conditions, under heat and cold, on mountain and +lowland, on deserts and marshes, most of the inhabitants within each great +class are plainly related; for they will generally be descendants of the same +progenitors and early colonists. On this same principle of former migration, +combined in most cases with modification, we can understand, by the aid of the +Glacial period, the identity of some few plants, and the close alliance of many +others, on the most distant mountains, under the most different climates; and +likewise the close alliance of some of the inhabitants of the sea in the +northern and southern temperate zones, though separated by the whole +intertropical ocean. Although two areas may present the same physical +conditions of life, we need feel no surprise at their inhabitants being widely +different, if they have been for a long period completely separated from each +other; for as the relation of organism to organism is the most important of all +relations, and as the two areas will have received colonists from some third +source or from each other, at various periods and in different proportions, the +course of modification in the two areas will inevitably be different. +</p> + +<p> +On this view of migration, with subsequent modification, we can see why oceanic +islands should be inhabited by few species, but of these, that many should be +peculiar. We can clearly see why those animals which cannot cross wide spaces +of ocean, as frogs and terrestrial mammals, should not inhabit oceanic islands; +and why, on the other hand, new and peculiar species of bats, which can +traverse the ocean, should so often be found on islands far distant from any +continent. Such facts +<a name="Page478"></a> +as the presence of peculiar species of bats, and the absence of all other +mammals, on oceanic islands, are utterly inexplicable on the theory of +independent acts of creation. +</p> + +<p> +The existence of closely allied or representative species in any two areas, +implies, on the theory of descent with modification, that the same parents +formerly inhabited both areas; and we almost invariably find that wherever many +closely allied species inhabit two areas, some identical species common to both +still exist. Wherever many closely allied yet distinct species occur, many +doubtful forms and varieties of the same species likewise occur. It is a rule +of high generality that the inhabitants of each area are related to the +inhabitants of the nearest source whence immigrants might have been derived. We +see this in nearly all the plants and animals of the Galapagos archipelago, of +Juan Fernandez, and of the other American islands being related in the most +striking manner to the plants and animals of the neighbouring American +mainland; and those of the Cape de Verde archipelago and other African islands +to the African mainland. It must be admitted that these facts receive no +explanation on the theory of creation. +</p> + +<p> +The fact, as we have seen, that all past and present organic beings constitute +one grand natural system, with group subordinate to group, and with extinct +groups often falling in between recent groups, is intelligible on the theory of +natural selection with its contingencies of extinction and divergence of +character. On these same principles we see how it is, that the mutual +affinities of the species and genera within each class are so complex and +circuitous. We see why certain characters are far more serviceable than others +for classification;—why adaptive characters, though of paramount +importance to the being, are of hardly any +<a name="Page479"></a> +importance in classification; why characters derived from rudimentary parts, +though of no service to the being, are often of high classificatory value; and +why embryological characters are the most valuable of all. The real affinities +of all organic beings are due to inheritance or community of descent. The +natural system is a genealogical arrangement, in which we have to discover the +lines of descent by the most permanent characters, however slight their vital +importance may be. +</p> + +<p> +The framework of bones being the same in the hand of a man, wing of a bat, fin +of the porpoise, and leg of the horse,—the same number of vertebræ +forming the neck of the giraffe and of the elephant,—and innumerable +other such facts, at once explain themselves on the theory of descent with slow +and slight successive modifications. The similarity of pattern in the wing and +leg of a bat, though used for such different purpose,—in the jaws and +legs of a crab,—in the petals, stamens, and pistils of a flower, is +likewise intelligible on the view of the gradual modification of parts or +organs, which were alike in the early progenitor of each class. On the +principle of successive variations not always supervening at an early age, and +being inherited at a corresponding not early period of life, we can clearly see +why the embryos of mammals, birds, reptiles, and fishes should be so closely +alike, and should be so unlike the adult forms. We may cease marvelling at the +embryo of an air-breathing mammal or bird having branchial slits and arteries +running in loops, like those in a fish which has to breathe the air dissolved +in water, by the aid of well-developed branchiæ. +</p> + +<p> +Disuse, aided sometimes by natural selection, will often tend to reduce an +organ, when it has become useless by changed habits or under changed conditions +<a name="Page480"></a> +of life; and we can clearly understand on this view the meaning of rudimentary +organs. But disuse and selection will generally act on each creature, when it +has come to maturity and has to play its full part in the struggle for +existence, and will thus have little power of acting on an organ during early +life; hence the organ will not be much reduced or rendered rudimentary at this +early age. The calf, for instance, has inherited teeth, which never cut through +the gums of the upper jaw, from an early progenitor having well-developed +teeth; and we may believe, that the teeth in the mature animal were reduced, +during successive generations, by disuse or by the tongue and palate having +been fitted by natural selection to browse without their aid; whereas in the +calf, the teeth have been left untouched by selection or disuse, and on the +principle of inheritance at corresponding ages have been inherited from a +remote period to the present day. On the view of each organic being and each +separate organ having been specially created, how utterly inexplicable it is +that parts, like the teeth in the embryonic calf or like the shrivelled wings +under the soldered wing-covers of some beetles, should thus so frequently bear +the plain stamp of inutility! Nature may be said to have taken pains to reveal, +by rudimentary organs and by homologous structures, her scheme of modification, +which it seems that we wilfully will not understand. +</p> + +<p class="p2"> +I have now recapitulated the chief facts and considerations which have +thoroughly convinced me that species have changed, and are still slowly +changing by the preservation and accumulation of successive slight favourable +variations. Why, it may be asked, have all the most eminent living naturalists +and geologists rejected this view of the mutability of species? It cannot be +<a name="Page481"></a> +asserted that organic beings in a state of nature are subject to no variation; +it cannot be proved that the amount of variation in the course of long ages is +a limited quantity; no clear distinction has been, or can be, drawn between +species and well-marked varieties. It cannot be maintained that species when +intercrossed are invariably sterile, and varieties invariably fertile; or that +sterility is a special endowment and sign of creation. The belief that species +were immutable productions was almost unavoidable as long as the history of the +world was thought to be of short duration; and now that we have acquired some +idea of the lapse of time, we are too apt to assume, without proof, that the +geological record is so perfect that it would have afforded us plain evidence +of the mutation of species, if they had undergone mutation. +</p> + +<p> +But the chief cause of our natural unwillingness to admit that one species has +given birth to other and distinct species, is that we are always slow in +admitting any great change of which we do not see the intermediate steps. The +difficulty is the same as that felt by so many geologists, when Lyell first +insisted that long lines of inland cliffs had been formed, and great valleys +excavated, by the slow action of the coast-waves. The mind cannot possibly +grasp the full meaning of the term of a hundred million years; it cannot add up +and perceive the full effects of many slight variations, accumulated during an +almost infinite number of generations. +</p> + +<p> +Although I am fully convinced of the truth of the views given in this volume +under the form of an abstract, I by no means expect to convince experienced +naturalists whose minds are stocked with a multitude of facts all viewed, +during a long course of years, from a point of view directly opposite to mine. +It is so easy +<a name="Page482"></a> +to hide our ignorance under such expressions as the “plan of +creation,” “unity of design,” etc., and to think that we give +an explanation when we only restate a fact. Any one whose disposition leads him +to attach more weight to unexplained difficulties than to the explanation of a +certain number of facts will certainly reject my theory. A few naturalists, +endowed with much flexibility of mind, and who have already begun to doubt on +the immutability of species, may be influenced by this volume; but I look with +confidence to the future, to young and rising naturalists, who will be able to +view both sides of the question with impartiality. Whoever is led to believe +that species are mutable will do good service by conscientiously expressing his +conviction; for only thus can the load of prejudice by which this subject is +overwhelmed be removed. +</p> + +<p> +Several eminent naturalists have of late published their belief that a +multitude of reputed species in each genus are not real species; but that other +species are real, that is, have been independently created. This seems to me a +strange conclusion to arrive at. They admit that a multitude of forms, which +till lately they themselves thought were special creations, and which are still +thus looked at by the majority of naturalists, and which consequently have +every external characteristic feature of true species,—they admit that +these have been produced by variation, but they refuse to extend the same view +to other and very slightly different forms. Nevertheless they do not pretend +that they can define, or even conjecture, which are the created forms of life, +and which are those produced by secondary laws. They admit variation as a +<i>vera causa</i> in one case, they arbitrarily reject it in another, without +assigning any distinction in the two cases. The day will come when this will be +given as a curious illustration of +<a name="Page483"></a> +the blindness of preconceived opinion. These authors seem no more startled at a +miraculous act of creation than at an ordinary birth. But do they really +believe that at innumerable periods in the earth’s history certain +elemental atoms have been commanded suddenly to flash into living tissues? Do +they believe that at each supposed act of creation one individual or many were +produced? Were all the infinitely numerous kinds of animals and plants created +as eggs or seed, or as full grown? and in the case of mammals, were they +created bearing the false marks of nourishment from the mother’s womb? +Although naturalists very properly demand a full explanation of every +difficulty from those who believe in the mutability of species, on their own +side they ignore the whole subject of the first appearance of species in what +they consider reverent silence. +</p> + +<p> +It may be asked how far I extend the doctrine of the modification of species. +The question is difficult to answer, because the more distinct the forms are +which we may consider, by so much the arguments fall away in force. But some +arguments of the greatest weight extend very far. All the members of whole +classes can be connected together by chains of affinities, and all can be +classified on the same principle, in groups subordinate to groups. Fossil +remains sometimes tend to fill up very wide intervals between existing orders. +Organs in a rudimentary condition plainly show that an early progenitor had the +organ in a fully developed state; and this in some instances necessarily +implies an enormous amount of modification in the descendants. Throughout whole +classes various structures are formed on the same pattern, and at an embryonic +age the species closely resemble each other. Therefore I cannot doubt that the +theory of descent with modification +<a name="Page484"></a> +embraces all the members of the same class. I believe that animals have +descended from at most only four or five progenitors, and plants from an equal +or lesser number. +</p> + +<p> +Analogy would lead me one step further, namely, to the belief that all animals +and plants have descended from some one prototype. But analogy may be a +deceitful guide. Nevertheless all living things have much in common, in their +chemical composition, their germinal vesicles, their cellular structure, and +their laws of growth and reproduction. We see this even in so trifling a +circumstance as that the same poison often similarly affects plants and +animals; or that the poison secreted by the gall-fly produces monstrous growths +on the wild rose or oak-tree. Therefore I should infer from analogy that +probably all the organic beings which have ever lived on this earth have +descended from some one primordial form, into which life was first breathed. +</p> + +<p class="p2"> +When the views entertained in this volume on the origin of species, or when +analogous views are generally admitted, we can dimly foresee that there will be +a considerable revolution in natural history. Systematists will be able to +pursue their labours as at present; but they will not be incessantly haunted by +the shadowy doubt whether this or that form be in essence a species. This I +feel sure, and I speak after experience, will be no slight relief. The endless +disputes whether or not some fifty species of British brambles are true species +will cease. Systematists will have only to decide (not that this will be easy) +whether any form be sufficiently constant and distinct from other forms, to be +capable of definition; and if definable, whether the differences be +sufficiently important to deserve a specific name. This latter point will +become a far more essential consideration +<a name="Page485"></a> +than it is at present; for differences, however slight, between any two forms, +if not blended by intermediate gradations, are looked at by most naturalists as +sufficient to raise both forms to the rank of species. Hereafter we shall be +compelled to acknowledge that the only distinction between species and +well-marked varieties is, that the latter are known, or believed, to be +connected at the present day by intermediate gradations, whereas species were +formerly thus connected. Hence, without quite rejecting the consideration of +the present existence of intermediate gradations between any two forms, we +shall be led to weigh more carefully and to value higher the actual amount of +difference between them. It is quite possible that forms now generally +acknowledged to be merely varieties may hereafter be thought worthy of specific +names, as with the primrose and cowslip; and in this case scientific and common +language will come into accordance. In short, we shall have to treat species in +the same manner as those naturalists treat genera, who admit that genera are +merely artificial combinations made for convenience. This may not be a cheering +prospect; but we shall at least be freed from the vain search for the +undiscovered and undiscoverable essence of the term species. +</p> + +<p> +The other and more general departments of natural history will rise greatly in +interest. The terms used by naturalists of affinity, relationship, community of +type, paternity, morphology, adaptive characters, rudimentary and aborted +organs, etc., will cease to be metaphorical, and will have a plain +signification. When we no longer look at an organic being as a savage looks at +a ship, as at something wholly beyond his comprehension; when we regard every +production of nature as one which has had a history; when we contemplate every +complex structure +<a name="Page486"></a> +and instinct as the summing up of many contrivances, each useful to the +possessor, nearly in the same way as when we look at any great mechanical +invention as the summing up of the labour, the experience, the reason, and even +the blunders of numerous workmen; when we thus view each organic being, how far +more interesting, I speak from experience, will the study of natural history +become! +</p> + +<p> +A grand and almost untrodden field of inquiry will be opened, on the causes and +laws of variation, on correlation of growth, on the effects of use and disuse, +on the direct action of external conditions, and so forth. The study of +domestic productions will rise immensely in value. A new variety raised by man +will be a far more important and interesting subject for study than one more +species added to the infinitude of already recorded species. Our +classifications will come to be, as far as they can be so made, genealogies; +and will then truly give what may be called the plan of creation. The rules for +classifying will no doubt become simpler when we have a definite object in +view. We possess no pedigrees or armorial bearings; and we have to discover and +trace the many diverging lines of descent in our natural genealogies, by +characters of any kind which have long been inherited. Rudimentary organs will +speak infallibly with respect to the nature of long-lost structures. Species +and groups of species, which are called aberrant, and which may fancifully be +called living fossils, will aid us in forming a picture of the ancient forms of +life. Embryology will reveal to us the structure, in some degree obscured, of +the prototypes of each great class. +</p> + +<p> +When we can feel assured that all the individuals of the same species, and all +the closely allied species of most genera, have within a not very remote period +descended +<a name="Page487"></a> +from one parent, and have migrated from some one birthplace; and when we better +know the many means of migration, then, by the light which geology now throws, +and will continue to throw, on former changes of climate and of the level of +the land, we shall surely be enabled to trace in an admirable manner the former +migrations of the inhabitants of the whole world. Even at present, by comparing +the differences of the inhabitants of the sea on the opposite sides of a +continent, and the nature of the various inhabitants of that continent in +relation to their apparent means of immigration, some light can be thrown on +ancient geography. +</p> + +<p> +The noble science of Geology loses glory from the extreme imperfection of the +record. The crust of the earth with its embedded remains must not be looked at +as a well-filled museum, but as a poor collection made at hazard and at rare +intervals. The accumulation of each great fossiliferous formation will be +recognised as having depended on an unusual concurrence of circumstances, and +the blank intervals between the successive stages as having been of vast +duration. But we shall be able to gauge with some security the duration of +these intervals by a comparison of the preceding and succeeding organic forms. +We must be cautious in attempting to correlate as strictly contemporaneous two +formations, which include few identical species, by the general succession of +their forms of life. As species are produced and exterminated by slowly acting +and still existing causes, and not by miraculous acts of creation and by +catastrophes; and as the most important of all causes of organic change is one +which is almost independent of altered and perhaps suddenly altered physical +conditions, namely, the mutual relation of organism to organism,—the +improvement of one being entailing the improvement or the extermination of +<a name="Page488"></a> +others; it follows, that the amount of organic change in the fossils of +consecutive formations probably serves as a fair measure of the lapse of actual +time. A number of species, however, keeping in a body might remain for a long +period unchanged, whilst within this same period, several of these species, by +migrating into new countries and coming into competition with foreign +associates, might become modified; so that we must not overrate the accuracy of +organic change as a measure of time. During early periods of the earth’s +history, when the forms of life were probably fewer and simpler, the rate of +change was probably slower; and at the first dawn of life, when very few forms +of the simplest structure existed, the rate of change may have been slow in an +extreme degree. The whole history of the world, as at present known, although +of a length quite incomprehensible by us, will hereafter be recognised as a +mere fragment of time, compared with the ages which have elapsed since the +first creature, the progenitor of innumerable extinct and living descendants, +was created. +</p> + +<p> +In the distant future I see open fields for far more important researches. +Psychology will be based on a new foundation, that of the necessary acquirement +of each mental power and capacity by gradation. Light will be thrown on the +origin of man and his history. +</p> + +<p> +Authors of the highest eminence seem to be fully satisfied with the view that +each species has been independently created. To my mind it accords better with +what we know of the laws impressed on matter by the Creator, that the +production and extinction of the past and present inhabitants of the world +should have been due to secondary causes, like those determining the birth and +death of the individual. When I view all beings not as special creations, but +as the lineal descendants of some few beings which lived long before the +<a name="Page489"></a> +first bed of the Silurian system was deposited, they seem to me to become +ennobled. Judging from the past, we may safely infer that not one living +species will transmit its unaltered likeness to a distant futurity. And of the +species now living very few will transmit progeny of any kind to a far distant +futurity; for the manner in which all organic beings are grouped, shows that +the greater number of species of each genus, and all the species of many +genera, have left no descendants, but have become utterly extinct. We can so +far take a prophetic glance into futurity as to foretel that it will be the +common and widely-spread species, belonging to the larger and dominant groups, +which will ultimately prevail and procreate new and dominant species. As all +the living forms of life are the lineal descendants of those which lived long +before the Silurian epoch, we may feel certain that the ordinary succession by +generation has never once been broken, and that no cataclysm has desolated the +whole world. Hence we may look with some confidence to a secure future of +equally inappreciable length. And as natural selection works solely by and for +the good of each being, all corporeal and mental endowments will tend to +progress towards perfection. +</p> + +<p> +It is interesting to contemplate an entangled bank, clothed with many plants of +many kinds, with birds singing on the bushes, with various insects flitting +about, and with worms crawling through the damp earth, and to reflect that +these elaborately constructed forms, so different from each other, and +dependent on each other in so complex a manner, have all been produced by laws +acting around us. These laws, taken in the largest sense, being Growth with +Reproduction; Inheritance which is almost implied by reproduction; Variability +from the indirect and direct action of the external conditions +<a name="Page490"></a> +of life, and from use and disuse; a Ratio of Increase so high as to lead to a +Struggle for Life, and as a consequence to Natural Selection, entailing +Divergence of Character and the Extinction of less-improved forms. Thus, from +the war of nature, from famine and death, the most exalted object which we are +capable of conceiving, namely, the production of the higher animals, directly +follows. There is grandeur in this view of life, with its several powers, +having been originally breathed into a few forms or into one; and that, whilst +this planet has gone cycling on according to the fixed law of gravity, from so +simple a beginning endless forms most beautiful and most wonderful have been, +and are being, evolved. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="Page491"></a><a name="chap15"></a>INDEX.</h2> + +<p class="noindent"> +Aberrant groups, <a href="#Page429">429</a>.<br /> +<br /> +Abyssinia, plants of, <a href="#Page375">375</a>.<br /> +<br /> +Acclimatisation, <a href="#Page139">139</a>.<br /> +<br /> +Affinities:<br /> +of extinct species, <a href="#Page329">329</a>.<br /> +of organic beings, <a href="#Page411">411</a>.<br /> +<br /> +Agassiz:<br /> +on Amblyopsis, <a href="#Page139">139</a>.<br /> +on groups of species suddenly appearing, <a href="#Page302">302</a>, <a href="#Page305">305</a>.<br /> +on embryological succession, <a href="#Page338">338</a>.<br /> +on the glacial period, <a href="#Page366">366</a>.<br /> +on embryological characters, <a href="#Page418">418</a>.<br /> +on the embryos of vertebrata, <a href="#Page439">439</a>.<br /> +on parallelism of embryological development and geological succession, <a href="#Page449">449</a>.<br /> +<br /> +Algæ of New Zealand, <a href="#Page376">376</a>.<br /> +<br /> +Alligators, males, fighting, <a href="#Page88">88.</a><br /> +<br /> +Amblyopsis, blind fish, <a href="#Page139">139</a>.<br /> +<br /> +America, North:<br /> +productions allied to those of Europe, <a href="#Page371">371</a>.<br /> +boulders and glaciers of, <a href="#Page373">373</a>.<br /> +South, no modern formations on west coast, <a href="#Page290">290</a>.<br /> +<br /> +Ammonites, sudden extinction of, <a href="#Page321">321</a>.<br /> +<br /> +Anagallis, sterility of, <a href="#Page247">247</a>.<br /> +<br /> +Analogy of variations, <a href="#Page159">159</a>.<br /> +<br /> +Ancylus, <a href="#Page386">386</a>.<br /> +<br /> +Animals:<br /> +not domesticated from being variable, <a href="#Page17">17</a>.<br /> +domestic, descended from several stocks, <a href="#Page19">19</a>.<br /> +acclimatisation of, <a href="#Page141">141</a>.<br /> +of Australia, <a href="#Page116">116</a>.<br /> +with thicker fur in cold climates, <a href="#Page133">133</a>.<br /> +blind, in caves, <a href="#Page137">137</a>.<br /> +extinct, of Australia, <a href="#Page339">339</a>.<br /> +<br /> +Anomma, <a href="#Page240">240</a>.<br /> +<br /> +Antarctic islands, ancient flora of, <a href="#Page399">399</a>.<br /> +<br /> +Antirrhinum, <a href="#Page161">161</a>.<br /> +<br /> +Ants:<br /> +attending aphides, <a href="#Page211">211</a>.<br /> +slave-making instinct, <a href="#Page219">219</a>.<br /> +<br /> +Ants, neuter, structure of, <a href="#Page236">236</a>.<br /> +<br /> +Aphides attended by ants, <a href="#Page211">211</a>.<br /> +<br /> +Aphis, development of, <a href="#Page442">442</a>.<br /> +<br /> +Apteryx, <a href="#Page182">182</a>.<br /> +<br /> +Arab horses, <a href="#Page35">35</a>.<br /> +<br /> +Aralo-Caspian Sea, <a href="#Page339">339</a>.<br /> +<br /> +Archiac, M. de, on the succession of species, <a href="#Page325">325</a>.<br /> +<br /> +Artichoke, Jerusalem, <a href="#Page142">142</a>.<br /> +<br /> +Ascension, plants of, <a href="#Page389">389</a>.<br /> +<br /> +Asclepias, pollen of, <a href="#Page193">193</a>.<br /> +<br /> +Asparagus, <a href="#Page359">359</a>.<br /> +<br /> +Aspicarpa, <a href="#Page417">417</a>.<br /> +<br /> +Asses, striped, <a href="#Page163">163</a>.<br /> +<br /> +Ateuchus, <a href="#Page135">135</a>.<br /> +<br /> +Audubon:<br /> +on habits of frigate-bird, <a href="#Page185">185</a>.<br /> +on variation in birds’-nests, <a href="#Page212">212</a>.<br /> +on heron eating seeds, <a href="#Page387">387</a>.<br /> +<br /> +Australia:<br /> +animals of, <a href="#Page116">116</a>.<br /> +dogs of, <a href="#Page215">215</a>.<br /> +extinct animals of, <a href="#Page339">339</a>.<br /> +European plants in, <a href="#Page375">375</a>.<br /> +<br /> +Azara on flies destroying cattle, <a href="#Page72">72</a>.<br /> +<br /> +Azores, flora of, <a href="#Page363">363</a>.<br /> +<br /> +Babington, Mr., on British plants, <a href="#Page48">48</a>.<br /> +<br /> +Balancement of growth, <a href="#Page147">147</a>.<br /> +<br /> +Bamboo with hooks, <a href="#Page197">197</a>.<br /> +<br /> +Barberry, flowers of, <a href="#Page98">98</a>.<br /> +<br /> +Barrande, M.:<br /> +on Silurian colonies, <a href="#Page313">313</a>.<br /> +on the succession of species, <a href="#Page325">325</a>.<br /> +on parallelism of palæozoic formations, <a href="#Page328">328</a>.<br /> +on affinities of ancient species, <a href="#Page330">330</a>.<br /> +<br /> +Barriers, importance of, <a href="#Page347">347</a>.<br /> +<br /> +Batrachians on islands, <a href="#Page393">393</a>.<br /> +<br /> +Bats:<br /> +how structure acquired, <a href="#Page180">180</a>.<br /> +distribution of, <a href="#Page394">394</a>.<br /> +<br /> +Bear, catching water-insects, <a href="#Page184">184</a>.<br /> +<br /> +Bee:<br /> +sting of, <a href="#Page202">202</a>.<br /> +queen, killing rivals, <a href="#Page202">202</a>.<br /> +<br /> +Bees fertilising flowers, <a href="#Page73">73</a>.<br /> +<br /> +Bees:<br /> +hive, not sucking the red clover, <a href="#Page95">95</a>.<br /> +cell-making instinct, <a href="#Page224">224</a>.<br /> +humble, cells of, <a href="#Page225">225</a>.<br /> +parasitic, <a href="#Page218">218</a>.<br /> +<br /> +Beetles:<br /> +wingless, in Madeira, <a href="#Page135">135</a>.<br /> +with deficient tarsi, <a href="#Page135">135</a>.<br /> +<br /> +Bentham, Mr.:<br /> +on British plants, <a href="#Page48">48</a>.<br /> +on classification, <a href="#Page419">419</a>.<br /> +<br /> +Berkeley, Mr., on seeds in salt-water, <a href="#Page358">358</a>.<br /> +<br /> +Bermuda, birds of, <a href="#Page391">391</a>.<br /> +<br /> +Birds:<br /> +acquiring fear, <a href="#Page212">212</a>.<br /> +annually cross the Atlantic, <a href="#Page364">364</a>.<br /> +colour of, on continents, <a href="#Page132">132</a>.<br /> +fossil, in caves of Brazil, <a href="#Page339">339</a>.<br /> +of Madeira, Bermuda, and Galapagos, <a href="#Page390">390</a>.<br /> +song of males, <a href="#Page89">89</a>.<br /> +transporting seeds, <a href="#Page361">361</a>.<br /> +waders, <a href="#Page386">386</a>.<br /> +wingless, <a href="#Page134">134</a>, <a href="#Page182">182</a>.<br /> +with traces of embryonic teeth, <a href="#Page451">451</a>.<br /> +<br /> +Bizcacha, <a href="#Page349">349</a>.<br /> +affinities of, <a href="#Page429">429</a>.<br /> +<br /> +Bladder for swimming in fish, <a href="#Page190">190</a>.<br /> +<br /> +Blindness of cave animals, <a href="#Page137">137</a>,<br /> +<br /> +Blyth, Mr.:<br /> +on distinctness of Indian cattle, <a href="#Page18">18</a>.<br /> +on striped Hemionus, <a href="#Page163">163</a>.<br /> +on crossed geese, <a href="#Page253">253</a>.<br /> +<br /> +Boar, shoulder-pad of, <a href="#Page88">88</a>.<br /> +<br /> +Borrow, Mr., on the Spanish pointer, <a href="#Page35">35</a>.<br /> +<br /> +Bory St. Vincent on Batrachians, <a href="#Page393">393</a>.<br /> +<br /> +Bosquet, M., on fossil Chthamalus, <a href="#Page304">304</a>.<br /> +<br /> +Boulders, erratic, on the Azores, <a href="#Page363">363</a>.<br /> +<br /> +Branchiæ, <a href="#Page190">190</a>.<br /> +<br /> +Brent, Mr.:<br /> +on house-tumblers, <a href="#Page214">214</a>.<br /> +on hawks killing pigeons, <a href="#Page362">362</a>.<br /> +<br /> +Brewer, Dr., on American cuckoo, <a href="#Page217">217</a>.<br /> +<br /> +Britain, mammals of, <a href="#Page395">395</a>.<br /> +<br /> +Bronn on duration of specific forms, <a href="#Page293">293</a>.<br /> +<br /> +Brown, Robert, on classification, <a href="#Page414">414</a>.<br /> +<br /> +Buckman on variation in plants, <a href="#Page10">10</a>.<br /> +<br /> +Buzareingues on sterility of varieties, <a href="#Page270">270</a>.<br /> +<br /> +Cabbage, varieties of, crossed, <a href="#Page99">99</a>.<br /> +<br /> +Calceolaria, <a href="#Page251">251</a>.<br /> +<br /> +Canary-birds, sterility of hybrids, <a href="#Page252">252</a>.<br /> +<br /> +Cape de Verde islands, <a href="#Page398">398</a>.<br /> +<br /> +Cape of Good Hope, plants of, <a href="#Page110">110</a>, <a href="#Page375">375</a>.<br /> +<br /> +Carrier-pigeons killed by hawks, <a href="#Page362">362</a>.<br /> +<br /> +Cassini on flowers of compositæ, <a href="#Page145">145</a>.<br /> +<br /> +Catasetum, <a href="#Page424">424</a>.<br /> +<br /> +Cats:<br /> +with blue eyes, deaf, <a href="#Page12">12</a>.<br /> +variation in habits of, <a href="#Page91">91</a>.<br /> +curling tail when going to spring, <a href="#Page201">201</a>.<br /> +<br /> +Cattle:<br /> +destroying fir-trees, <a href="#Page71">71</a>.<br /> +destroyed by flies in La Plata, <a href="#Page72">72</a>.<br /> +breeds of, locally extinct, <a href="#Page111">111</a>.<br /> +fertility of Indian and European breeds, <a href="#Page254">254</a>.<br /> +<br /> +Cave, inhabitants of, blind, <a href="#Page137">137</a>.<br /> +<br /> +Centres of creation, <a href="#Page352">352</a>.<br /> +<br /> +Cephalopodæ, development of, <a href="#Page442">442</a>.<br /> +<br /> +Cervulus, <a href="#Page253">253</a>.<br /> +<br /> +Cetacea, teeth and hair, <a href="#Page144">144</a>.<br /> +<br /> +Ceylon, plants of, <a href="#Page375">375</a>.<br /> +<br /> +Chalk formation, <a href="#Page322">322</a>.<br /> +<br /> +Characters:<br /> +divergence of, <a href="#Page111">111</a>.<br /> +sexual, variable, <a href="#Page156">156</a>.<br /> +adaptive or analogical, <a href="#Page427">427</a>.<br /> +<br /> +Charlock, <a href="#Page76">76</a>.<br /> +<br /> +Checks:<br /> +to increase, <a href="#Page67">67</a>.<br /> +mutual, <a href="#Page71">71</a>.<br /> +<br /> +Chickens, instinctive tameness of, <a href="#Page216">216</a>.<br /> +<br /> +Chthamalinæ, <a href="#Page288">288</a>.<br /> +<br /> +Chthamalus, cretacean species of, <a href="#Page304">304</a>.<br /> +<br /> +Circumstances favourable:<br /> +to selection of domestic products, <a href="#Page40">40</a>.<br /> +to natural selection, <a href="#Page101">101</a>.<br /> +<br /> +Cirripedes:<br /> +capable of crossing, <a href="#Page101">101</a>.<br /> +carapace aborted, <a href="#Page148">148</a>.<br /> +their ovigerous frena, <a href="#Page192">192</a>.<br /> +fossil, <a href="#Page304">304</a>.<br /> +larvæ of, <a href="#Page440">440</a>.<br /> +<br /> +Classification, <a href="#Page413">413</a>.<br /> +<br /> +Clift, Mr., on the succession of types, <a href="#Page339">339</a>.<br /> +<br /> +Climate:<br /> +effects of, in checking increase of beings, <a href="#Page68">68</a>.<br /> +adaptation of, to organisms, <a href="#Page139">139</a>.<br /> +<br /> +Cobites, intestine of, <a href="#Page190">190</a>.<br /> +<br /> +Cockroach, <a href="#Page76">76</a>.<br /> +<br /> +Collections, palæontological, poor, <a href="#Page287">287</a>.<br /> +<br /> +Colour:<br /> +influenced by climate, <a href="#Page132">132</a>.<br /> +in relation to attacks by flies, <a href="#Page198">198</a>.<br /> +<br /> +Columba livia, parent of domestic pigeons, <a href="#Page23">23</a>.<br /> +<br /> +Colymbetes, <a href="#Page386">386</a>.<br /> +<br /> +Compensation of growth, <a href="#Page147">147</a>.<br /> +<br /> +Compositæ:<br /> +outer and inner florets of, <a href="#Page144">144</a>.<br /> +male flowers of, <a href="#Page451">451</a>.<br /> +<br /> +Conclusion, general, <a href="#Page480">480</a>.<br /> +<br /> +Conditions, slight changes in, favourable to fertility, <a href="#Page267">267</a>.<br /> +<br /> +Coot, <a href="#Page185">185</a>.<br /> +<br /> +Coral:<br /> +islands, seeds drifted to, <a href="#Page360">360</a>.<br /> +reefs, indicating movements of earth, <a href="#Page309">309</a>.<br /> +<br /> +Corn-crake, <a href="#Page185">185</a>.<br /> +<br /> +Correlation:<br /> +of growth in domestic productions, <a href="#Page11">11</a>.<br /> +of growth, <a href="#Page143">143</a>, <a href="#Page198">198</a>.<br /> +<br /> +Cowslip, <a href="#Page49">49</a>.<br /> +<br /> +Creation, single centres of, <a href="#Page352">352</a>.<br /> +<br /> +Crinum, <a href="#Page250">250</a>.<br /> +<br /> +Crosses, reciprocal, <a href="#Page258">258</a>.<br /> +<br /> +Crossing:<br /> +of domestic animals, importance in altering breeds, <a href="#Page20">20</a>.<br /> +advantages of, <a href="#Page96">96</a>.<br /> +unfavourable to selection, <a href="#Page102">102</a>.<br /> +<br /> +Crustacea of New Zealand, <a href="#Page376">376</a>.<br /> +<br /> +Crustacean, blind, <a href="#Page137">137</a>.<br /> +<br /> +Cryptocerus, <a href="#Page238">238</a>.<br /> +<br /> +Ctenomys, blind, <a href="#Page137">137</a>.<br /> +<br /> +Cuckoo, instinct of, <a href="#Page216">216</a>.<br /> +<br /> +Currants, grafts of, <a href="#Page262">262</a>.<br /> +<br /> +Currents of sea, rate of, <a href="#Page359">359</a>.<br /> +<br /> +Cuvier:<br /> +on conditions of existence, <a href="#Page206">206</a>.<br /> +on fossil monkeys, <a href="#Page303">303</a>.<br /> +<br /> +Cuvier, Fred., on instinct, <a href="#Page208">208</a>.<br /> +<br /> +Dana, Professor:<br /> +on blind cave-animals, <a href="#Page139">139</a>.<br /> +on relations of crustaceans of Japan, <a href="#Page372">372</a>.<br /> +on crustaceans of New Zealand, <a href="#Page376">376</a>.<br /> +<br /> +De Candolle:<br /> +on struggle for existence, <a href="#Page62">62</a>.<br /> +on umbelliferæ, <a href="#Page146">146</a>.<br /> +on general affinities, <a href="#Page430">430</a>.<br /> +<br /> +De Candolle, Alph.:<br /> +on low plants, widely dispersed, <a href="#Page406">406</a>.<br /> +on widely-ranging plants being variable, <a href="#Page53">53</a>.<br /> +on naturalisation, <a href="#Page115">115</a>.<br /> +on winged seeds, <a href="#Page146">146</a>.<br /> +on Alpine species suddenly becoming rare, <a href="#Page175">175</a>.<br /> +on distribution of plants with large seeds, <a href="#Page360">360</a>.<br /> +on vegetation of Australia, <a href="#Page379">379</a>.<br /> +on fresh-water plants, <a href="#Page386">386</a>.<br /> +on insular plants, <a href="#Page389">389</a>.<br /> +<br /> +Degradation of coast-rocks, <a href="#Page282">282</a>.<br /> +<br /> +Denudation:<br /> +rate of, <a href="#Page285">285</a>.<br /> +of oldest rocks, <a href="#Page308">308</a>.<br /> +<br /> +Development of ancient forms, <a href="#Page336">336</a>.<br /> +<br /> +Devonian system, <a href="#Page334">334</a>.<br /> +<br /> +Dianthus, fertility of crosses, <a href="#Page256">256</a>.<br /> +<br /> +Dirt on feet of birds, <a href="#Page362">362</a>.<br /> +<br /> +Dispersal:<br /> +means of, <a href="#Page356">356</a>.<br /> +during glacial period, <a href="#Page365">365</a>.<br /> +<br /> +Distribution:<br /> +geographical, <a href="#Page346">346</a>.<br /> +means of, <a href="#Page356">356</a>.<br /> +<br /> +Disuse, effects of, under nature, <a href="#Page134">134</a>.<br /> +<br /> +Divergence of character, <a href="#Page111">111</a>.<br /> +<br /> +Division, physiological, of labour, <a href="#Page115">115</a>.<br /> +<br /> +Dogs:<br /> +hairless, with imperfect teeth, <a href="#Page12">12</a>.<br /> +descended from several wild stocks, <a href="#Page18">18</a>.<br /> +domestic instincts of, <a href="#Page213">213</a>.<br /> +inherited civilisation of, <a href="#Page215">215</a>.<br /> +fertility of breeds together, <a href="#Page254">254</a>.<br /> +of crosses, <a href="#Page268">268</a>.<br /> +proportions of, when young, <a href="#Page444">444</a>.<br /> +<br /> +Domestication, variation under, <a href="#Page7">7</a>.<br /> +<br /> +Downing, Mr., on fruit-trees in America, <a href="#Page85">85</a>.<br /> +<br /> +Downs, North and South, <a href="#Page285">285</a>.<br /> +<br /> +Dragon-flies, intestines of, <a href="#Page190">190</a>.<br /> +<br /> +Drift-timber, <a href="#Page360">360</a>.<br /> +<br /> +Driver-ant, <a href="#Page240">240</a>.<br /> +<br /> +Drones killed by other bees, <a href="#Page202">202</a>.<br /> +<br /> +Duck:<br /> +domestic, wings of, reduced, <a href="#Page11">11</a>.<br /> +logger-headed, <a href="#Page182">182</a>.<br /> +<br /> +Duckweed, <a href="#Page385">385</a>.<br /> +<br /> +Dugong, affinities of, <a href="#Page414">414</a>.<br /> +<br /> +Dung-beetles with deficient tarsi, <a href="#Page135">135</a>.<br /> +<br /> +Dyticus, <a href="#Page386">386</a>.<br /> +<br /> +Earl, Mr. W., on the Malay Archipelago, <a href="#Page395">395</a>.<br /> +<br /> +Ears:<br /> +drooping, in domestic animals, <a href="#Page11">11</a>.<br /> +rudimentary, <a href="#Page454">454</a>.<br /> +<br /> +Earth, seeds in roots of trees, <a href="#Page361">361</a>.<br /> +<br /> +Eciton, <a href="#Page238">238</a>.<br /> +<br /> +Economy of organisation, <a href="#Page147">147</a>.<br /> +<br /> +Edentata:<br /> +teeth and hair, <a href="#Page144">144</a>.<br /> +fossil species of, <a href="#Page339">339</a>.<br /> +<br /> +Edwards, Milne:<br /> +on physiological divisions of labour, <a href="#Page115">115</a>.<br /> +on gradations of structure, <a href="#Page194">194</a>.<br /> +on embryological characters, <a href="#Page418">418</a>.<br /> +<br /> +Eggs, young birds escaping from, <a href="#Page87">87</a>.<br /> +<br /> +Electric organs, <a href="#Page192">192</a>.<br /> +<br /> +Elephant:<br /> +rate of increase, <a href="#Page64">64</a>.<br /> +of glacial period, <a href="#Page141">141</a>.<br /> +<br /> +Embryology, <a href="#Page439">439</a>.<br /> +<br /> +Existence:<br /> +struggle for, <a href="#Page60">60</a>.<br /> +conditions of, <a href="#Page206">206</a>.<br /> +<br /> +Extinction:<br /> +as bearing on natural selection, <a href="#Page109">109</a>.<br /> +of domestic varieties, <a href="#Page111">111</a>.<br /> +<a href="#Page317">317</a>.<br /> +<br /> +Eye:<br /> +structure of, <a href="#Page187">187</a>.<br /> +correction for aberration, <a href="#Page202">202</a>.<br /> +<br /> +Eyes reduced in moles, <a href="#Page137">137</a>.<br /> +<br /> +Fabre, M., on parasitic sphex, <a href="#Page218">218</a>.<br /> +<br /> +Falconer, Dr.:<br /> +on naturalization of plants in India, <a href="#Page65">65</a>.<br /> +on fossil crocodile, <a href="#Page313">313</a>.<br /> +on elephants and mastodons, <a href="#Page334">334</a>.<br /> +and Cautley on mammals of sub-Himalayan beds, <a href="#Page340">340</a>.<br /> +<br /> +Falkland Island, wolf of, <a href="#Page393">393</a>.<br /> +<br /> +Faults, <a href="#Page285">285</a>.<br /> +<br /> +Faunas, marine, <a href="#Page348">348</a>.<br /> +<br /> +Fear, instinctive, in birds, <a href="#Page212">212</a>.<br /> +<br /> +Feet of birds, young molluscs adhering to, <a href="#Page385">385</a>.<br /> +<br /> +Fertility:<br /> +of hybrids, <a href="#Page249">249</a>.<br /> +from slight changes in conditions, <a href="#Page267">267</a>.<br /> +of crossed varieties, <a href="#Page267">267</a>.<br /> +<br /> +Fir-trees:<br /> +destroyed by cattle, <a href="#Page71">71</a>.<br /> +pollen of, <a href="#Page203">203</a>.<br /> +<br /> +Fish:<br /> +flying, <a href="#Page182">182</a>.<br /> +teleostean, sudden appearance of, <a href="#Page305">305</a>.<br /> +eating seeds, <a href="#Page362">362</a>, <a href="#Page387">387</a>.<br /> +fresh-water, distribution of, <a href="#Page384">384</a>.<br /> +<br /> +Fishes:<br /> +ganoid, now confined to fresh water, <a href="#Page107">107</a>.<br /> +electric organs of, <a href="#Page192">192</a>.<br /> +ganoid, living in fresh water, <a href="#Page321">321</a>.<br /> +of southern hemisphere, <a href="#Page376">376</a>.<br /> +<br /> +Flight, powers of, how acquired, <a href="#Page182">182</a>.<br /> +<br /> +Flowers:<br /> +structure of, in relation to crossing, <a href="#Page97">97</a>.<br /> +of compositæ and umbelliferæ, <a href="#Page144">144</a>.<br /> +<br /> +Forbes, E.:<br /> +on colours of shells, <a href="#Page132">132</a>.<br /> +on abrupt range of shells in depth, <a href="#Page175">175</a>.<br /> +on poorness of palæontological collections, <a href="#Page287">287</a>.<br /> +on continuous succession of genera, <a href="#Page316">316</a>.<br /> +on continental extensions, <a href="#Page357">357</a>.<br /> +on distribution during glacial period, <a href="#Page366">366</a><br /> +on parallelism in time and space, <a href="#Page409">409</a>.<br /> +<br /> +Forests, changes in, in America, <a href="#Page74">74</a>.<br /> +<br /> +Formation, Devonian, <a href="#Page334">334</a>.<br /> +<br /> +Formations:<br /> +thickness of, in Britain, <a href="#Page284">284</a>.<br /> +intermittent, <a href="#Page290">290</a>.<br /> +<br /> +Formica rufescens, <a href="#Page219">219</a>.<br /> +<br /> +Formica sanguinea, <a href="#Page219">219</a>.<br /> +<br /> +Formica flava, neuter of, <a href="#Page239">239</a>.<br /> +<br /> +Frena, ovigerous, of cirripedes, <a href="#Page192">192</a>.<br /> +<br /> +Fresh-water productions, dispersal of, <a href="#Page383">383</a>.<br /> +<br /> +Fries on species in large genera being closely allied to other species, <a href="#Page57">57</a>.<br /> +<br /> +Frigate-bird, <a href="#Page185">185</a>.<br /> +<br /> +Frogs on islands, <a href="#Page393">393</a>.<br /> +<br /> +Fruit-trees:<br /> +gradual improvement of, <a href="#Page37">37</a>.<br /> +in United States, <a href="#Page85">85</a>.<br /> +varieties of, acclimatised in United States, <a href="#Page142">142</a>.<br /> +<br /> +Fuci, crossed, <a href="#Page258">258</a>.<br /> +<br /> +Fur, thicker in cold climates, <a href="#Page133">133</a>.<br /> +<br /> +Furze, <a href="#Page439">439</a>.<br /> +<br /> +Galapagos Archipelago:<br /> +birds of, <a href="#Page390">390</a>.<br /> +productions of, <a href="#Page398">398</a>, <a href="#Page400">400</a>.<br /> +<br /> +Galeopithecus, <a href="#Page181">181</a>.<br /> +<br /> +Game, increase of, checked by vermin, <a href="#Page68">68</a>.<br /> +<br /> +Gärtner:<br /> +on sterility of hybrids, <a href="#Page247">247</a>, <a href="#Page255">255</a>.<br /> +on reciprocal crosses, <a href="#Page258">258</a>.<br /> +on crossed maize and verbascum, <a href="#Page270">270</a>.<br /> +on comparison of hybrids and mongrels, <a href="#Page272">272</a>.<br /> +<br /> +Geese:<br /> +fertility when crossed, <a href="#Page253">253</a>.<br /> +upland, <a href="#Page185">185</a>.<br /> +<br /> +Genealogy important in classification, <a href="#Page425">425</a>.<br /> +<br /> +Geoffrey St. Hilaire:<br /> +on balancement, <a href="#Page147">147</a>.<br /> +on homologous organs, <a href="#Page434">434</a>.<br /> +<br /> +Geoffrey St. Hilaire, Isidore:<br /> +on variability of repeated parts, <a href="#Page149">149</a>.<br /> +on correlation in monstrosities, <a href="#Page11">11</a>.<br /> +on correlation, <a href="#Page144">144</a>.<br /> +on variable parts being often monstrous, <a href="#Page155">155</a>.<br /> +<br /> +Geographical distribution, <a href="#Page346">346</a>.<br /> +<br /> +Geography, ancient, <a href="#Page487">487</a>.<br /> +<br /> +Geology:<br /> +future progress of, <a href="#Page487">487</a>.<br /> +imperfection of the record, <a href="#Page279">279</a>.<br /> +<br /> +Giraffe, tail of, <a href="#Page195">195</a>.<br /> +<br /> +Glacial period, <a href="#Page365">365</a>.<br /> +<br /> +Gmelin on distribution, <a href="#Page365">365</a>.<br /> +<br /> +Gnathodon, fossil, <a href="#Page368">368</a>.<br /> +<br /> +Godwin-Austen, Mr., on the Malay Archipelago, <a href="#Page299">299</a>.<br /> +<br /> +Goethe on compensation of growth, <a href="#Page147">147</a>.<br /> +<br /> +Gooseberry, grafts of, <a href="#Page262">262</a>.<br /> +<br /> +Gould, Dr. A., on land-shells, <a href="#Page397">397</a>.<br /> +<br /> +Gould, Mr.:<br /> +on colours of birds, <a href="#Page132">132</a>.<br /> +on birds of the Galapagos, <a href="#Page398">398</a>.<br /> +on distribution of genera of birds, <a href="#Page404">404</a>.<br /> +<br /> +Gourds, crossed, <a href="#Page270">270</a>.<br /> +<br /> +Grafts, capacity of, <a href="#Page261">261</a>.<br /> +<br /> +Grasses, varieties of, <a href="#Page113">113</a>.<br /> +<br /> +Gray, Dr. Asa:<br /> +on trees of United States, <a href="#Page100">100</a>.<br /> +on naturalised plants in the United States, <a href="#Page115">115</a>.<br /> +on rarity of intermediate varieties, <a href="#Page176">176</a>.<br /> +on Alpine plants, <a href="#Page365">365</a>.<br /> +<br /> +Gray, Dr. J. E., on striped mule, <a href="#Page165">165</a>.<br /> +<br /> +Grebe, <a href="#Page185">185</a>.<br /> +<br /> +Groups, aberrant, <a href="#Page429">429</a>.<br /> +<br /> +Grouse:<br /> +colours of, <a href="#Page84">84</a>.<br /> +red, a doubtful species, <a href="#Page49">49</a>.<br /> +<br /> +Growth:<br /> +compensation of, <a href="#Page147">147</a>.<br /> +correlation of, in domestic products, <a href="#Page11">11</a>.<br /> +correlation of, <a href="#Page143">143</a>.<br /> +<br /> +Habit:<br /> +effect of, under domestication, <a href="#Page11">11</a>.<br /> +effect of, under nature, <a href="#Page134">134</a>.<br /> +diversified, of same species, <a href="#Page183">183</a>.<br /> +<br /> +Hair and teeth, correlated, <a href="#Page144">144</a>.<br /> +<br /> +Harcourt, Mr. E. V., on the birds of Madeira, <a href="#Page391">391</a>.<br /> +<br /> +Hartung, M., on boulders in the Azores, <a href="#Page363">363</a>.<br /> +<br /> +Hazel-nuts, <a href="#Page359">359</a>.<br /> +<br /> +Hearne on habits of bears, <a href="#Page184">184</a>.<br /> +<br /> +Heath, changes in vegetation, <a href="#Page72">72</a>,<br /> +<br /> +Heer, O., on plants of Madeira, <a href="#Page107">107</a>.<br /> +<br /> +Helix pomatia, <a href="#Page397">397</a>.<br /> +<br /> +Helosciadium, <a href="#Page359">359</a>.<br /> +<br /> +Hemionus, striped, <a href="#Page163">163</a>.<br /> +<br /> +Herbert, W.:<br /> +on struggle for existence, <a href="#Page62">62</a>.<br /> +on sterility of hybrids, <a href="#Page249">249</a>.<br /> +<br /> +Hermaphrodites crossing, <a href="#Page96">96</a>.<br /> +<br /> +Heron eating seed, <a href="#Page387">387</a>.<br /> +<br /> +Heron, Sir R., on peacocks, <a href="#Page89">89</a>.<br /> +<br /> +Heusinger on white animals not poisoned by certain plants, <a href="#Page12">12</a>.<br /> +<br /> +Hewitt, Mr., on sterility of first crosses, <a href="#Page264">264</a>.<br /> +<br /> +Himalaya:<br /> +glaciers of, <a href="#Page373">373</a>.<br /> +plants of, <a href="#Page375">375</a>.<br /> +<br /> +Hippeastrum, <a href="#Page250">250</a>.<br /> +<br /> +Holly-trees, sexes of, <a href="#Page93">93</a>.<br /> +<br /> +Hollyhock, varieties of, crossed, <a href="#Page271">271</a>.<br /> +<br /> +Hooker, Dr., on trees of New Zealand, <a href="#Page100">100</a>.<br /> +<br /> +Hooker, Dr.:<br /> +on acclimatisation of Himalayan trees, <a href="#Page140">140</a>.<br /> +on flowers of umbelliferæ, <a href="#Page145">145</a>.<br /> +on glaciers of Himalaya, <a href="#Page373">373</a>.<br /> +on algæ of New Zealand, <a href="#Page376">376</a>.<br /> +on vegetation at the base of the Himalaya, <a href="#Page378">378</a>.<br /> +on plants of Tierra del Fuego, <a href="#Page374">374</a>, <a href="#Page378">378</a>.<br /> +on Australian plants, <a href="#Page375">375</a>, <a href="#Page399">399</a>.<br /> +on relations of flora of South America, <a href="#Page379">379</a>.<br /> +on flora of the Antarctic lands, <a href="#Page381">381</a>, <a href="#Page399">399</a>.<br /> +on the plants of the Galapagos, <a href="#Page391">391</a>, <a href="#Page398">398</a>.<br /> +<br /> +Hooks:<br /> +on bamboos, <a href="#Page197">197</a>.<br /> +to seeds on islands, <a href="#Page392">392</a>.<br /> +<br /> +Horner, Mr., on the antiquity of Egyptians, <a href="#Page18">18</a>.<br /> +<br /> +Horns, rudimentary, <a href="#Page454">454</a>.<br /> +<br /> +Horse, fossil, in La Plata, <a href="#Page318">318</a>.<br /> +<br /> +Horses:<br /> +destroyed by flies in La Plata, <a href="#Page72">72</a>.<br /> +striped, <a href="#Page163">163</a>.<br /> +proportions of, when young, <a href="#Page445">445</a>.<br /> +<br /> +Horticulturists, selection applied by, <a href="#Page32">32</a>.<br /> +<br /> +Huber on cells of bees, <a href="#Page230">230</a>.<br /> +<br /> +Huber, P.:<br /> +on reason blended with instinct, <a href="#Page208">208</a>.<br /> +on habitual nature of instincts, <a href="#Page208">208</a>.<br /> +on slave making ants, <a href="#Page219">219</a>.<br /> +on Melipona domestica, <a href="#Page225">225</a>.<br /> +<br /> +Humble-bees, cells of, <a href="#Page225">225</a>.<br /> +<br /> +Hunter, J., on secondary sexual characters, <a href="#Page150">150</a>.<br /> +<br /> +Hutton, Captain, on crossed geese, <a href="#Page253">253</a>.<br /> +<br /> +Huxley, Professor:<br /> +on structure of hermaphrodites, <a href="#Page101">101</a>.<br /> +on embryological succession, <a href="#Page338">338</a>.<br /> +on homologous organs, <a href="#Page438">438</a>.<br /> +on the development of aphis, <a href="#Page442">442</a>.<br /> +<br /> +Hybrids and mongrels compared, <a href="#Page272">272</a>.<br /> +<br /> +Hybridism, <a href="#Page245">245</a>.<br /> +<br /> +Hydra, structure of, <a href="#Page190">190</a>.<br /> +<br /> +Ibla, <a href="#Page148">148</a>.<br /> +<br /> +Icebergs transporting seeds, <a href="#Page363">363</a>.<br /> +<br /> +Increase, rate of, <a href="#Page63">63</a>.<br /> +<br /> +Individuals:<br /> +numbers favourable to selection, <a href="#Page102">102</a>.<br /> +many, whether simultaneously created, <a href="#Page356">356</a>.<br /> +<br /> +Inheritance:<br /> +laws of, <a href="#Page12">12</a>.<br /> +at corresponding ages, <a href="#Page14">14</a>, <a href="#Page86">86</a>.<br /> +<br /> +Insects:<br /> +colour of, fitted for habitations, <a href="#Page84">84</a>.<br /> +sea-side, colours of, <a href="#Page132">132</a>.<br /> +blind, in caves, <a href="#Page138">138</a>.<br /> +luminous, <a href="#Page193">193</a>.<br /> +neuter, <a href="#Page236">236</a>.<br /> +<br /> +Instinct, <a href="#Page207">207</a>.<br /> +<br /> +Instincts, domestic, <a href="#Page213">213</a>.<br /> +<br /> +Intercrossing, advantages of, <a href="#Page96">96</a>.<br /> +<br /> +Islands, oceanic, <a href="#Page388">388</a>.<br /> +<br /> +Isolation favourable to selection, <a href="#Page104">104</a>.<br /> +<br /> +Japan, productions of, <a href="#Page372">372</a>.<br /> +<br /> +Java, plants of, <a href="#Page375">375</a>.<br /> +<br /> +Jones, Mr. J. M., on the birds of Bermuda, <a href="#Page391">391</a>.<br /> +<br /> +Jussieu on classification, <a href="#Page417">417</a>.<br /> +<br /> +Kentucky, caves of, <a href="#Page137">137</a>.<br /> +<br /> +Kerguelen-land, flora of, <a href="#Page381">381</a>, <a href="#Page399">399</a>.<br /> +<br /> +Kidney-bean, acclimatisation of, <a href="#Page142">142</a>.<br /> +<br /> +Kidneys of birds, <a href="#Page144">144</a>.<br /> +<br /> +Kirby on tarsi deficient in beetles, <a href="#Page135">135</a>.<br /> +<br /> +Knight, Andrew, on cause of variation, <a href="#Page7">7</a>.<br /> +<br /> +Kölreuter:<br /> +on the barberry, <a href="#Page98">98</a>.<br /> +on sterility of hybrids, <a href="#Page247">247</a>.<br /> +on reciprocal crosses, <a href="#Page258">258</a>.<br /> +on crossed varieties of nicotiana, <a href="#Page271">271</a>.<br /> +on crossing male and hermaphrodite flowers, <a href="#Page451">451</a>.<br /> +<br /> +Lamarck on adaptive characters, <a href="#Page427">427</a>.<br /> +<br /> +Land-shells:<br /> +distribution of, <a href="#Page397">397</a>.<br /> +of Madeira, naturalised, <a href="#Page402">402</a>.<br /> +<br /> +Languages, classification of, <a href="#Page422">422</a>.<br /> +<br /> +Lapse, great, of time, <a href="#Page282">282</a>.<br /> +<br /> +Larvæ, <a href="#Page440">440</a>.<br /> +<br /> +Laurel, nectar secreted by the leaves, <a href="#Page92">92</a>.<br /> +<br /> +Laws of variation, <a href="#Page131">131</a>.<br /> +<br /> +Leech, varieties of, <a href="#Page76">76</a>.<br /> +<br /> +Leguminosæ, nectar secreted by glands, <a href="#Page92">92</a>.<br /> +<br /> +Lepidosiren, <a href="#Page107">107</a>, <a href="#Page330">330</a>.<br /> +<br /> +Life, struggle for, <a href="#Page60">60</a>.<br /> +<br /> +Lingula, Silurian, <a href="#Page306">306</a>.<br /> +<br /> +Linnæus, aphorism of, <a href="#Page413">413</a>.<br /> +<br /> +Lion:<br /> +mane of, <a href="#Page88">88</a>.<br /> +young of, striped, <a href="#Page439">439</a>.<br /> +<br /> +Lobelia fulgens, <a href="#Page73">73</a>, <a href="#Page98">98</a>.<br /> +<br /> +Lobelia, sterility of crosses, <a href="#Page250">250</a>.<br /> +<br /> +Loess of the Rhine, <a href="#Page384">384</a>.<br /> +<br /> +Lowness of structure connected with variability, <a href="#Page149">149</a>.<br /> +<br /> +Lowness, related to wide distribution, <a href="#Page406">406</a>.<br /> +<br /> +Lubbock, Mr., on the nerves of coccus, <a href="#Page46">46</a>.<br /> +<br /> +Lucas, Dr. P.:<br /> +on inheritance, <a href="#Page12">12</a>.<br /> +on resemblance of child to parent, <a href="#Page275">275</a>.<br /> +<br /> +Lund and Clausen on fossils of Brazil, <a href="#Page339">339</a>.<br /> +<br /> +Lyell, Sir C.:<br /> +on the struggle for existence, <a href="#Page62">62</a>.<br /> +on modern changes of the earth, <a href="#Page95">95</a>.<br /> +on measure of denudation, <a href="#Page283">283</a>.<br /> +on a carboniferous land-shell, <a href="#Page289">289</a>.<br /> +on fossil whales, <a href="#Page303">303</a>.<br /> +on strata beneath Silurian system, <a href="#Page307">307</a>.<br /> +on the imperfection of the geological record, <a href="#Page310">310</a>.<br /> +on the appearance of species, <a href="#Page312">312</a>.<br /> +on Barrande’s colonies, <a href="#Page313">313</a>.<br /> +on tertiary formations of Europe and North America, <a href="#Page323">323</a>.<br /> +on parallelism of tertiary formations, <a href="#Page328">328</a>.<br /> +on transport of seeds by icebergs, <a href="#Page363">363</a>.<br /> +on great alternations of climate, <a href="#Page382">382</a>.<br /> +on the distribution of fresh-water shells, <a href="#Page385">385</a>.<br /> +on land-shells of Madeira, <a href="#Page402">402</a>.<br /> +<br /> +Lyell and Dawson on fossilized trees in Nova Scotia, <a href="#Page296">296</a>.<br /> +<br /> +Macleay on analogical characters, <a href="#Page427">427</a>.<br /> +<br /> +Madeira:<br /> +plants of, <a href="#Page107">107</a>.<br /> +beetles of, wingless, <a href="#Page135">135</a>.<br /> +fossil land-shells of, <a href="#Page339">339</a>.<br /> +birds of, <a href="#Page390">390</a>.<br /> +<br /> +Magpie tame in Norway, <a href="#Page212">212</a>.<br /> +<br /> +Maize, crossed, <a href="#Page270">270</a>.<br /> +<br /> +Malay Archipelago:<br /> +compared with Europe, <a href="#Page299">299</a>.<br /> +mammals of, <a href="#Page395">395</a>.<br /> +<br /> +Malpighiaceæ, <a href="#Page417">417</a>.<br /> +<br /> +Mammæ, rudimentary, <a href="#Page451">451</a>.<br /> +<br /> +Mammals:<br /> +fossil, in secondary formation, <a href="#Page303">303</a>.<br /> +insular, <a href="#Page393">393</a>.<br /> +<br /> +Man, origin of races of, <a href="#Page199">199</a>.<br /> +<br /> +Manatee, rudimentary nails of, <a href="#Page454">454</a>.<br /> +<br /> +Marsupials:<br /> +of Australia, <a href="#Page116">116</a>.<br /> +fossil species of, <a href="#Page339">339</a>.<br /> +<br /> +Martens, M., experiment on seeds, <a href="#Page360">360</a>.<br /> +<br /> +Martin, Mr. W. C., on striped mules, <a href="#Page165">165</a>.<br /> +<br /> +Matteuchi on the electric organs of rays, <a href="#Page193">193</a>.<br /> +<br /> +Matthiola, reciprocal crosses of, <a href="#Page258">258</a>.<br /> +<br /> +Means of dispersal, <a href="#Page356">356</a>.<br /> +<br /> +Melipona domestica, <a href="#Page225">225</a>.<br /> +<br /> +Metamorphism of oldest rocks, <a href="#Page308">308</a>.<br /> +<br /> +Mice:<br /> +destroying bees, <a href="#Page74">74</a>.<br /> +acclimatisation of, <a href="#Page141">141</a>.<br /> +<br /> +Migration, bears on first appearance of fossils, <a href="#Page296">296</a>.<br /> +<br /> +Miller, Professor, on the cells of bees, <a href="#Page226">226</a>.<br /> +<br /> +Mirabilis, crosses of, <a href="#Page258">258</a>.<br /> +<br /> +Missel-thrush, <a href="#Page76">76</a>.<br /> +<br /> +Misseltoe, complex relations of, <a href="#Page3">3</a>.<br /> +<br /> +Mississippi, rate of deposition at mouth, <a href="#Page284">284</a>.<br /> +<br /> +Mocking-thrush of the Galapagos, <a href="#Page402">402</a>.<br /> +<br /> +Modification of species, how far applicable, <a href="#Page483">483</a>.<br /> +<br /> +Moles, blind, <a href="#Page137">137</a>.<br /> +<br /> +Mongrels:<br /> +fertility and sterility of, <a href="#Page267">267</a>.<br /> +and hybrids compared, <a href="#Page272">272</a>.<br /> +<br /> +Monkeys, fossil, <a href="#Page303">303</a>,<br /> +<br /> +Monocanthus, <a href="#Page424">424</a>.<br /> +<br /> +Mons, Van, on the origin of fruit-trees, <a href="#Page29">29</a>, <a href="#Page39">39</a>.<br /> +<br /> +Moquin-Tandon on sea-side plants, <a href="#Page132">132</a>.<br /> +<br /> +Morphology, <a href="#Page434">434</a>.<br /> +<br /> +Mozart, musical powers of, <a href="#Page209">209</a>.<br /> +<br /> +Mud, seeds in, <a href="#Page386">386</a>.<br /> +<br /> +Mules, striped, <a href="#Page165">165</a>.<br /> +<br /> +Müller, Dr. F., on Alpine Australian plants, <a href="#Page375">375</a>.<br /> +<br /> +Murchison, Sir R.:<br /> +on the formations of Russia, <a href="#Page289">289</a>.<br /> +on azoic formations, <a href="#Page307">307</a>.<br /> +on extinction, <a href="#Page317">317</a>.<br /> +<br /> +Mustela vison, <a href="#Page179">179</a>.<br /> +<br /> +Myanthus, <a href="#Page424">424</a>.<br /> +<br /> +Myrmecocystus, <a href="#Page238">238</a>.<br /> +<br /> +Myrmica, eyes of, <a href="#Page240">240</a>.<br /> +<br /> +Nails, rudimentary, <a href="#Page453">453</a>.<br /> +<br /> +Natural history:<br /> +future progress of, <a href="#Page484">484</a>.<br /> +selection, <a href="#Page80">80</a>.<br /> +system, <a href="#Page413">413</a>.<br /> +<br /> +Naturalisation:<br /> +of forms distinct from the indigenous species, <a href="#Page115">115</a>.<br /> +in New Zealand, <a href="#Page201">201</a>.<br /> +<br /> +Nautilus, Silurian, <a href="#Page306">306</a>.<br /> +<br /> +Nectar of plants, <a href="#Page92">92</a>.<br /> +<br /> +Nectaries, how formed, <a href="#Page92">92</a>.<br /> +<br /> +Nelumbium luteum, <a href="#Page387">387</a>.<br /> +<br /> +Nests, variation in, <a href="#Page212">212</a>.<br /> +<br /> +Neuter insects, <a href="#Page236">236</a>.<br /> +<br /> +Newman, Mr., on humble-bees, <a href="#Page74">74</a>.<br /> +<br /> +New Zealand:<br /> +productions of, not perfect, <a href="#Page201">201</a>.<br /> +naturalised products of, <a href="#Page337">337</a>.<br /> +fossil birds of, <a href="#Page339">339</a>.<br /> +glacial action in, <a href="#Page373">373</a>.<br /> +crustaceans of, <a href="#Page376">376</a>.<br /> +algæ of, <a href="#Page376">376</a>.<br /> +number of plants of, <a href="#Page389">389</a>.<br /> +flora of, <a href="#Page399">399</a>.<br /> +<br /> +Nicotiana:<br /> +crossed varieties of, <a href="#Page271">271</a>.<br /> +certain species very sterile, <a href="#Page257">257</a>.<br /> +<br /> +Noble, Mr., on fertility of Rhododendron, <a href="#Page251">251</a>.<br /> +<br /> +Nodules, phosphatic, in azoic rocks, <a href="#Page307">307</a>.<br /> +<br /> +Oak, varieties of, <a href="#Page50">50</a>.<br /> +<br /> +Onites apelles, <a href="#Page135">135</a>.<br /> +<br /> +Orchis, pollen of, <a href="#Page193">193</a>.<br /> +<br /> +Organs:<br /> +of extreme perfection, <a href="#Page186">186</a>.<br /> +electric, of fishes, <a href="#Page192">192</a>.<br /> +of little importance, <a href="#Page194">194</a>.<br /> +homologous, <a href="#Page434">434</a>.<br /> +rudiments of, <a href="#Page450">450</a>.<br /> +<br /> +Ornithorhynchus, <a href="#Page107">107</a>, <a href="#Page416">416</a>.<br /> +<br /> +Ostrich:<br /> +not capable of flight, <a href="#Page134">134</a>.<br /> +habit of laying eggs together, <a href="#Page218">218</a>.<br /> +American, two species of, <a href="#Page349">349</a>.<br /> +<br /> +Otter, habits of, how acquired, <a href="#Page179">179</a>.<br /> +<br /> +Ouzel, water, <a href="#Page185">185</a>.<br /> +<br /> +Owen, Professor:<br /> +on birds not flying, <a href="#Page134">134</a>.<br /> +on vegetative repetition, <a href="#Page149">149</a>.<br /> +on variable length of arms in ourang-outang, <a href="#Page150">150</a>.<br /> +on the swim-bladder of fishes, <a href="#Page191">191</a>.<br /> +on electric organs, <a href="#Page192">192</a>.<br /> +on fossil horse of La Plata, <a href="#Page319">319</a>.<br /> +on relations of ruminants and pachyderms, <a href="#Page329">329</a>.<br /> +on fossil birds of New Zealand, <a href="#Page339">339</a>.<br /> +on succession of types, <a href="#Page339">339</a>.<br /> +on affinities of the dugong, <a href="#Page414">414</a>.<br /> +on homologous organs, <a href="#Page435">435</a>.<br /> +on the metamorphosis of cephalopods and spiders, <a href="#Page442">442</a>.<br /> +<br /> +Pacific Ocean, faunas of, <a href="#Page348">348</a>.<br /> +<br /> +Paley on no organ formed to give pain, <a href="#Page201">201</a>.<br /> +<br /> +Pallas on the fertility of the wild stocks of domestic animals, <a href="#Page253">253</a>.<br /> +<br /> +Paraguay, cattle destroyed by flies, <a href="#Page72">72</a>.<br /> +<br /> +Parasites, <a href="#Page217">217</a>.<br /> +<br /> +Partridge, dirt on feet, <a href="#Page362">362</a>.<br /> +<br /> +Parts:<br /> +greatly developed, variable, <a href="#Page150">150</a>.<br /> +degrees of utility of, <a href="#Page201">201</a>.<br /> +<br /> +Parus major, <a href="#Page183">183</a>.<br /> +<br /> +Passiflora, <a href="#Page251">251</a>.<br /> +<br /> +Peaches in United States, <a href="#Page85">85</a>.<br /> +<br /> +Pear, grafts of, <a href="#Page261">261</a>.<br /> +<br /> +Pelargonium:<br /> +flowers of, <a href="#Page145">145</a>.<br /> +sterility of, <a href="#Page251">251</a>.<br /> +<br /> +Pelvis of women, <a href="#Page144">144</a>.<br /> +<br /> +Peloria, <a href="#Page145">145</a>.<br /> +<br /> +Period, glacial, <a href="#Page365">365</a>.<br /> +<br /> +Petrels, habits of, <a href="#Page184">184</a>.<br /> +<br /> +Phasianus, fertility of hybrids, <a href="#Page253">253</a>.<br /> +<br /> +Pheasant, young, wild, <a href="#Page216">216</a>.<br /> +<br /> +Philippi on tertiary species in Sicily, <a href="#Page312">312</a>.<br /> +<br /> +Pictet, Professor:<br /> +on groups of species suddenly appearing, <a href="#Page302">302</a>, <a href="#Page305">305</a>.<br /> +on rate of organic change, <a href="#Page313">313</a>.<br /> +on continuous succession of genera, <a href="#Page316">316</a>.<br /> +on close alliance of fossils in consecutive formations, <a href="#Page335">335</a>.<br /> +on embryological succession, <a href="#Page338">338</a>.<br /> +<br /> +Pierce, Mr., on varieties of wolves, <a href="#Page91">91</a>.<br /> +<br /> +Pigeons:<br /> +with feathered feet and skin between toes, <a href="#Page12">12</a>.<br /> +breeds described, and origin of, <a href="#Page20">20</a>.<br /> +breeds of, how produced, <a href="#Page39">39</a>, <a href="#Page42">42</a>.<br /> +tumbler, not being able to get out of egg, <a href="#Page87">87</a>.<br /> +reverting to blue colour, <a href="#Page160">160</a>.<br /> +instinct of tumbling, <a href="#Page214">214</a>.<br /> +carriers, killed by hawks, <a href="#Page362">362</a>.<br /> +young of, <a href="#Page445">445</a>.<br /> +<br /> +Pistil, rudimentary, <a href="#Page451">451</a>.<br /> +<br /> +Plants:<br /> +poisonous, not affecting certain coloured animals, <a href="#Page12">12</a>.<br /> +selection applied to, <a href="#Page32">32</a>.<br /> +gradual improvement of, <a href="#Page37">37</a>.<br /> +not improved in barbarous countries, <a href="#Page38">38</a>.<br /> +destroyed by insects, <a href="#Page67">67</a>.<br /> +in midst of range, have to struggle with other plants, <a href="#Page77">77</a>.<br /> +nectar of, <a href="#Page92">92</a>.<br /> +fleshy, on sea-shores, <a href="#Page132">132</a>.<br /> +fresh-water, distribution of, <a href="#Page386">386</a>.<br /> +low in scale, widely distributed, <a href="#Page406">406</a>.<br /> +<br /> +Plumage, laws of change in sexes of birds, <a href="#Page89">89</a>.<br /> +<br /> +Plums in the United States, <a href="#Page85">85</a>.<br /> +<br /> +Pointer dog:<br /> +origin of, <a href="#Page35">35</a>.<br /> +habits of, <a href="#Page213">213</a>.<br /> +<br /> +Poison not affecting certain coloured animals, <a href="#Page12">12</a>.<br /> +<br /> +Poison, similar effect of, on animals and plants, <a href="#Page484">484</a>.<br /> +<br /> +Pollen of fir-trees, <a href="#Page203">203</a>,<br /> +<br /> +Poole, Col., on striped hemionus, <a href="#Page163">163</a>.<br /> +<br /> +Potamogeton, <a href="#Page387">387</a>.<br /> +<br /> +Prestwich, Mr., on English and French eocene formations, <a href="#Page328">328</a>.<br /> +<br /> +Primrose, <a href="#Page49">49</a>.<br /> +sterility of, <a href="#Page247">247</a>.<br /> +<br /> +Primula, varieties of, <a href="#Page49">49</a>.<br /> +<br /> +Proteolepas, <a href="#Page148">148</a>.<br /> +<br /> +Proteus, <a href="#Page139">139</a>.<br /> +<br /> +Psychology, future progress of, <a href="#Page488">488</a>.<br /> +<br /> +Quagga, striped, <a href="#Page165">165</a>.<br /> +<br /> +Quince, grafts of, <a href="#Page261">261</a>.<br /> +<br /> +Rabbit, disposition of young, <a href="#Page215">215</a>.<br /> +<br /> +Races, domestic, characters of, <a href="#Page16">16</a>.<br /> +<br /> +Race-horses:<br /> +Arab, <a href="#Page35">35</a>.<br /> +English, <a href="#Page356">356</a>.<br /> +<br /> +Ramond on plants of Pyrenees, <a href="#Page368">368</a>.<br /> +<br /> +Ramsay, Professor:<br /> +on thickness of the British formations, <a href="#Page284">284</a>.<br /> +on faults, <a href="#Page285">285</a>.<br /> +<br /> +Ratio of increase, <a href="#Page63">63</a>.<br /> +<br /> +Rats:<br /> +supplanting each other, <a href="#Page76">76</a>.<br /> +acclimatisation of, <a href="#Page141">141</a>.<br /> +blind in cave, <a href="#Page137">137</a>.<br /> +<br /> +Rattle-snake, <a href="#Page201">201</a>.<br /> +<br /> +Reason and instinct, <a href="#Page208">208</a>.<br /> +<br /> +Recapitulation, general, <a href="#Page459">459</a>.<br /> +<br /> +Reciprocity of crosses, <a href="#Page258">258</a>.<br /> +<br /> +Record, geological, imperfect, <a href="#Page279">279</a>.<br /> +<br /> +Rengger on flies destroying cattle, <a href="#Page72">72</a>.<br /> +<br /> +Reproduction, rate of, <a href="#Page63">63</a>.<br /> +<br /> +Resemblance to parents in mongrels and hybrids, <a href="#Page273">273</a>.<br /> +<br /> +Reversion:<br /> +law of inheritance, <a href="#Page14">14</a>.<br /> +in pigeons to blue colour, <a href="#Page160">160</a>.<br /> +<br /> +Rhododendron, sterility of, <a href="#Page251">251</a>.<br /> +<br /> +Richard, Professor, on Aspicarpa, <a href="#Page417">417</a>.<br /> +<br /> +Richardson, Sir J.:<br /> +on structure of squirrels, <a href="#Page180">180</a>.<br /> +on fishes of the southern hemisphere, <a href="#Page376">376</a>.<br /> +<br /> +Robinia, grafts of, <a href="#Page262">262</a>.<br /> +<br /> +Rodents, blind, <a href="#Page137">137</a>.<br /> +<br /> +Rudimentary organs, <a href="#Page450">450</a>.<br /> +<br /> +Rudiments important for classification, <a href="#Page416">416</a>.<br /> +<br /> +Sageret on grafts, <a href="#Page262">262</a>.<br /> +<br /> +Salmons, males fighting, and hooked jaws of, <a href="#Page88">88</a>.<br /> +<br /> +Salt-water, how far injurious to seeds, <a href="#Page358">358</a>.<br /> +<br /> +Saurophagus sulphuratus, <a href="#Page183">183</a>.<br /> +<br /> +Schiödte on blind insects, <a href="#Page138">138</a>.<br /> +<br /> +Schlegel on snakes, <a href="#Page144">144</a>.<br /> +<br /> +Sea-water, how far injurious to seeds, <a href="#Page358">358</a>.<br /> +<br /> +Sebright, Sir J.:<br /> +on crossed animals, <a href="#Page20">20</a>.<br /> +on selection of pigeons, <a href="#Page31">31</a>.<br /> +<br /> +Sedgwick, Professor, on groups of species suddenly appearing, <a href="#Page302">302</a>.<br /> +<br /> +Seedlings destroyed by insects, <a href="#Page67">67</a>.<br /> +<br /> +Seeds:<br /> +nutriment in, <a href="#Page77">77</a>.<br /> +winged, <a href="#Page146">146</a>.<br /> +power of resisting salt-water, <a href="#Page358">358</a>.<br /> +in crops and intestines of birds, <a href="#Page361">361</a>.<br /> +eaten by fish, <a href="#Page362">362</a>, <a href="#Page387">387</a>.<br /> +in mud, <a href="#Page386">386</a>.<br /> +hooked, on islands, <a href="#Page392">392</a>.<br /> +<br /> +Selection:<br /> +of domestic products, <a href="#Page29">29</a>.<br /> +principle not of recent origin, <a href="#Page33">33</a>.<br /> +unconscious, <a href="#Page34">34</a>.<br /> +natural, <a href="#Page80">80</a>.<br /> +sexual, <a href="#Page87">87</a>.<br /> +natural, circumstances favourable to, <a href="#Page101">101</a>.<br /> +<br /> +Sexes, relations of, <a href="#Page87">87</a>.<br /> +<br /> +Sexual:<br /> +characters variable, <a href="#Page156">156</a>.<br /> +selection, <a href="#Page87">87</a>.<br /> +<br /> +Sheep:<br /> +Merino, their selection, <a href="#Page31">31</a>.<br /> +two sub-breeds unintentionally produced, <a href="#Page36">36</a>.<br /> +mountain, varieties of, <a href="#Page76">76</a>.<br /> +<br /> +Shells:<br /> +colours of, <a href="#Page132">132</a>.<br /> +littoral, seldom embedded, <a href="#Page288">288</a>.<br /> +fresh-water, dispersal of, <a href="#Page385">385</a>.<br /> +of Madeira, <a href="#Page391">391</a>.<br /> +land, distribution of, <a href="#Page397">397</a>.<br /> +<br /> +Silene, fertility of crosses, <a href="#Page257">257</a>.<br /> +<br /> +Silliman, Professor, on blind rat, <a href="#Page137">137</a>.<br /> +<br /> +Skulls of young mammals, <a href="#Page197">197</a>, <a href="#Page437">437</a>.<br /> +<br /> +Slave-making instinct, <a href="#Page219">219</a>.<br /> +<br /> +Smith, Col. Hamilton, on striped horses, <a href="#Page164">164</a>.<br /> +<br /> +Smith, Mr. Fred.:<br /> +on slave-making ants, <a href="#Page219">219</a>.<br /> +on neuter ants, <a href="#Page239">239</a>.<br /> +<br /> +Smith, Mr., of Jordan Hill, on the degradation of coast-rocks, <a href="#Page283">283</a>.<br /> +<br /> +Snap-dragon, <a href="#Page161">161</a>.<br /> +<br /> +Somerville, Lord, on selection of sheep, <a href="#Page31">31</a>.<br /> +<br /> +Sorbus, grafts of, <a href="#Page262">262</a>.<br /> +<br /> +Spaniel, King Charles’s breed, <a href="#Page35">35</a>.<br /> +<br /> +Species:<br /> +polymorphic, <a href="#Page46">46</a>.<br /> +common, variable, <a href="#Page53">53</a>.<br /> +in large genera variable, <a href="#Page54">54</a>.<br /> +groups of, suddenly appearing, <a href="#Page302">302</a>, <a href="#Page306">306</a>.<br /> +beneath Silurian formations, <a href="#Page306">306</a>.<br /> +successively appearing, <a href="#Page312">312</a>.<br /> +changing simultaneously throughout the world, <a href="#Page322">322</a>.<br /> +<br /> +Spencer, Lord, on increase in size of cattle, <a href="#Page35">35</a>.<br /> +<br /> +Sphex, parasitic, <a href="#Page218">218</a>.<br /> +<br /> +Spiders, development of, <a href="#Page442">442</a>.<br /> +<br /> +Spitz-dog crossed with fox, <a href="#Page268">268</a>.<br /> +<br /> +Sports in plants, <a href="#Page9">9</a>.<br /> +<br /> +Sprengel, C. C.:<br /> +on crossing, <a href="#Page98">98</a>.<br /> +on ray-florets, <a href="#Page145">145</a>.<br /> +<br /> +Squirrels, gradations in structure, <a href="#Page180">180</a>.<br /> +<br /> +Staffordshire, heath, changes in, <a href="#Page72">72</a>.<br /> +<br /> +Stag-beetles, fighting, <a href="#Page88">88</a>.<br /> +<br /> +Sterility:<br /> +from changed conditions of life, <a href="#Page9">9</a>.<br /> +of hybrids, <a href="#Page246">246</a>.<br /> +laws of, <a href="#Page254">254</a>.<br /> +causes of, <a href="#Page263">263</a>.<br /> +from unfavourable conditions, <a href="#Page265">265</a>.<br /> +of certain varieties, <a href="#Page269">269</a>.<br /> +<br /> +St. Helena, productions of, <a href="#Page389">389</a>.<br /> +<br /> +St. Hilaire, Aug., on classification, <a href="#Page418">418</a>.<br /> +<br /> +St. John, Mr., on habits of cats, <a href="#Page91">91</a>.<br /> +<br /> +Sting of bee, <a href="#Page202">202</a>.<br /> +<br /> +Stocks, aboriginal, of domestic animals, <a href="#Page18">18</a>,<br /> +<br /> +Strata, thickness of, in Britain, <a href="#Page284">284</a>.<br /> +<br /> +Stripes on horses, <a href="#Page163">163</a>.<br /> +<br /> +Structure, degrees of utility of, <a href="#Page201">201</a>.<br /> +<br /> +Struggle for existence, <a href="#Page60">60</a>.<br /> +<br /> +Succession, geological, <a href="#Page312">312</a>.<br /> +<br /> +Succession of types in same areas, <a href="#Page338">338</a>.<br /> +<br /> +Swallow, one species supplanting another, <a href="#Page76">76</a>.<br /> +<br /> +Swim-bladder, <a href="#Page190">190</a>.<br /> +<br /> +System, natural, <a href="#Page413">413</a>.<br /> +<br /> +Tail:<br /> +of giraffe, <a href="#Page195">195</a>.<br /> +of aquatic animals, <a href="#Page196">196</a>.<br /> +rudimentary, <a href="#Page454">454</a>.<br /> +<br /> +Tarsi deficient, <a href="#Page135">135</a>.<br /> +<br /> +Tausch on umbelliferous flowers, <a href="#Page146">146</a>.<br /> +<br /> +Teeth and hair:<br /> +correlated, <a href="#Page144">144</a>.<br /> +embryonic, traces of, in birds, <a href="#Page451">451</a>.<br /> +rudimentary, in embryonic calf, <a href="#Page450">450</a>, <a href="#Page480">480</a>.<br /> +<br /> +Tegetmeier, Mr., on cells of bees, <a href="#Page228">228</a>, <a href="#Page233">233</a>.<br /> +<br /> +Temminck on distribution aiding classification, <a href="#Page419">419</a>.<br /> +<br /> +Thouin on grafts, <a href="#Page262">262</a>.<br /> +<br /> +Thrush:<br /> +aquatic species of, <a href="#Page185">185</a>.<br /> +mocking, of the Galapagos, <a href="#Page402">402</a>.<br /> +young of, spotted, <a href="#Page439">439</a>.<br /> +nest of, <a href="#Page243">243</a>.<br /> +<br /> +Thuret, >M., on crossed fuci, <a href="#Page258">258</a>.<br /> +<br /> +Thwaites, Mr., on acclimatisation, <a href="#Page140">140</a>.<br /> +<br /> +Tierra del Fuego:<br /> +dogs of, <a href="#Page215">215</a>.<br /> +plants of, <a href="#Page374">374</a>, <a href="#Page378">378</a>.<br /> +<br /> +Timber-drift, <a href="#Page360">360</a>.<br /> +<br /> +Time, lapse of, <a href="#Page282">282</a>.<br /> +<br /> +Titmouse, <a href="#Page183">183</a>.<br /> +<br /> +Toads on islands, <a href="#Page393">393</a>.<br /> +<br /> +Tobacco, crossed varieties of, <a href="#Page271">271</a>.<br /> +<br /> +Tomes, Mr., on the distribution of bats, <a href="#Page394">394</a>.<br /> +<br /> +Transitions in varieties rare, <a href="#Page172">172</a>.<br /> +<br /> +Trees:<br /> +on islands belong to peculiar orders, <a href="#Page392">392</a>.<br /> +with separated sexes, <a href="#Page99">99</a>.<br /> +<br /> +Trifolium pratense, <a href="#Page73">73</a>, <a href="#Page94">94</a>.<br /> +<br /> +Trifolium incarnatum, <a href="#Page94">94</a>.<br /> +<br /> +Trigonia, <a href="#Page321">321</a>.<br /> +<br /> +Trilobites, <a href="#Page306">306</a>.<br /> +sudden extinction of, <a href="#Page321">321</a>.<br /> +<br /> +Troglodytes, <a href="#Page243">243</a>.<br /> +<br /> +Tucutucu, blind, <a href="#Page137">137</a>.<br /> +<br /> +Tumbler pigeons:<br /> +habits of, hereditary, <a href="#Page214">214</a>.<br /> +young of, <a href="#Page446">446</a>.<br /> +<br /> +Turkey-cock, brush of hair on breast, <a href="#Page90">90</a>.<br /> +<br /> +Turkey:<br /> +naked skin on head, <a href="#Page197">197</a>.<br /> +young, wild, <a href="#Page216">216</a>.<br /> +<br /> +Turnip and cabbage, analogous variations of, <a href="#Page159">159</a>.<br /> +<br /> +Type, unity of, <a href="#Page206">206</a>.<br /> +<br /> +Types, succession of, in same areas, <a href="#Page338">338</a>.<br /> +<br /> +Udders:<br /> +enlarged by use, <a href="#Page11">11</a>.<br /> +rudimentary, <a href="#Page451">451</a>.<br /> +<br /> +Ulex, young leaves of, <a href="#Page439">439</a>.<br /> +<br /> +Umbelliferæ, outer and inner florets of, <a href="#Page144">144</a>.<br /> +<br /> +Unity of type, <a href="#Page206">206</a>.<br /> +<br /> +Use:<br /> +effects of, under domestication, <a href="#Page11">11</a>.<br /> +effects of, in a state of nature, <a href="#Page134">134</a>.<br /> +<br /> +Utility, how far important in the construction of each part, <a href="#Page199">199</a>.<br /> +<br /> +Valenciennes on fresh-water fish, <a href="#Page384">384</a>.<br /> +<br /> +Variability of mongrels and hybrids, <a href="#Page274">274</a>.<br /> +<br /> +Variation:<br /> +under domestication, <a href="#Page7">7</a>.<br /> +caused by reproductive system being affected by conditions of life, <a href="#Page8">8</a>.<br /> +under nature, <a href="#Page44">44</a>.<br /> +laws of, <a href="#Page131">131</a>.<br /> +<br /> +Variations:<br /> +appear at corresponding ages, <a href="#Page14">14</a>, <a href="#Page86">86</a>.<br /> +analogous in distinct species, <a href="#Page159">159</a>.<br /> +<br /> +Varieties:<br /> +natural, <a href="#Page44">44</a>.<br /> +struggle between, <a href="#Page75">75</a>.<br /> +domestic, extinction of, <a href="#Page111">111</a>.<br /> +transitional, rarity of, <a href="#Page172">172</a>.<br /> +when crossed, fertile, <a href="#Page267">267</a>.<br /> +when crossed, sterile, <a href="#Page269">269</a>.<br /> +classification of, <a href="#Page423">423</a>.<br /> +<br /> +Verbascum:<br /> +sterility of, <a href="#Page251">251</a>.<br /> +varieties of, crossed, <a href="#Page270">270</a>.<br /> +<br /> +Verneuil, M. de, on the succession of species, <a href="#Page325">325</a>.<br /> +<br /> +Viola tricolor, <a href="#Page73">73</a>.<br /> +<br /> +Volcanic islands, denudation of, <a href="#Page284">284</a>.<br /> +<br /> +Vulture, naked skin on head, <a href="#Page197">197</a>.<br /> +<br /> +Wading-birds, <a href="#Page386">386</a>.<br /> +<br /> +Wallace, Mr.:<br /> +on origin of species, <a href="#Page2">2</a>.<br /> +on law of geographical distribution, <a href="#Page355">355</a>.<br /> +on the Malay Archipelago, <a href="#Page395">395</a>.<br /> +<br /> +Wasp, sting of, <a href="#Page202">202</a>.<br /> +<br /> +Water, fresh, productions of, <a href="#Page383">383</a>.<br /> +<br /> +Water-hen, <a href="#Page185">185</a>.<br /> +<br /> +Waterhouse, Mr.:<br /> +on Australian marsupials, <a href="#Page116">116</a>.<br /> +on greatly developed parts being variable, <a href="#Page150">150</a>.<br /> +on the cells of bees, <a href="#Page225">225</a>.<br /> +on general affinities, <a href="#Page429">429</a>.<br /> +<br /> +Water-ouzel, <a href="#Page185">185</a>.<br /> +<br /> +Watson, Mr. H. C.:<br /> +on range of varieties of British plants, <a href="#Page58">58</a>.<br /> +on acclimatisation, <a href="#Page140">140</a>.<br /> +on flora of Azores, <a href="#Page363">363</a>.<br /> +on Alpine plants, <a href="#Page367">367</a>, <a href="#Page376">376</a>.<br /> +on rarity of intermediate varieties, <a href="#Page176">176</a>.<br /> +<br /> +Weald, denudation of, <a href="#Page285">285</a>.<br /> +<br /> +Web of feet in water-birds, <a href="#Page185">185</a>.<br /> +<br /> +West Indian islands, mammals of, <a href="#Page395">395</a>.<br /> +<br /> +Westwood:<br /> +on species in large genera being closely allied to others, <a href="#Page57">57</a>.<br /> +on the tarsi of Engidæ, <a href="#Page157">157</a>.<br /> +on the antennæ of hymenopterous insects, <a href="#Page416">416</a>.<br /> +<br /> +Whales, fossil, <a href="#Page303">303</a>.<br /> +<br /> +Wheat, varieties of, <a href="#Page113">113</a>.<br /> +<br /> +White Mountains, flora of, <a href="#Page365">365</a>.<br /> +<br /> +Wings, reduction of size, <a href="#Page134">134</a>.<br /> +<br /> +Wings:<br /> +of insects homologous with branchiæ, <a href="#Page191">191</a>.<br /> +rudimentary, in insects, <a href="#Page451">451</a>.<br /> +<br /> +Wolf:<br /> +crossed with dog, <a href="#Page214">214</a>.<br /> +of Falkland Isles, <a href="#Page393">393</a>.<br /> +<br /> +Wollaston, Mr.:<br /> +on varieties of insects, <a href="#Page48">48</a>.<br /> +on fossil varieties of land-shells in Madeira, <a href="#Page52">52</a>.<br /> +on colours of insects on sea-shore, <a href="#Page132">132</a>.<br /> +on wingless beetles, <a href="#Page135">135</a>.<br /> +on rarity of intermediate varieties, <a href="#Page176">176</a>.<br /> +on insular insects, <a href="#Page389">389</a>.<br /> +on land-shells of Madeira, naturalised, <a href="#Page402">402</a>.<br /> +<br /> +Wolves, varieties of, <a href="#Page90">90</a>.<br /> +<br /> +Woodpecker:<br /> +habits of, <a href="#Page184">184</a>.<br /> +green colour of, <a href="#Page197">197</a>.<br /> +<br /> +Woodward, Mr.:<br /> +on the duration of specific forms, <a href="#Page293">293</a>.<br /> +on the continuous succession of genera, <a href="#Page316">316</a>.<br /> +on the succession of types, <a href="#Page339">339</a>.<br /> +<br /> +World, species changing simultaneously throughout, <a href="#Page322">322</a>.<br /> +<br /> +Wrens, nest of, <a href="#Page243">243</a>.<br /> +<br /> +Youatt, Mr.:<br /> +on selection, <a href="#Page31">31</a>.<br /> +on sub-breeds of sheep, <a href="#Page36">36</a>.<br /> +on rudimentary horns in young cattle, <a href="#Page454">454</a>.<br /> +<br /> +Zebra, stripes on, <a href="#Page163">163</a>. +</p> + +</div><!--end chapter--> + +<div>*** END OF THE PROJECT GUTENBERG EBOOK 1228 ***</div> +</body> + +</html> + + |