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left:0.4em;} + .nobo {border: thin;} + .red {color: red;} + .figure, .figcenter, .figright, .figleft + {padding: 1em; margin: 0; text-align: center; font-size: 0.8em;} + .figure img, .figcenter img, .figright img, .figleft img + {border: none;} + .figure p, .figcenter p, .figright p, .figleft p + {margin: 0; text-indent: 1em;} + .figure p.in, .figcenter p.in, .figright p.in, .figleft p.in + {margin: 0; text-indent: 8em;} + .figcenter p.poem + {margin-left: 1em; text-align: left; text-indent: 0;} + .figcenter {margin: auto;} + .figright {float: right;} + .figleft {float: left;} + img.middle { border: none; vertical-align: middle } + + </style> + </head> +<body> + +<div style='text-align:center; font-size:1.2em; font-weight:bold;'>The Project Gutenberg eBook of On the Origin of Species by Means of Natural Selection, by Charles Darwin</div> +<div style='display:block; margin:1em 0'> +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online +at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you +are not located in the United States, you will have to check the laws of the +country where you are located before using this eBook. +</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: On the Origin of Species by Means of Natural Selection +or the Preservation of Favoured Races in the Struggle for Life. (2nd edition)</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Charles Darwin</div> +<div style='display:block;margin:1em 0'>Release Date: September 25, 2007 [eBook #22764]</div> +<div style='display:block;margin:1em 0'>Language: English</div> +<div style='display:block;margin:1em 0'>Character set encoding: UTF-8</div> +<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Steven Gibbs, Keith Edkins and the Online Distributed Proofreading Team</div> +<div style='margin-top:2em;margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK ORIGIN OF SPECIES ***</div> + +<h4>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. +</h4> + + +<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> + + +<table border="0" cellpadding="10" style="background-color: #ccccff;"> +<tr> +<td style="width:25%; vertical-align:top"> +Transcriber's note: +</td> +<td> +A few typographical errors have been corrected. They +appear in the text <span class="correction" title="explanation will pop up">like this</span>, and the +explanation will appear when the mouse pointer is moved over the marked +passage. +</td> +</tr> +</table> + +<h3>ON THE</h3> + +<h1>ORIGIN OF SPECIES.</h1> + +<hr /> + + <p>"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="author"><span class="sc">Whewell</span>: <i>Bridgewater Treatise</i>.</p> + + <p>"The only distinct meaning of the word 'natural' is <i>stated</i>, + <i>fixed</i>, or <i>settled</i>; since what is natural as much requires + and presupposes an intelligent agent to render it so, <i>i.e.</i> to + effect it continually or at stated times, as what is supernatural or + miraculous does to effect it for once."</p> + + <p class="author"><span class="sc">Butler</span>: <i>Analogy of Revealed Religion</i>.</p> + + <p>"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="author"><span class="sc">Bacon</span>: <i>Advancement of Learning</i>.</p> + +<hr /> + + <div class="poem"> + <div class="stanza"> + <p><i>Down, Bromley, Kent,</i></p> + <p class="i4"><i>October 1st, 1859.</i> (<i>1st Thousand</i>).</p> + </div> + </div> +<hr /> + +<h3>ON</h3> + +<h1>THE ORIGIN OF SPECIES</h1> + +<h2>BY MEANS OF NATURAL SELECTION,</h2> + +<p class="cenhead">OR THE</p> + +<h3>PRESERVATION OF FAVOURED RACES IN THE STRUGGLE<br /> +FOR LIFE.</h3> + +<h2><span class="sc">By</span> CHARLES DARWIN, M.A.,</h2> + +<p class="cenhead">FELLOW OF THE ROYAL, GEOLOGICAL, LINNEAN, ETC., SOCIETIES;</p> + +<p class="cenhead">AUTHOR OF 'JOURNAL OF RESEARCHES DURING H. M. S. BEAGLE'S VOYAGE<br /> +ROUND THE WORLD.'</p> + +<h3><i>FIFTH THOUSAND.</i></h3> + +<h3>LONDON:<br /> +JOHN MURRAY, ALBEMARLE STREET.<br /> +1860.</h3> + +<p class="cenhead"><i>The right of Translation is reserved.</i></p> + +<hr /> + +<p class="cenhead">LONDON: PRINTED BY W. CLOWES AND SONS, STAMFORD STREET,<br /> +AND CHARING CROSS.</p> + +<hr /> + +<p><!-- Page v --><span class="pagenum"><a name="pagev"></a>[v]</span></p> + +<h2>CONTENTS.</h2> + +<hr /> + + <p><span class="sc">Introduction</span></p> + + <p class="author">Page <a href="#page1">1</a></p> + +<h3>CHAPTER I.</h3> + +<p class="cenhead"><span class="sc">Variation under Domestication.</span></p> + + <p>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 class="author"><a href="#page7">7</a>-<a href="#page43">43</a></p> + +<h3>CHAPTER II.</h3> + +<p class="cenhead"><span class="sc">Variation under Nature.</span></p> + + <p>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 class="author"><a href="#page44">44</a>-<a href="#page59">59</a></p> + +<p><!-- Page vi --><span class="pagenum"><a name="pagevi"></a>[vi]</span></p> + +<h3>CHAPTER III.</h3> + +<p class="cenhead"><span class="sc">Struggle for Existence.</span></p> + + <p>Its bearing 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 class="author"><a href="#page60">60</a>-<a href="#page79">79</a></p> + +<h3>CHAPTER IV.</h3> + +<p class="cenhead"><span class="sc">Natural Selection.</span></p> + + <p>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 class="author"><a href="#page80">80</a>-<a href="#page130">130</a></p> + +<h3>CHAPTER V.</h3> + +<p class="cenhead"><span class="sc">Laws of Variation.</span></p> + + <p>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 class="author"><a href="#page131">131</a>-<a href="#page170">170</a></p> + +<p><!-- Page vii --><span class="pagenum"><a name="pagevii"></a>[vii]</span></p> + +<h3>CHAPTER VI.</h3> + +<p class="cenhead"><span class="sc">Difficulties on Theory.</span></p> + + <p>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 class="author"><a href="#page171">171</a>-<a href="#page206">206</a></p> + +<h3>CHAPTER VII.</h3> + +<p class="cenhead"><span class="sc">Instinct.</span></p> + + <p>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 class="author"><a href="#page207">207</a>-<a href="#page244">244</a></p> + +<h3>CHAPTER VIII.</h3> + +<p class="cenhead"><span class="sc">Hybridism.</span></p> + + <p>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 class="author"><a href="#page245">245</a>-<a href="#page278">278</a></p> + +<p><!-- Page viii --><span class="pagenum"><a name="pageviii"></a>[viii]</span></p> + +<h3>CHAPTER IX.</h3> + +<p class="cenhead"><span class="sc">On the Imperfection of the Geological Record.</span></p> + + <p>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 class="author"><a href="#page279">279</a>-<a href="#page311">311</a></p> + +<h3>CHAPTER X.</h3> + +<p class="cenhead"><span class="sc">On the Geological Succession of Organic Beings.</span></p> + + <p>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 class="author"><a href="#page312">312</a>-<a href="#page345">345</a></p> + +<h3>CHAPTER XI.</h3> + +<p class="cenhead"><span class="sc">Geographical Distribution.</span></p> + + <p>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 class="author"><a href="#page346">346</a>-<a href="#page382">382</a></p> + +<h3>CHAPTER XII.</h3> + +<p class="cenhead"><span class="sc">Geographical Distribution</span>—<i>continued</i>.</p> + + <p>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 class="author"><a href="#page383">383</a>-<a href="#page410">410</a></p> + +<h3>CHAPTER XIII.</h3> + +<p class="cenhead"><span class="sc">Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.</span></p> + + <p><span class="sc">Classification</span>, 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—<span + class="sc">Morphology</span>, between members of the same class, between + parts of the same individual—<span class="sc">Embryology</span>, + laws of, explained by variations not supervening at an early age, and + being inherited at a corresponding age—<span class="sc">Rudimentary + organs</span>; their origin explained—Summary</p> + + <p class="author"><a href="#page411">411</a>-<a href="#page458">458</a></p> + +<h3>CHAPTER XIV.</h3> + +<p class="cenhead"><span class="sc">Recapitulation and Conclusion.</span></p> + + <p>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 class="author"><a href="#page459">459</a>-<a href="#page490">490</a></p> + +<hr /> + +<p><!-- Page 1 --><span class="pagenum"><a name="page1"></a>[1]</span></p> + +<h1>ON THE ORIGIN OF SPECIES.</h1> + +<hr /> + +<h3>INTRODUCTION.</h3> + + <p>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 <!-- Page 2 --><span + class="pagenum"><a name="page2"></a>[2]</span>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 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, <!-- Page 3 --><span class="pagenum"><a + name="page3"></a>[3]</span>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, &c., 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 <!-- Page 4 --><span class="pagenum"><a + name="page4"></a>[4]</span>generations, some bird had given birth to a + woodpecker, and some plant to the missletoe, 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 the high geometrical ratio of their <!-- Page 5 --><span + class="pagenum"><a name="page5"></a>[5]</span>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 leads to 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 <!-- Page 6 --><span class="pagenum"><a + name="page6"></a>[6]</span>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> + +<hr /> + +<p><!-- Page 7 --><span class="pagenum"><a name="page7"></a>[7]</span></p> + +<h3>CHAPTER I.</h3> + +<p class="cenhead"><span class="sc">Variation under Domestication.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 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 great 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. <!-- + Page 8 --><span class="pagenum"><a name="page8"></a>[8]</span>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 function 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 discovered + 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 <!-- Page 9 --><span class="pagenum"><a + name="page9"></a>[9]</span>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.</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 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. + <!-- Page 10 --><span class="pagenum"><a + name="page10"></a>[10]</span>Such buds can be propagated by grafting, + &c., 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, 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, &c., 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 are 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 <!-- Page 11 --><span class="pagenum"><a + name="page11"></a>[11]</span>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 or 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 <!-- Page 12 --><span + class="pagenum"><a name="page12"></a>[12]</span>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, &c.; + 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 any + deviation of structure often appears, and we see it in the <!-- Page 13 + --><span class="pagenum"><a name="page13"></a>[13]</span>father and + child, we cannot tell whether it may not be due to the same cause having + acted 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, &c., 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 a + peculiarity in different individuals of the same species, or 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 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 not + be otherwise: thus the inherited peculiarities in the horns of cattle + could appear only in <!-- Page 14 --><span class="pagenum"><a + name="page14"></a>[14]</span>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 single variety should be turned loose in its + new home. Nevertheless, as our varieties certainly do occasionally <!-- + Page 15 --><span class="pagenum"><a name="page15"></a>[15]</span>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 the same conditions, + and whilst kept in a considerable body, so that free intercrossing might + check, by blending together, any slight deviations in their 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 closely allied species, we + generally perceive in each domestic race, as already remarked, less + uniformity of character than in true species. Domestic races of the same + species, also, often have a somewhat monstrous character; by which I + mean, that, although differing <!-- Page 16 --><span class="pagenum"><a + name="page16"></a>[16]</span>from each other, and from 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 + 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 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, + bloodhound, terrier, spaniel, and bull-dog, which we all know propagate + their kind so truly, were the <!-- Page 17 --><span class="pagenum"><a + name="page17"></a>[17]</span>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 natural species—for instance, of + the many foxes—inhabiting different quarters of the world. I do not + believe, as we shall presently see, that the whole amount of difference + between the several breeds of the dog has been produced under + domestication; I believe that some small part of the difference is due to + their being descended from distinct species. In the case of some other + domesticated species, there is presumptive, or even strong evidence, that + all the breeds have descended from a single wild stock.</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 reindeer, + 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 wild species. The argument mainly + relied on by those who believe in the multiple origin <!-- Page 18 + --><span class="pagenum"><a name="page18"></a>[18]</span>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. Knowing, as we do, that savages + are very fond of taming animals, it seems to me unlikely, in the case of + the dog-genus, which is distributed in a wild state throughout the world, + that since man first appeared one single species alone should have been + domesticated. 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, &c., 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 <!-- Page 19 --><span + class="pagenum"><a name="page19"></a>[19]</span>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 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 have all 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, &c., but that each of these kingdoms possesses several + peculiar breeds of cattle, sheep, &c., 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, &c.—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 <!-- Page 20 --><span + class="pagenum"><a name="page20"></a>[20]</span>been produced by the + crossing of a few aboriginal species; but by crossing we can only get + forms in some degree intermediate between their parents; and if we + 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, &c., 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 Hon. W. Elliot from India, + and by the Hon. C. Murray from Persia. Many treatises in different + languages have been published on pigeons, and some of them are very + important, as being of <!-- Page 21 --><span class="pagenum"><a + name="page21"></a>[21]</span>considerable antiquity. I have associated + with several eminent fanciers, and have been permitted to join two 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 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 œsophagus. 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 <!-- Page 22 + --><span class="pagenum"><a name="page22"></a>[22]</span>carried so erect + that in good birds the head and tail touch; the oil-gland is quite + aborted. Several other less distinct breeds might be 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 œsophagus; 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 the <!-- Page 23 + --><span class="pagenum"><a name="page23"></a>[23]</span>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 <!-- Page 24 --><span class="pagenum"><a + name="page24"></a>[24]</span>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. <!-- Page 25 + --><span class="pagenum"><a name="page25"></a>[25]</span></p> + + <p>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 subspecies, 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, <!-- Page 26 --><span + class="pagenum"><a name="page26"></a>[26]</span>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 <!-- Page 27 --><span + class="pagenum"><a name="page27"></a>[27]</span>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 + varieties, 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 Ægyptian + 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 <!-- Page 28 + --><span class="pagenum"><a name="page28"></a>[28]</span>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 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. <!-- Page 29 + --><span class="pagenum"><a name="page29"></a>[29]</span>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 <!-- Page 30 --><span class="pagenum"><a + name="page30"></a>[30]</span>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 <!-- Page 31 --><span class="pagenum"><a + name="page31"></a>[31]</span>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 agriculturists 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; <!-- Page 32 --><span + class="pagenum"><a name="page32"></a>[32]</span>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 <!-- Page 33 --><span + class="pagenum"><a name="page33"></a>[33]</span>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 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 barbarous periods <!-- Page 34 --><span + class="pagenum"><a name="page34"></a>[34]</span>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 doubt that this process, + continued during centuries, <!-- Page 35 --><span class="pagenum"><a + name="page35"></a>[35]</span>would improve and modify any breed, in the + same way as Bakewell, Collins, &c., 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 and + tumblers with these breeds as now existing in Britain, <!-- Page 36 + --><span class="pagenum"><a name="page36"></a>[36]</span>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 at their first appearance as distinct + varieties, and whether <!-- Page 37 --><span class="pagenum"><a + name="page37"></a>[37]</span>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 the 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 or modify most of our plants up to their + present <!-- Page 38 --><span class="pagenum"><a + name="page38"></a>[38]</span>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 <!-- Page 39 --><span + class="pagenum"><a name="page39"></a>[39]</span>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 œsophagus,—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 <!-- Page 40 --><span + class="pagenum"><a name="page40"></a>[40]</span>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 civilization 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 <!-- Page 41 + --><span class="pagenum"><a name="page41"></a>[41]</span>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 <!-- Page 42 --><span + class="pagenum"><a name="page42"></a>[42]</span>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, &c., 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>To sum up on the origin of our Domestic Races of <!-- Page 43 --><span + class="pagenum"><a name="page43"></a>[43]</span>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, &c., 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> + +<hr /> + +<p><!-- Page 44 --><span class="pagenum"><a name="page44"></a>[44]</span></p> + +<h3>CHAPTER II.</h3> + +<p class="cenhead"><span class="sc">Variation under Nature.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 <!-- Page 45 --><span + class="pagenum"><a name="page45"></a>[45]</span>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 <!-- + Page 46 --><span class="pagenum"><a name="page46"></a>[46]</span>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. <!-- Page 47 --><span class="pagenum"><a + name="page47"></a>[47]</span></p> + + <p>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. <!-- Page 48 --><span + class="pagenum"><a name="page48"></a>[48]</span></p> + + <p>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 <!-- + Page 49 --><span class="pagenum"><a name="page49"></a>[49]</span>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, &c., + 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 vulgaris and veris. 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 <!-- Page 50 --><span + class="pagenum"><a name="page50"></a>[50]</span>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 <!-- Page 51 --><span class="pagenum"><a + name="page51"></a>[51]</span>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 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 <!-- Page 52 + --><span class="pagenum"><a name="page52"></a>[52]</span>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 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. <!-- Page 53 --><span class="pagenum"><a + name="page53"></a>[53]</span></p> + + <p>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 <!-- Page 54 + --><span class="pagenum"><a name="page54"></a>[54]</span>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, 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 <!-- Page 55 --><span class="pagenum"><a + name="page55"></a>[55]</span>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 + <!-- Page 56 --><span class="pagenum"><a + name="page56"></a>[56]</span>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 <!-- Page 57 --><span + class="pagenum"><a name="page57"></a>[57]</span>should 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 confirm the view. I have also consulted some + sagacious and 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, <!-- Page 58 --><span + class="pagenum"><a name="page58"></a>[58]</span>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> </p> + + <p>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 <!-- Page 59 + --><span class="pagenum"><a name="page59"></a>[59]</span>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 or 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> + +<hr /> + +<p><!-- Page 60 --><span class="pagenum"><a name="page60"></a>[60]</span></p> + +<h3>CHAPTER III.</h3> + +<p class="cenhead"><span class="sc">Struggle for Existence</span>.</p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 + <!-- Page 61 --><span class="pagenum"><a + name="page61"></a>[61]</span>plainly in the woodpecker and missletoe; and + only a little less plainly in the humblest parasite which clings 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 + 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 <!-- + Page 62 --><span class="pagenum"><a + name="page62"></a>[62]</span>immeasurably superior to man's feeble + efforts, as the works of Nature are to those of Art.</p> + + <p>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 <!-- Page + 63 --><span class="pagenum"><a name="page63"></a>[63]</span>for life + against the drought, though more properly it should be said to be + dependent on the moisture. 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 <!-- Page 64 --><span + class="pagenum"><a name="page64"></a>[64]</span>can be no artificial + increase of food, and no prudential restraint from marriage. Although + some species may be 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 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 incredible. So it is with plants: + cases could be given of <!-- Page 65 --><span class="pagenum"><a + name="page65"></a>[65]</span>introduced plants which have become common + throughout whole islands in a period of less than ten years. Several of + the plants, such as the cardoon and a tall thistle, 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 <!-- Page 66 + --><span class="pagenum"><a name="page66"></a>[66]</span>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 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 <!-- + Page 67 --><span class="pagenum"><a name="page67"></a>[67]</span>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 destruction ever so little, and the number of the species + will almost instantaneously increase to any amount.</p> + + <p>The causes which check the natural tendency of each species to + increase in number are 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, the more + vigorous plants <!-- Page 68 --><span class="pagenum"><a + name="page68"></a>[68]</span>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 cold, <!-- Page 69 --><span + class="pagenum"><a name="page69"></a>[69]</span>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 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. <!-- Page 70 --><span + class="pagenum"><a name="page70"></a>[70]</span></p> + + <p>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, &c., 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 the species from utter destruction. I should add + that the good effects of frequent intercrossing, and <!-- Page 71 + --><span class="pagenum"><a name="page71"></a>[71]</span>the ill effects + 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. <!-- Page 72 --><span class="pagenum"><a + name="page72"></a>[72]</span>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, with + twenty-six rings of growth, had during many 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 became feral, and this would + certainly greatly <!-- Page 73 --><span class="pagenum"><a + name="page73"></a>[73]</span>alter (as 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 lately tried, I have + found that the visits of bees are necessary for the fertilisation of some + kinds of clover; but humble-bees alone visit the red clover (Trifolium + pratense), as other bees cannot reach the nectar. Hence I have very + little doubt, that if the <!-- Page 74 --><span class="pagenum"><a + name="page74"></a>[74]</span>whole genus of humble-bees became 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 concur 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 ancient Indian ruins in the Southern United + States, which must formerly have been cleared of trees, now display the + same beautiful diversity and proportion of kinds as in the surrounding + <!-- Page 75 --><span class="pagenum"><a + name="page75"></a>[75]</span>virgin forests. What a struggle between the + several kinds of trees 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 <!-- Page 76 --><span + class="pagenum"><a name="page76"></a>[76]</span>coloured sweet-peas, they + must be each year harvested separately, and the seed then mixed in due + proportion, 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; <!-- Page 77 + --><span class="pagenum"><a name="page77"></a>[77]</span>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 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 <!-- Page 78 --><span class="pagenum"><a + name="page78"></a>[78]</span>that it can perfectly well withstand a + little more heat or cold, dampness or dryness, for elsewhere it ranges + 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 <!-- Page 79 --><span class="pagenum"><a + name="page79"></a>[79]</span>organic being is striving to increase at a + geometrical 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> + +<hr /> + +<p><!-- Page 80 --><span class="pagenum"><a name="page80"></a>[80]</span></p> + +<h3>CHAPTER IV.</h3> + +<p class="cenhead"><span class="sc">Natural Selection.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 <!-- Page 81 --><span class="pagenum"><a + name="page81"></a>[81]</span>(remembering 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 + 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 <!-- Page 82 --><span class="pagenum"><a + name="page82"></a>[82]</span>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 <!-- Page 83 --><span class="pagenum"><a + name="page83"></a>[83]</span>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 struggle for life, and so be <!-- Page 84 + --><span class="pagenum"><a name="page84"></a>[84]</span>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 metaphorically 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 + <!-- Page 85 --><span class="pagenum"><a + name="page85"></a>[85]</span>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 quite unimportant, we + must not forget that climate, food, &c., 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 <!-- Page 86 + --><span class="pagenum"><a name="page86"></a>[86]</span>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 variations profitable 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 <!-- Page 87 + --><span class="pagenum"><a name="page87"></a>[87]</span>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, 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> </p> + + <p><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 <!-- Page 88 --><span + class="pagenum"><a name="page88"></a>[88]</span>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 depends 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 the 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, <!-- Page 89 --><span + class="pagenum"><a name="page89"></a>[89]</span>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 <!-- Page 90 --><span class="pagenum"><a + name="page90"></a>[90]</span>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, &c.), 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> </p> + + <p><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 <!-- Page 91 --><span + class="pagenum"><a name="page91"></a>[91]</span>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 <!-- Page 92 --><span class="pagenum"><a + name="page92"></a>[92]</span>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. <!-- Page 93 --><span + class="pagenum"><a name="page93"></a>[93]</span></p> + + <p>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 a rather 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 <!-- Page 94 --><span + class="pagenum"><a name="page94"></a>[94]</span>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, &c., 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 <!-- + Page 95 --><span class="pagenum"><a name="page95"></a>[95]</span>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 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 + 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 <!-- Page 96 + --><span class="pagenum"><a name="page96"></a>[96]</span>beings, or of + any great and sudden modification in their structure.</p> + + <p> </p> + + <p><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 + (with the exception of the curious and not well-understood cases of + parthenogenesis) 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 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 + <!-- Page 97 --><span class="pagenum"><a + name="page97"></a>[97]</span>fertility to the offspring; and on the other + hand, that <i>close</i> interbreeding diminishes vigour and fertility; + that 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 <!-- Page 98 --><span class="pagenum"><a + name="page98"></a>[98]</span>one flower and then the stigma of another + with the same brush to ensure fertilisation; but it must not be 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 in this very genus, which seems to have a special contrivance for + self-fertilisation, it is well known that if 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 <!-- Page + 99 --><span class="pagenum"><a name="page99"></a>[99]</span>be no special + mechanical contrivance to prevent the stigma of a flower receiving its + own pollen, yet, as 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 <!-- Page 100 --><span + class="pagenum"><a name="page100"></a>[100]</span>innumerable flowers, it + may be objected that pollen could seldom be carried from tree to tree, + and at most only from flower 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 + <!-- Page 101 --><span class="pagenum"><a + name="page101"></a>[101]</span>currents in the water offer an obvious + means for an occasional cross. And, as in the case of flowers, I have as + yet 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. <!-- Page 102 --><span class="pagenum"><a + name="page102"></a>[102]</span></p> + + <p> </p> + + <p><i>Circumstances favourable to Natural Selection.</i>—This 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 + <!-- Page 103 --><span class="pagenum"><a + name="page103"></a>[103]</span>this case the effects of intercrossing can + hardly be counterbalanced 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 physical conditions at + least 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 <!-- Page 104 --><span class="pagenum"><a + name="page104"></a>[104]</span>obviously thus act far more efficiently + with those animals 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, &c.; 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 <!-- Page 105 --><span + class="pagenum"><a name="page105"></a>[105]</span>modifications 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 <!-- Page 106 + --><span class="pagenum"><a name="page106"></a>[106]</span>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 <!-- Page 107 --><span + class="pagenum"><a name="page107"></a>[107]</span>extermination. 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, + is the most favourable for the production of many new forms of life, + likely to endure long and to spread widely. For the area first 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 <!-- Page 108 --><span + class="pagenum"><a name="page108"></a>[108]</span>prevented, 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 always + acts 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, <!-- Page 109 --><span + class="pagenum"><a name="page109"></a>[109]</span>intermittent 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> </p> + + <p><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 ratio 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 <!-- Page 110 --><span class="pagenum"><a + name="page110"></a>[110]</span>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 <!-- Page + 111 --><span class="pagenum"><a name="page111"></a>[111]</span>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> </p> + + <p><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 <!-- Page 112 + --><span class="pagenum"><a name="page112"></a>[112]</span>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. <!-- Page + 113 --><span class="pagenum"><a name="page113"></a>[113]</span></p> + + <p>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. + <!-- Page 114 --><span class="pagenum"><a + name="page114"></a>[114]</span>Consequently, 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 <!-- Page 115 + --><span class="pagenum"><a name="page115"></a>[115]</span>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 may gain some crude idea in what manner some of + the natives would have to be modified, in order to gain an advantage over + the other natives; and we may at least safely infer that diversification + of structure, amounting to new generic differences, would be 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 Milne <!-- Page 116 --><span class="pagenum"><a + name="page116"></a>[116]</span>Edwards. No physiologist doubts that a + stomach 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.</p> + + <p>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 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, + <!-- Page 117 --><span class="pagenum"><a + name="page117"></a>[117]</span>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> + + <div class="figcenter" style="width:90%;"> + <a href="images/fp117.png"><img style="width:100%" src="images/fp117.png" + alt="Facing page 117." title="Facing page 117." /></a> + </div> + <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, <!-- + Page 118 --><span class="pagenum"><a name="page118"></a>[118]</span>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 + 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. <!-- Page 119 --><span class="pagenum"><a + name="page119"></a>[119]</span>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 <!-- Page 120 --><span class="pagenum"><a + name="page120"></a>[120]</span>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 + <!-- Page 121 --><span class="pagenum"><a + name="page121"></a>[121]</span>between 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 to + transmit 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 <!-- Page 122 + --><span class="pagenum"><a name="page122"></a>[122]</span>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. <!-- Page 123 --><span class="pagenum"><a + name="page123"></a>[123]</span></p> + + <p>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 alone, 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 <!-- Page 124 --><span class="pagenum"><a + name="page124"></a>[124]</span>more parent-species are supposed to have + descended from some one species of an earlier genus. In our diagram, this + is indicated by the broken lines, beneath the capital letters, converging + in sub-branches 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 <span class="scac">F</span><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 + (<span class="scac">F</span><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 <!-- Page 125 --><span class="pagenum"><a + name="page125"></a>[125]</span>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 <!-- Page 126 --><span + class="pagenum"><a name="page126"></a>[126]</span>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 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> </p> + + <p><i>Summary of Chapter.</i>—If during the long course of ages and + under varying conditions of life, organic beings <!-- Page 127 --><span + class="pagenum"><a name="page127"></a>[127]</span>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 ratio of increase of each + species, a severe struggle for life at some age, season, or year, 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 manner 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; and it leads to the + improvement of each creature in relation to its organic and inorganic + conditions of life.</p> + + <p>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 <!-- Page 128 + --><span class="pagenum"><a name="page128"></a>[128]</span>has acted in + the world's history, geology plainly declares. Natural selection, also, + leads to divergence of 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 to the inhabitants of any + small spot or to 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 for life. Thus the small differences + distinguishing varieties of the same species, 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 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 <!-- Page 129 --><span class="pagenum"><a + name="page129"></a>[129]</span>sub-families, families, orders, + sub-classes, and classes. The several subordinate groups in any class + cannot be 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 <!-- Page 130 --><span class="pagenum"><a + name="page130"></a>[130]</span>sizes may represent those whole orders, + families, and genera which have now no living representatives, and 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> + +<hr /> + +<p><!-- Page 131 --><span class="pagenum"><a name="page131"></a>[131]</span></p> + +<h3>CHAPTER V.</h3> + +<p class="cenhead"><span class="sc">Laws of Variation</span>.</p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 <!-- Page 132 --><span class="pagenum"><a + name="page132"></a>[132]</span>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, &c., 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, + &c.: 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 <!-- Page 133 + --><span class="pagenum"><a name="page133"></a>[133]</span>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 <span class="correction" title="text reads `slighest'" + >slightest</span> 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 <!-- Page 134 + --><span class="pagenum"><a name="page134"></a>[134]</span>the conditions + of life 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> </p> + + <p><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 <!-- + Page 135 --><span class="pagenum"><a + name="page135"></a>[135]</span>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 me 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 frequently + blown to sea and perish; that the beetles in Madeira, as observed by Mr. + Wollaston, lie much concealed, <!-- Page 136 --><span class="pagenum"><a + name="page136"></a>[136]</span>until the wind lulls and the sun shines; + that the proportion of wingless beetles is larger on the exposed Desertas + 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 would 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. <!-- Page 137 --><span class="pagenum"><a + name="page137"></a>[137]</span></p> + + <p>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 <!-- Page 138 --><span class="pagenum"><a + name="page138"></a>[138]</span>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 <!-- Page 139 --><span class="pagenum"><a + name="page139"></a>[139]</span>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> </p> + + <p><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, &c., 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. <!-- Page 140 --><span class="pagenum"><a + name="page140"></a>[140]</span>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 <!-- Page 141 --><span + class="pagenum"><a name="page141"></a>[141]</span>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 <!-- Page 142 --><span class="pagenum"><a + name="page142"></a>[142]</span>cautious 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, <!-- Page 143 + --><span class="pagenum"><a name="page143"></a>[143]</span>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 + variations.</p> + + <p> </p> + + <p><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 <!-- Page 144 --><span class="pagenum"><a + name="page144"></a>[144]</span>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 + covering, viz. Cetacea (whales) and Edentata (armadilloes, scaly + anteaters, &c.), 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 <!-- Page 145 --><span class="pagenum"><a + name="page145"></a>[145]</span>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 <!-- Page + 146 --><span class="pagenum"><a name="page146"></a>[146]</span>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 cœlospermous 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 <!-- Page + 147 --><span class="pagenum"><a name="page147"></a>[147]</span>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 <!-- Page 148 + --><span class="pagenum"><a name="page148"></a>[148]</span>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. <!-- Page 149 --><span class="pagenum"><a + name="page149"></a>[149]</span></p> + + <p>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 Prof. 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 <!-- Page 150 --><span class="pagenum"><a + name="page150"></a>[150]</span>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> </p> + + <p><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 <!-- Page 151 --><span class="pagenum"><a + name="page151"></a>[151]</span>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 <!-- Page 152 --><span class="pagenum"><a + name="page152"></a>[152]</span>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, &c., + 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 <!-- Page 153 --><span class="pagenum"><a + name="page153"></a>[153]</span>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 <!-- Page 154 + --><span class="pagenum"><a name="page154"></a>[154]</span>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 <!-- Page 155 --><span class="pagenum"><a + name="page155"></a>[155]</span>return 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 <!-- Page 156 --><span class="pagenum"><a + name="page156"></a>[156]</span>inheritance from a common 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 <!-- Page 157 + --><span class="pagenum"><a name="page157"></a>[157]</span>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 order to fit <!-- + Page 158 --><span class="pagenum"><a name="page158"></a>[158]</span>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 slight 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 having + been thus adapted for secondary sexual, and for ordinary specific + purposes. <!-- Page 159 --><span class="pagenum"><a + name="page159"></a>[159]</span></p> + + <p> </p> + + <p><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 <i>vera causa</i> 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 <!-- Page 160 --><span class="pagenum"><a + name="page160"></a>[160]</span>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 <!-- Page 161 --><span class="pagenum"><a + name="page161"></a>[161]</span>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 <!-- + Page 162 --><span class="pagenum"><a + name="page162"></a>[162]</span>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 <!-- Page 163 --><span + class="pagenum"><a name="page163"></a>[163]</span>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 the 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 <!-- Page 164 --><span + class="pagenum"><a name="page164"></a>[164]</span>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 racehorse 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 <!-- Page + 165 --><span class="pagenum"><a name="page165"></a>[165]</span>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, &c., + 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: according to Mr. Gosse, in + certain parts of the United States about nine out of ten mules have + striped legs. I once saw a mule with its legs so much striped that any + one would at first 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 Morton'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 + his 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 <!-- Page 166 --><span + class="pagenum"><a name="page166"></a>[166]</span>accident, that I was + led solely from the occurrence of the face-stripes on this hybrid from + the ass and hemionus 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 <!-- Page 167 --><span class="pagenum"><a + name="page167"></a>[167]</span>centuries, species; and how exactly + parallel is the case with that of the species of the horse-genus! 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> </p> + + <p><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 + <!-- Page 168 --><span class="pagenum"><a + name="page168"></a>[168]</span>climate and food, &c., seem to have + induced some slight modifications. Habit in producing constitutional + differences, 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; <!-- Page 169 --><span + class="pagenum"><a name="page169"></a>[169]</span>for in a district where + many species of any genus are found—that is, where there has been + much former 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 <!-- Page 170 --><span + class="pagenum"><a name="page170"></a>[170]</span>variation, such + modifications will add to the beautiful and harmonious diversity of + nature.</p> + + <p>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> + +<hr /> + +<p><!-- Page 171 --><span class="pagenum"><a name="page171"></a>[171]</span></p> + +<h3>CHAPTER VI.</h3> + +<p class="cenhead"><span class="sc">Difficulties on Theory.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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:—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 <!-- Page 172 --><span class="pagenum"><a + name="page172"></a>[172]</span>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 has 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> </p> + + <p><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 <!-- Page 173 --><span class="pagenum"><a + name="page173"></a>[173]</span>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 <!-- Page 174 + --><span class="pagenum"><a name="page174"></a>[174]</span>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 <!-- + Page 175 --><span class="pagenum"><a name="page175"></a>[175]</span>it is + quite remarkable how abruptly, as Alph. de Candolle has observed, a + common alpine species disappears. The same fact has been noticed by E. + 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, <!-- Page 176 --><span + class="pagenum"><a name="page176"></a>[176]</span>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 <!-- Page 177 --><span class="pagenum"><a + name="page177"></a>[177]</span>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 <!-- Page 178 --><span + class="pagenum"><a name="page178"></a>[178]</span>variations chance to + occur, and until a place in the natural polity 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 <!-- Page 179 --><span + class="pagenum"><a name="page179"></a>[179]</span>tend to connect. From + this cause alone the intermediate 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> </p> + + <p><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 <!-- + Page 180 --><span class="pagenum"><a name="page180"></a>[180]</span>dives + for and preys on fish, but during the long winter 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 <!-- Page 181 + --><span class="pagenum"><a name="page181"></a>[181]</span>rodents or new + beasts of prey immigrate, or old ones 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 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 + was useful to its possessor. Nor can I see any insuperable difficulty in + further believing it possible that the membrane-connected fingers and + forearm 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. <!-- Page 182 --><span class="pagenum"><a + name="page182"></a>[182]</span></p> + + <p>If about a dozen genera of birds had become extinct or were unknown, + who would have ventured to have 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 <!-- Page 183 --><span class="pagenum"><a + name="page183"></a>[183]</span>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 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 <!-- Page 184 + --><span class="pagenum"><a name="page184"></a>[184]</span>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 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, almost like a whale, insects + in the water.</p> + + <p>As we sometimes see individuals of a species following habits widely + different from those 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 berardi, in its general + habits, in its astonishing power of diving, its manner of swimming, and + of flying when <!-- Page 185 --><span class="pagenum"><a + name="page185"></a>[185]</span>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 Creator to cause a being of one + type to take the place of one of another type; but this seems to me only + <!-- Page 186 --><span class="pagenum"><a + name="page186"></a>[186]</span>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, 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> </p> + + <p><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 selection, though insuperable by our imagination, can + hardly be considered real. How a nerve comes to be sensitive to <!-- Page + 187 --><span class="pagenum"><a name="page187"></a>[187]</span>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 and must act by convergence; and at their lower ends + there seems to be an imperfect vitreous substance. <!-- Page 188 --><span + class="pagenum"><a name="page188"></a>[188]</span>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 part of this layer to be continually changing <!-- + Page 189 --><span class="pagenum"><a + name="page189"></a>[189]</span>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 <!-- Page 190 --><span + class="pagenum"><a name="page190"></a>[190]</span>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 <!-- Page 191 --><span class="pagenum"><a + name="page191"></a>[191]</span>which 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, <!-- Page 192 --><span class="pagenum"><a + name="page192"></a>[192]</span>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, <!-- Page 193 --><span class="pagenum"><a + name="page193"></a>[193]</span>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 + <span class="correction" title="text reads `Matteucei' - index has Matteucci" + >Matteucci</span> 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 <!-- Page 194 --><span class="pagenum"><a + name="page194"></a>[194]</span>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 beings 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 organs could have arrived at their 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 but somewhat exaggerated 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 commonly 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> </p> + + <p><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 <!-- Page 195 --><span + class="pagenum"><a name="page195"></a>[195]</span>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 its + 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 + 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 <!-- Page 196 --><span class="pagenum"><a + name="page196"></a>[196]</span>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 + swimbladders 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, &c., 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 <!-- Page 197 + --><span class="pagenum"><a name="page197"></a>[197]</span>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 <!-- Page 198 --><span + class="pagenum"><a name="page198"></a>[198]</span>immediately made + conscious of this by reflecting on the differences in the breeds of our + domesticated animals in different countries,—more especially in the + less civilised 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 ignorance of the precise cause <!-- + Page 199 --><span class="pagenum"><a name="page199"></a>[199]</span>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 <!-- Page 200 --><span class="pagenum"><a + name="page200"></a>[200]</span>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 nipper 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 + nipper, 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, + &c. 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 <!-- Page 201 --><span class="pagenum"><a + name="page201"></a>[201]</span><span class="correction" title="text reads `depo-sisted' across page break" + >deposited</span> 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 <!-- + Page 202 --><span class="pagenum"><a name="page202"></a>[202]</span>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 <!-- Page 203 + --><span class="pagenum"><a name="page203"></a>[203]</span>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> </p> + + <p><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 serious; 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 <!-- Page 204 --><span class="pagenum"><a + name="page204"></a>[204]</span>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 <!-- Page 205 --><span class="pagenum"><a + name="page205"></a>[205]</span>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 <!-- Page 206 --><span class="pagenum"><a + name="page206"></a>[206]</span>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> + +<hr /> + +<p><!-- Page 207 --><span class="pagenum"><a name="page207"></a>[207]</span></p> + +<h3>CHAPTER VII.</h3> + +<p class="cenhead"><span class="sc">Instinct.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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. <!-- Page 208 --><span + class="pagenum"><a name="page208"></a>[208]</span>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. <!-- Page 209 --><span + class="pagenum"><a name="page209"></a>[209]</span></p> + + <p>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 <!-- Page 210 + --><span class="pagenum"><a name="page210"></a>[210]</span>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. 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 &c.; 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, and prevented their + attendance during several hours. After this interval, I felt sure that + the aphides <!-- Page 211 --><span class="pagenum"><a + name="page211"></a>[211]</span>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 be here given; + but want of space prevents me. I can only assert, that instincts + certainly do vary—for instance, the migratory instinct, both in + extent and direction, and in its total loss. So it is with the nests of + birds, which vary partly <!-- Page 212 --><span class="pagenum"><a + name="page212"></a>[212]</span>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 the 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 domestication. We shall thus also be enabled to see the + respective parts which habit and the selection of <!-- Page 213 --><span + class="pagenum"><a name="page213"></a>[213]</span>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 are + inherited, and how curiously they become mingled, is well shown when + different breeds of dogs are <!-- Page 214 --><span class="pagenum"><a + name="page214"></a>[214]</span>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: the act of pointing is probably, as many have thought, + only the exaggerated pause of an animal preparing to spring on its prey. + When the first tendency to point was once displayed, methodical selection + and the inherited effects of compulsory training in each successive + generation would soon complete the <!-- Page 215 --><span + class="pagenum"><a name="page215"></a>[215]</span>work; and unconscious + 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 + <!-- Page 216 --><span class="pagenum"><a + name="page216"></a>[216]</span>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 <!-- Page 217 --><span class="pagenum"><a + name="page217"></a>[217]</span>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, + <!-- Page 218 --><span class="pagenum"><a + name="page218"></a>[218]</span>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 <!-- Page 219 --><span + class="pagenum"><a name="page219"></a>[219]</span>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> </p> + + <p><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 larvae 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>Another species, 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 <!-- Page 220 --><span class="pagenum"><a + name="page220"></a>[220]</span>the British 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 + (F. fusca) 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, through 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 <!-- Page + 221 --><span class="pagenum"><a name="page221"></a>[221]</span>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 witnessed a migration of F. sanguinea from one + nest to another, and it was a most interesting spectacle to behold the + masters carefully carrying (instead of being carried by, as in the case + of F. rufescens) 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. <!-- Page 222 --><span class="pagenum"><a + name="page222"></a>[222]</span></p> + + <p>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 returning home and entering their nests, carrying + the dead bodies of F. fusca (showing that it was not a migration) and + numerous pupæ. I traced a long file of ants 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 <!-- Page 223 --><span class="pagenum"><a + name="page223"></a>[223]</span>agitation, and one was perched motionless + with its own pupa in its mouth on the top of a spray of heath, an image + of despair, 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 continental 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 such pupæ originally stored as food might + become developed; and the foreign ants thus unintentionally reared would + then follow their proper instincts, and do <!-- Page 224 --><span + class="pagenum"><a name="page224"></a>[224]</span>what work they could. + If their presence proved useful to the species which had seized + them—if it were more advantageous to this species to capture + workers than to procreate them—the habit of collecting pupae + 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> </p> + + <p><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 <!-- Page 225 --><span class="pagenum"><a + name="page225"></a>[225]</span>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>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 fit 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 <!-- Page 226 --><span + class="pagenum"><a name="page226"></a>[226]</span>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 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 bases 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 × √2, or radius × 1.41421 (or at some lesser distance), from + the centres of the six surrounding spheres in the same <!-- Page 227 + --><span class="pagenum"><a name="page227"></a>[227]</span>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 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 <!-- Page 228 --><span + class="pagenum"><a name="page228"></a>[228]</span>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 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 scalloped 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 <!-- Page 229 --><span class="pagenum"><a + name="page229"></a>[229]</span>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 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 <!-- Page 230 --><span class="pagenum"><a + name="page230"></a>[230]</span>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 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 + <!-- Page 231 --><span class="pagenum"><a + name="page231"></a>[231]</span>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 + 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, <!-- Page 232 --><span class="pagenum"><a + name="page232"></a>[232]</span>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 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 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 <!-- Page 233 --><span class="pagenum"><a + name="page233"></a>[233]</span>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 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 <!-- Page 234 + --><span class="pagenum"><a name="page234"></a>[234]</span>are consumed + by a hive of bees for the secretion of each pound of wax; to that a + prodigious quantity of fluid nectar must be collected and consumed by the + bees in a hive for 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 <!-- Page 235 --><span + class="pagenum"><a name="page235"></a>[235]</span>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 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> </p> + + <p>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 <!-- Page 236 + --><span class="pagenum"><a name="page236"></a>[236]</span>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 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 <!-- Page 237 --><span class="pagenum"><a + name="page237"></a>[237]</span>in the ordinary state, I should have + unhesitatingly assumed that all its characters had been slowly acquired + through natural selection; namely, by an individual 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 <!-- + Page 238 --><span class="pagenum"><a + name="page238"></a>[238]</span>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 + 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 <!-- Page 239 --><span + class="pagenum"><a name="page239"></a>[239]</span>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, 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 <!-- Page 240 --><span + class="pagenum"><a name="page240"></a>[240]</span>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 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 <!-- Page 241 --><span class="pagenum"><a + name="page241"></a>[241]</span>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 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 <!-- Page 242 --><span + class="pagenum"><a name="page242"></a>[242]</span>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 + labour is useful to civilised man. As ants work by inherited instincts + and by inherited organs or tools, 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 affect 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> </p> + + <p><i>Summary.</i>—I have endeavoured briefly in this chapter <!-- + Page 243 --><span class="pagenum"><a name="page243"></a>[243]</span>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 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 <!-- Page 244 --><span + class="pagenum"><a name="page244"></a>[244]</span>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 cuckoo + ejecting its foster-brothers,—ants making slaves,—the larvae + 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> + +<hr /> + +<p><!-- Page 245 --><span class="pagenum"><a name="page245"></a>[245]</span></p> + +<h3>CHAPTER VIII.</h3> + +<p class="cenhead"><span class="sc">Hybridism.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 species <!-- Page 246 --><span class="pagenum"><a + name="page246"></a>[246]</span>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 unhesitatingly ranks <!-- Page 247 --><span + class="pagenum"><a name="page247"></a>[247]</span>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 cœrulea), 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. <!-- Page 248 --><span + class="pagenum"><a name="page248"></a>[248]</span></p> + + <p>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 <!-- Page 249 --><span class="pagenum"><a + name="page249"></a>[249]</span>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 a 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 Hon. and <!-- Page 250 --><span + class="pagenum"><a name="page250"></a>[250]</span>Rev. 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 of certain species of Lobelia + and of some other genera, which can be far more easily fertilised by the + pollen of another and distinct species, than by their own pollen; and all + the individuals of nearly all the species of Hippeastrum seem to be in + this predicament. 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 <!-- Page 251 --><span class="pagenum"><a + name="page251"></a>[251]</span>species: the result was that "the ovaries + of the three first flowers soon ceased to grow, and after 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, &c., 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 <!-- Page 252 --><span + class="pagenum"><a name="page252"></a>[252]</span>are perfectly fertile. + Mr. C. Noble, for instance, informs me that he raises stocks for grafting + from a hybrid between Rhod. 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, <!-- + Page 253 --><span class="pagenum"><a + name="page253"></a>[253]</span>should be perfectly fertile. Again, with + respect to the fertility in successive generations of the more fertile + 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. + There is no doubt that these three pheasants, namely, the common, the + true ring-necked, and the Japan, intercross, and are becoming blended + together in the woods of several parts of England. 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 <!-- Page 254 --><span class="pagenum"><a + name="page254"></a>[254]</span>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 largely accepted by + modern naturalists; namely, that most of our domestic animals have + descended from two or more wild 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 <!-- Page 255 --><span class="pagenum"><a + name="page255"></a>[255]</span>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> </p> + + <p><i>Laws governing the Sterility of first Crosses and of + Hybrids.</i>—We will now consider a little more in detail the + 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 <!-- + Page 256 --><span class="pagenum"><a name="page256"></a>[256]</span>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 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. <!-- Page 257 + --><span class="pagenum"><a name="page257"></a>[257]</span></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 + 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 <!-- Page 258 + --><span class="pagenum"><a name="page258"></a>[258]</span>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>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 jalapa 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. + jalapa, 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 <!-- + Page 259 --><span class="pagenum"><a name="page259"></a>[259]</span>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 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 <!-- Page 260 --><span + class="pagenum"><a name="page260"></a>[260]</span>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 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, <!-- Page 261 + --><span class="pagenum"><a name="page261"></a>[261]</span>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, 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, &c.; 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 <!-- Page 262 --><span + class="pagenum"><a name="page262"></a>[262]</span>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 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, &c., which + seeded much more freely when fertilised with the pollen of distinct + species, than when self-fertilised with their own pollen. <!-- Page 263 + --><span class="pagenum"><a name="page263"></a>[263]</span></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 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> </p> + + <p><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 <!-- Page + 264 --><span class="pagenum"><a name="page264"></a>[264]</span>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 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 <!-- Page 265 --><span + class="pagenum"><a name="page265"></a>[265]</span>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 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 exotic 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 <!-- Page 266 --><span + class="pagenum"><a name="page266"></a>[266]</span>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 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 <!-- Page 267 + --><span class="pagenum"><a name="page267"></a>[267]</span>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 farmers and gardeners in their + frequent exchanges of seed, tubers, &c., 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 <!-- Page 268 --><span + class="pagenum"><a name="page268"></a>[268]</span>common but unknown + bond, which is essentially related to the principle of life.</p> + + <p> </p> + + <p><i>Fertility of Varieties when crossed, and of their Mongrel + offspring.</i>—It may be urged, as a most forcible argument, 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 every one, 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 <!-- Page 269 --><span class="pagenum"><a + name="page269"></a>[269]</span>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 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 <!-- Page 270 + --><span class="pagenum"><a name="page270"></a>[270]</span>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 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 <!-- Page 271 --><span class="pagenum"><a + name="page271"></a>[271]</span>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 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 similarly 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 <!-- Page 272 --><span class="pagenum"><a + name="page272"></a>[272]</span>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 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> </p> + + <p><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 + <!-- Page 273 --><span class="pagenum"><a + name="page273"></a>[273]</span>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; 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 superadded 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 <!-- Page 274 --><span + class="pagenum"><a name="page274"></a>[274]</span>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 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 <!-- Page 275 --><span class="pagenum"><a + name="page275"></a>[275]</span>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 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, <!-- Page 276 + --><span class="pagenum"><a name="page276"></a>[276]</span>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 astonishing fact. + But it harmonises perfectly with the view that there is no essential + distinction between species and varieties.</p> + + <p> </p> + + <p><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 + <!-- Page 277 --><span class="pagenum"><a + name="page277"></a>[277]</span>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 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 from it, 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 <!-- Page 278 --><span class="pagenum"><a + name="page278"></a>[278]</span>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 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> + +<hr /> + +<p><!-- Page 279 --><span class="pagenum"><a name="page279"></a>[279]</span></p> + +<h3>CHAPTER IX.</h3> + +<p class="cenhead"><span class="sc">On the Imperfection of the Geological Record.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 <!-- Page 280 --><span class="pagenum"><a + name="page280"></a>[280]</span>depends 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 <!-- Page 281 --><span class="pagenum"><a + name="page281"></a>[281]</span>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 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 <!-- Page 282 --><span class="pagenum"><a + name="page282"></a>[282]</span>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> </p> + + <p><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 incomprehensively 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 <!-- Page 283 --><span + class="pagenum"><a name="page283"></a>[283]</span>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 good evidence 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. In the Cordillera I estimated + one pile of conglomerate at ten thousand feet in thickness. Let the <!-- + Page 284 --><span class="pagenum"><a + name="page284"></a>[284]</span>observer remember Lyell's profound remark + that the thickness and extent of sedimentary formations 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> + + +<table width="43%" class="nobctr" summary="thickness of strata" title="thickness of strata"> + <tr> + <td class="spacsingle" style="text-align:right" colspan="2"> + <p>Feet.</p> + </td> + </tr> + + <tr> + <td class="spacsingle" style="text-align:left; width:85%"> + <p>Palæozoic strata (not including igneous beds)</p> + </td> + <td class="spacsingle" style="text-align:right; width:14%"> + <p>57,154</p> + </td> + </tr> + + <tr> + <td class="spacsingle" style="text-align:left"> + <p>Secondary strata</p> + </td> + <td class="spacsingle" style="text-align:right"> + <p>13,190</p> + </td> + </tr> + + <tr> + <td class="spacsingle" style="text-align:left"> + <p>Tertiary strata</p> + </td> + <td class="spacsingle" style="text-align:right"> + <p>2,240</p> + </td> + </tr> +</table> + + <p>—making altogether 72,584 feet; that is, very nearly thirteen + and three-quarters British miles. Some of the 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 has no pretension to strict + exactness; 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 <!-- Page 285 --><span class="pagenum"><a + name="page285"></a>[285]</span>having been much struck with the evidence + of denudation, when viewing volcanic islands, which have been 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. Prof. 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 Prof. Ramsay's masterly memoir on this subject: + yet it is an admirable lesson to stand on the intermediate hilly country + and look on the one hand at the North Downs, and <!-- Page 286 --><span + class="pagenum"><a name="page286"></a>[286]</span>on the other hand at + the 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 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 Prof. 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 <!-- Page 287 --><span + class="pagenum"><a name="page287"></a>[287]</span>must remember that + almost all strata contain harder layers or nodules, which from long + resisting attrition form a breakwater at the base. We may at least + confidently believe that no rocky coast 500 feet in height commonly + yields at the rate of a foot per century; for this would be the same in + amount as a cliff one yard in height retreating twelve yards in + twenty-two years; and no one, I think, who has carefully observed the + shape of old fallen fragments at the base of cliffs, will admit any near + approach to such rapid wearing away. Hence, under ordinary circumstances, + I should infer that for a cliff 500 feet in height, a denudation of one + inch per century for the whole length would be a sufficient 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. But + perhaps it would be safer to allow two or three inches per century, and + this would reduce the number of years to one hundred and fifty or one + 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 it is not + improbable that a 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, <!-- Page 288 --><span class="pagenum"><a + name="page288"></a>[288]</span>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> </p> + + <p><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 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 + <!-- Page 289 --><span class="pagenum"><a + name="page289"></a>[289]</span>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 subfamily 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 remains is fragmentary in an extreme degree. For + instance, not a land shell is known belonging to either of these vast + periods, with the exception of one species discovered by Sir C. Lyell and + Dr. Dawson in the carboniferous strata of North America, of which shell + several specimens have now been collected. 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 <!-- Page 290 --><span class="pagenum"><a + name="page290"></a>[290]</span>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 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 poorly developed, that no + record of several <!-- Page 291 --><span class="pagenum"><a + name="page291"></a>[291]</span>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, 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 <!-- Page + 292 --><span class="pagenum"><a name="page292"></a>[292]</span>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 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 subsequently but 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 <!-- Page 293 --><span class="pagenum"><a + name="page293"></a>[293]</span>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, 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 <!-- Page 294 --><span class="pagenum"><a + name="page294"></a>[294]</span>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 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; <!-- Page 295 --><span class="pagenum"><a + name="page295"></a>[295]</span>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 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 <!-- Page 296 --><span class="pagenum"><a + name="page296"></a>[296]</span>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 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 <!-- Page 297 --><span + class="pagenum"><a name="page297"></a>[297]</span>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 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 <!-- Page 298 --><span class="pagenum"><a + name="page298"></a>[298]</span>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, 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 <!-- Page 299 --><span + class="pagenum"><a name="page299"></a>[299]</span>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 + 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 <!-- Page 300 --><span + class="pagenum"><a name="page300"></a>[300]</span>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, whilst most + of our formations were accumulating. The Malay Archipelago is one of the + richest regions of the 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>I believe that fossiliferous formations could be formed <!-- Page 301 + --><span class="pagenum"><a name="page301"></a>[301]</span>in the + archipelago, of thickness sufficient 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 + 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 <!-- Page 302 --><span + class="pagenum"><a name="page302"></a>[302]</span>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, 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> </p> + + <p><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 <!-- Page 303 --><span + class="pagenum"><a name="page303"></a>[303]</span>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 + probably elapsed between our consecutive formations,—longer perhaps + in most 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 <!-- Page 304 + --><span class="pagenum"><a name="page304"></a>[304]</span>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, for its thickness, 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. Had it not been for the rare + accident of the preservation of footsteps in the new red sandstone of the + United States, who would have ventured to suppose that, besides reptiles, + no less than at least thirty kinds of birds, some of gigantic size, + existed during that period? Not a fragment of bone has been discovered in + these beds. Notwithstanding that the number of joints shown in the fossil + impressions correspond with the number in the several toes of living + birds' feet, some authors doubt whether the animals which left the + impressions were really birds. Until quite recently these authors might + have maintained, and some have maintained, that the whole class of birds + came suddenly into existence during an early tertiary period; but now we + know, on the authority of Professor Owen (as may be seen in Lyell's + 'Manual'), that a bird certainly lived during the deposition of the upper + greensand.</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 <!-- Page 305 --><span class="pagenum"><a + name="page305"></a>[305]</span>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 + then 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 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 <!-- Page 306 --><span class="pagenum"><a + name="page306"></a>[306]</span>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 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 <!-- Page 307 --><span class="pagenum"><a + name="page307"></a>[307]</span>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> </p> + + <p><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, + &c., 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. 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. <!-- Page 308 --><span + class="pagenum"><a name="page308"></a>[308]</span></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 E. 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 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 always 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 <!-- Page 309 + --><span class="pagenum"><a name="page309"></a>[309]</span>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 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 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 <!-- Page 310 --><span + class="pagenum"><a name="page310"></a>[310]</span>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 the beginning of this world? 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 + 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> </p> + + <p>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 <!-- Page + 311 --><span class="pagenum"><a name="page311"></a>[311]</span>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, Agassiz, Barrande, Falconer, E. Forbes, + &c., and all our greatest geologists, as Lyell, Murchison, Sedgwick, + &c., 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 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 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> + +<hr /> + +<p><!-- Page 312 --><span class="pagenum"><a name="page312"></a>[312]</span></p> + +<h3>CHAPTER X.</h3> + +<p class="cenhead"><span class="sc">On the Geological Succession of Organic Beings.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 <!-- Page 313 --><span class="pagenum"><a + name="page313"></a>[313]</span>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. <!-- Page 314 --><span + class="pagenum"><a name="page314"></a>[314]</span></p> + + <p>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, <!-- Page 315 --><span + class="pagenum"><a name="page315"></a>[315]</span>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 + exterminated by its improved offspring, it is quite <!-- Page 316 + --><span class="pagenum"><a name="page316"></a>[316]</span>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 <span + class="correction" title="text reads `faintail'">fantail</span> 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 number of the species of + a genus, or the number of <!-- Page 317 --><span class="pagenum"><a + name="page317"></a>[317]</span>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> </p> + + <p><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, &c., 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 finally from + the world. Both single species and whole <!-- Page 318 --><span + class="pagenum"><a name="page318"></a>[318]</span>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 how utterly groundless was my astonishment! <!-- Page 319 + --><span class="pagenum"><a name="page319"></a>[319]</span>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 + been exterminated, either locally or wholly, through <!-- Page 320 + --><span class="pagenum"><a name="page320"></a>[320]</span>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 specific 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. <!-- Page 321 --><span class="pagenum"><a + name="page321"></a>[321]</span>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 <!-- Page 322 + --><span class="pagenum"><a name="page322"></a>[322]</span>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> </p> + + <p><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 <!-- Page 323 --><span class="pagenum"><a + name="page323"></a>[323]</span>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 co-existed + with still living sea-shells; but as these anomalous monsters co-existed + with the <!-- Page 324 --><span class="pagenum"><a + name="page324"></a>[324]</span>Mastodon and Horse, it might at least have + been inferred that they had lived during one of the later 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. <!-- Page 325 --><span class="pagenum"><a + name="page325"></a>[325]</span>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 <!-- Page 326 --><span + class="pagenum"><a name="page326"></a>[326]</span>dominant, 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 + <!-- Page 327 --><span class="pagenum"><a + name="page327"></a>[327]</span>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 <!-- Page 328 --><span + class="pagenum"><a name="page328"></a>[328]</span>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 occur 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 <!-- Page 329 --><span class="pagenum"><a + name="page329"></a>[329]</span>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> </p> + + <p><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 + palaeontologist, 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 + <!-- Page 330 --><span class="pagenum"><a + name="page330"></a>[330]</span>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 Palaeozoic + 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 <!-- Page 331 --><span + class="pagenum"><a name="page331"></a>[331]</span>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 modified in relation to its slightly + altered conditions of <!-- Page 332 --><span class="pagenum"><a + name="page332"></a>[332]</span>life, and yet retain throughout a vast + period the same general characteristics. This is represented in the + diagram by the letter <span class="scac">F</span><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 No. VI.—but none + from beneath this line, then only the two families on the left hand + (namely, <i>a</i><sup>14</sup>, &c., and + <i>b</i><sup>14</sup>, &c.) 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 discovery of the fossils. If, for + instance, we suppose the existing genera of the two families to differ + from each <!-- Page 333 --><span class="pagenum"><a + name="page333"></a>[333]</span>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 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 <!-- Page 334 --><span + class="pagenum"><a name="page334"></a>[334]</span>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 in any one region for + the immigration of new forms from other regions, 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 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 <!-- Page 335 --><span class="pagenum"><a + name="page335"></a>[335]</span>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 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 <!-- Page + 336 --><span class="pagenum"><a name="page336"></a>[336]</span>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> </p> + + <p><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. The best definition probably is, that the higher forms have their + organs more distinctly specialised for different functions; and as such + division of physiological labour seems to be an advantage to each being, + natural selection will constantly tend in so far to make the later and + more modified forms higher than their early progenitors, or than the + slightly modified descendants of such progenitors. In a more general + sense the <!-- Page 337 --><span class="pagenum"><a + name="page337"></a>[337]</span>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 <!-- Page 338 --><span class="pagenum"><a + name="page338"></a>[338]</span>species 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> </p> + + <p><i>On the Succession of the same Types within the same <!-- Page 339 + --><span class="pagenum"><a name="page339"></a>[339]</span>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, <!-- Page 340 --><span class="pagenum"><a + name="page340"></a>[340]</span>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. <!-- Page 341 --><span class="pagenum"><a + name="page341"></a>[341]</span></p> + + <p>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 old 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> </p> + + <p><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 <!-- Page 342 --><span class="pagenum"><a + name="page342"></a>[342]</span>only 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 <!-- Page + 343 --><span class="pagenum"><a name="page343"></a>[343]</span>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 <!-- Page 344 --><span + class="pagenum"><a name="page344"></a>[344]</span>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 <!-- Page 345 --><span class="pagenum"><a + name="page345"></a>[345]</span>become 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> + +<hr /> + +<p><!-- Page 346 --><span class="pagenum"><a name="page346"></a>[346]</span></p> + +<h3>CHAPTER XI.</h3> + +<p class="cenhead"><span class="sc">Geographical Distribution.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 <!-- Page 347 --><span + class="pagenum"><a name="page347"></a>[347]</span>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 + <!-- Page 348 --><span class="pagenum"><a + name="page348"></a>[348]</span>lofty and continuous mountain-ranges, and + of great deserts, and sometimes even of large rivers, we find different + productions; though as mountain-chains, deserts, &c., 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, or continuous coasts, + 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 <!-- Page 349 --><span class="pagenum"><a + name="page349"></a>[349]</span>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 emu, 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 <!-- Page 350 --><span + class="pagenum"><a name="page350"></a>[350]</span>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, <!-- Page 351 --><span + class="pagenum"><a name="page351"></a>[351]</span>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 <!-- Page 352 + --><span class="pagenum"><a name="page352"></a>[352]</span>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 <!-- Page 353 --><span class="pagenum"><a + name="page353"></a>[353]</span>Australia or South America? The conditions + of life are 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 <!-- Page 354 --><span class="pagenum"><a + name="page354"></a>[354]</span>formerly continuous range of many species. + So that we are reduced to consider whether the exceptions to 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 + <!-- Page 355 --><span class="pagenum"><a + name="page355"></a>[355]</span>most of its inhabitants are closely + related to, or belong to the same genera with the species of a second + region, 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 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 <!-- Page 356 --><span class="pagenum"><a + name="page356"></a>[356]</span>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> </p> + + <p><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 <!-- Page 357 --><span + class="pagenum"><a name="page357"></a>[357]</span>possibly even + continents together, and thus have allowed terrestrial productions to + pass from one to the other. 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 <!-- Page 358 --><span class="pagenum"><a + name="page358"></a>[358]</span>recent period continents which are now + quite separate, have been continuously, or almost continuously, united + 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 <!-- Page 359 + --><span class="pagenum"><a name="page359"></a>[359]</span>out of 87 + kinds, 64 germinated after an immersion of 28 days, and a few survived an + immersion of 137 days. 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, &c., and some of these floated for a long time. It + is well known what a difference there is in the buoyancy of green and + seasoned timber; and it occurred to me that floods might wash down plants + or branches, and that these might be dried on the banks, and then by a + fresh rise in the stream be washed into the sea. Hence I was led to dry + stems and branches of 94 plants with ripe fruit, and to place them on + sea-water. The majority sank quickly, but some which whilst green floated + for a very short time, when dried floated much longer; for instance, ripe + hazel-nuts sank immediately, but when dried they floated for 90 days, and + afterwards when planted they germinated; an asparagus plant with ripe + berries floated for 23 days, when dried it floated for 85 days, and the + seeds afterwards 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 <!-- Page 360 --><span + class="pagenum"><a name="page360"></a>[360]</span>rate of the several + Atlantic currents is 33 miles per diem (some currents running at the rate + of 60 miles 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, <!-- Page 361 --><span + class="pagenum"><a name="page361"></a>[361]</span>even on those in the + midst of the widest oceans; and the natives of the coral-islands in the + Pacific, procure 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 + 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. <!-- Page 362 --><span class="pagenum"><a + name="page362"></a>[362]</span>But the following fact is more important: + the crops of birds do not secrete gastric juice, and do not in the 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 <!-- Page 363 --><span class="pagenum"><a + name="page363"></a>[363]</span>of dry argillaceous earth from one foot of + a partridge, and in this earth there was a pebble quite as large as 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 <!-- Page 364 --><span + class="pagenum"><a name="page364"></a>[364]</span>doubt remain to be + discovered, have been in action year after year, for centuries and tens + of thousands of 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 + sea-water; 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 <!-- Page 365 --><span + class="pagenum"><a name="page365"></a>[365]</span>(and it would be very + difficult to prove this), received within the last few centuries, through + occasional means 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, if fitted for the climate, would be sure to germinate + and survive.</p> + + <p> </p> + + <p><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 <!-- Page 366 --><span class="pagenum"><a + name="page366"></a>[366]</span>same species must have been independently + created at several distinct points; and we might have remained 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 <!-- Page 367 --><span class="pagenum"><a + name="page367"></a>[367]</span>its maximum, we should have a uniform + arctic fauna and flora, covering the central parts of Europe, as far + 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 <!-- Page + 368 --><span class="pagenum"><a name="page368"></a>[368]</span>have been + due south and north. The Alpine plants, for example, of Scotland, as + remarked by Mr. H. C. Watson, 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, <!-- Page 369 --><span class="pagenum"><a + name="page369"></a>[369]</span>first at the bases and ultimately on the + summits of the mountains, the case will have been somewhat different; 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 <!-- Page 370 --><span + class="pagenum"><a name="page370"></a>[370]</span>Atlantic Ocean and by + the extreme northern part of the Pacific. During the Glacial period, when + the inhabitants 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 <!-- Page 371 + --><span class="pagenum"><a name="page371"></a>[371]</span>New Worlds, + began slowly to migrate southwards as the climate became less warm, long + before the commencement 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 intermigration 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 + <!-- Page 372 --><span class="pagenum"><a + name="page372"></a>[372]</span>the now living productions of the + temperate regions of the New and Old Worlds, we find very few identical + 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. <!-- Page 373 --><span class="pagenum"><a + name="page373"></a>[373]</span></p> + + <p>But we must return to our more immediate subject, the Glacial period. + I am convinced that Forbes's view 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 + that 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°; 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 Chili 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 <!-- Page 374 --><span + class="pagenum"><a name="page374"></a>[374]</span>opposite sides of the + world. But we have good evidence in almost every case, that the epoch was + included within 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 + <!-- Page 375 --><span class="pagenum"><a + name="page375"></a>[375]</span>intervening hot countries. So on the Silla + of Caraccas the illustrious Humboldt long ago found species belonging 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 <!-- Page 376 --><span + class="pagenum"><a name="page376"></a>[376]</span>more lofty mountains, + and on the temperate lowlands of the northern and southern hemispheres, + are sometimes 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, Prof. Dana, that "it is certainly a + wonderful fact that New Zealand should have a closer resemblance in its + Crustacea to Great Britain, its antipode, than to any other part of the + world." Sir J. Richardson, also, speaks of the reappearance on the shores + of New Zealand, Tasmania, &c., 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 <!-- Page 377 --><span + class="pagenum"><a name="page377"></a>[377]</span>body of geological + evidence, that the whole world, or a large part of it, was during the + Glacial period simultaneously much colder than at present. The Glacial + period, as measured by years, must have been very long; and when we + remember over what vast spaces some naturalised plants and animals have + spread 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 lowest, most protected, and warmest districts. 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 <!-- Page 378 --><span class="pagenum"><a + name="page378"></a>[378]</span>even crossed the equator. The invasion + would, of course, have been greatly favoured by high land, and perhaps 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 <!-- Page 379 --><span class="pagenum"><a + name="page379"></a>[379]</span>lowlands; those which had not reached the + equator would re-migrate northward or southward towards their former + 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 + <!-- Page 380 --><span class="pagenum"><a + name="page380"></a>[380]</span>were enabled to beat the less powerful + southern forms. Just in the same manner as we see at the present day, + 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 <!-- Page 381 --><span class="pagenum"><a + name="page381"></a>[381]</span>indicate the exact lines and means of + migration, or the reason why certain species and not others have + migrated; 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 <!-- Page 382 --><span + class="pagenum"><a name="page382"></a>[382]</span>and isolated flora. I + suspect that before this flora was exterminated by the Glacial epoch, a + few forms were 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: 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> + +<hr /> + +<p><!-- Page 383 --><span class="pagenum"><a name="page383"></a>[383]</span></p> + +<h3>CHAPTER XII.</h3> + +<p class="cenhead"><span class="sc">Geographical Distribution</span>—<i>continued</i>.</p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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, &c., 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 <!-- + Page 384 --><span class="pagenum"><a name="page384"></a>[384]</span>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 + <!-- Page 385 --><span class="pagenum"><a + name="page385"></a>[385]</span>subsequently 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 duckweed 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 <!-- Page 386 + --><span class="pagenum"><a name="page386"></a>[386]</span>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 6¾ ounces; I kept it covered up in my study for six months, + pulling up and counting each plant as it grew; the plants were <!-- Page + 387 --><span class="pagenum"><a name="page387"></a>[387]</span>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, <!-- Page 388 + --><span class="pagenum"><a name="page388"></a>[388]</span>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, freshwater 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> </p> + + <p><i>On the Inhabitants of Oceanic Islands.</i>—We now come to the + last of the three classes of facts, which I <!-- Page 389 --><span + class="pagenum"><a name="page389"></a>[389]</span>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 plants; <!-- Page 390 --><span + class="pagenum"><a name="page390"></a>[390]</span>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 partly 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; + and partly on the frequent arrival of unmodified immigrants from the + mother-country, and the consequent intercrossing with them. With respect + to the effects of this intercrossing, <!-- Page 391 --><span + class="pagenum"><a name="page391"></a>[391]</span>it should be remembered + that the offspring of such crosses would almost certainly gain in vigour; + so that even an occasional cross would produce more effect than might at + first have been anticipated. To give a few examples: in the Galapagos + Islands nearly every land-bird, but only two out of the eleven marine + birds, are peculiar; and it is obvious that 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. Any tendency to modification + will, also, have been checked by intercrossing with the unmodified + immigrants from the mother-country. 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 + sea-shells are dispersed, yet we can see that their eggs or larvae, + perhaps attached to seaweed or floating timber, or to the feet of + wading-birds, might be transported far more easily than <!-- Page 392 + --><span class="pagenum"><a name="page392"></a>[392]</span>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 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 <!-- Page 393 --><span class="pagenum"><a + name="page393"></a>[393]</span>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 oceanic 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 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 <!-- Page 394 --><span class="pagenum"><a + name="page394"></a>[394]</span>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 + 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, aërial 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 <!-- Page 395 + --><span class="pagenum"><a name="page395"></a>[395]</span>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 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 <!-- + Page 396 --><span class="pagenum"><a + name="page396"></a>[396]</span>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 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 aërial bats,—the singular + proportions of certain orders of plants,—herbaceous forms having + been developed into trees, &c.,—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 <!-- Page 397 --><span + class="pagenum"><a name="page397"></a>[397]</span>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. 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. <!-- Page 398 --><span + class="pagenum"><a name="page398"></a>[398]</span></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 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 <span + class="correction" title="text reads `volanic'">volcanic</span> 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 <!-- Page 399 --><span class="pagenum"><a + name="page399"></a>[399]</span>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 + 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 <!-- Page 400 --><span + class="pagenum"><a name="page400"></a>[400]</span>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, 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, &c., 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 as 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 <!-- Page 401 --><span class="pagenum"><a + name="page401"></a>[401]</span>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 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 <!-- Page 402 + --><span class="pagenum"><a name="page402"></a>[402]</span>species, both + those found in other parts of the world and those confined to the + archipelago, are common to 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 <!-- Page 403 --><span class="pagenum"><a + name="page403"></a>[403]</span>subject; namely, that <span + class="correction" title="text reads `Maderia'">Madeira</span> and the + adjoining islet of 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 <span + class="correction" title="text reads `Maderia'">Madeira</span>, 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, <!-- Page + 404 --><span class="pagenum"><a name="page404"></a>[404]</span>&c., + all of strictly American forms, and it is obvious 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 <!-- Page 405 --><span class="pagenum"><a + name="page405"></a>[405]</span>enormous ranges. It is not meant that in + world-ranging 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 <!-- Page 406 --><span class="pagenum"><a + name="page406"></a>[406]</span>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 or readiest source, together with + the subsequent modification and better adaptation of the colonists to + their new homes.</p> + + <p> </p> + + <p><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 <!-- Page 407 --><span class="pagenum"><a + name="page407"></a>[407]</span>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 great.</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. + <!-- Page 408 --><span class="pagenum"><a + name="page408"></a>[408]</span></p> + + <p>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 + relation of organism to organism is 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 <!-- Page 409 --><span class="pagenum"><a + name="page409"></a>[409]</span>developed 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 aërial 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 <!-- Page 410 --><span class="pagenum"><a + name="page410"></a>[410]</span>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> + +<hr /> + +<p><!-- Page 411 --><span class="pagenum"><a name="page411"></a>[411]</span></p> + +<h3>CHAPTER XIII.</h3> + +<p class="cenhead"><span class="sc">Mutual Affinities of Organic Beings: Morphology: Embryology: Rudimentary Organs.</span></p> + +<blockquote class="b1n"> + + <p><span class="sc">Classification</span>, 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—<span + class="sc">Morphology</span>, between members of the same class, between + parts of the same individual—<span class="sc">Embryology</span>, + laws of, explained by variations not supervening at an early age, and + being inherited at a corresponding age—<span class="sc">Rudimentary + organs</span>; their origin explained—Summary.</p> + +</blockquote> + + <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 + sub-group 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 <!-- Page 412 + --><span class="pagenum"><a name="page412"></a>[412]</span>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 <!-- Page 413 --><span class="pagenum"><a + name="page413"></a>[413]</span>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 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 + <!-- Page 414 --><span class="pagenum"><a + name="page414"></a>[414]</span>modification, 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 <!-- Page 415 --><span + class="pagenum"><a name="page415"></a>[415]</span>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 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; <!-- Page 416 + --><span class="pagenum"><a name="page416"></a>[416]</span>in another + division they differ much, and the differences are of quite subordinate + value in classification; yet no one probably will say that the antennae + 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 <!-- Page 417 --><span class="pagenum"><a + name="page417"></a>[417]</span>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 <!-- Page 418 + --><span class="pagenum"><a name="page418"></a>[418]</span>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 aërating 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 <!-- Page 419 --><span + class="pagenum"><a name="page419"></a>[419]</span>embryonic 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. <!-- Page 420 --><span + class="pagenum"><a name="page420"></a>[420]</span></p> + + <p>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 <!-- Page 421 --><span class="pagenum"><a + name="page421"></a>[421]</span>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 <span + class="scac">F</span><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 <!-- Page 422 --><span + class="pagenum"><a name="page422"></a>[422]</span>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 <!-- Page 423 --><span class="pagenum"><a + name="page423"></a>[423]</span>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, &c.; 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 <!-- Page 424 --><span + class="pagenum"><a name="page424"></a>[424]</span>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. <!-- Page 425 --><span class="pagenum"><a + name="page425"></a>[425]</span></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 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. <!-- Page + 426 --><span class="pagenum"><a name="page426"></a>[426]</span>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. + 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 <!-- Page + 427 --><span class="pagenum"><a name="page427"></a>[427]</span>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 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 <!-- Page 428 --><span class="pagenum"><a + name="page428"></a>[428]</span>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 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 class; 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 <!-- Page 429 --><span + class="pagenum"><a name="page429"></a>[429]</span>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 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 <!-- Page 430 --><span class="pagenum"><a + name="page430"></a>[430]</span>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 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 <!-- Page 431 --><span class="pagenum"><a + name="page431"></a>[431]</span>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 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 <!-- Page 432 --><span class="pagenum"><a + name="page432"></a>[432]</span>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 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, + <!-- Page 433 --><span class="pagenum"><a + name="page433"></a>[433]</span>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, &c., 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 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> </p> + + <p><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 <!-- Page 434 --><span class="pagenum"><a + name="page434"></a>[434]</span>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 similar 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 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 <!-- Page 435 + --><span class="pagenum"><a name="page435"></a>[435]</span>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 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, acting on some originally created form, 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 <!-- + Page 436 --><span class="pagenum"><a + name="page436"></a>[436]</span>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 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 + extraordinary 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 <!-- Page 437 --><span class="pagenum"><a + name="page437"></a>[437]</span>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 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 other and distinct species, we can indicate but + few serial homologies; that is, we are seldom enabled to say that one + <!-- Page 438 --><span class="pagenum"><a + name="page438"></a>[438]</span>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, &c.,—as having + been metamorphosed, not one from the other, but from some common element. + Naturalists, however, use such language only in a metaphorical sense: + they are far from meaning that during a long course of descent, + primordial organs of any kind—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 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> </p> + + <p><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 + <!-- Page 439 --><span class="pagenum"><a + name="page439"></a>[439]</span>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, &c., 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 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, <!-- Page 440 --><span + class="pagenum"><a name="page440"></a>[440]</span>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 stages barely + distinguishable.</p> + + <p>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 <!-- Page 441 --><span class="pagenum"><a + name="page441"></a>[441]</span>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>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 + <!-- Page 442 --><span class="pagenum"><a + name="page442"></a>[442]</span>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 + <span class="correction" title="text reads `indivividual'" + >individual</span> 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 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 + embryos 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 <!-- Page 443 + --><span class="pagenum"><a name="page443"></a>[443]</span>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 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 + <!-- Page 444 --><span class="pagenum"><a + name="page444"></a>[444]</span>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 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 <!-- Page 445 --><span class="pagenum"><a + name="page445"></a>[445]</span>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. 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 <!-- Page 446 --><span + class="pagenum"><a name="page446"></a>[446]</span>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 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 + have 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 <!-- Page 447 + --><span class="pagenum"><a name="page447"></a>[447]</span>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 of our 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 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 <!-- Page 448 + --><span class="pagenum"><a name="page448"></a>[448]</span>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, &c., 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 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; <!-- Page 449 --><span class="pagenum"><a + name="page449"></a>[449]</span>and in so far it reveals the structure of + its progenitor. In two groups of animals, 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 existing embryos, has not been obliterated, either by the + successive variations in a long course of modification having supervened + 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 <!-- Page 450 --><span class="pagenum"><a + name="page450"></a>[450]</span>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> </p> + + <p><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 fœtal 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 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 <!-- Page 451 --><span + class="pagenum"><a name="page451"></a>[451]</span>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 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 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 nearly rudimentary for its proper function + of giving buoyancy, but has become converted into a <!-- Page 452 + --><span class="pagenum"><a name="page452"></a>[452]</span>nascent + breathing organ or lung. Other similar instances could be given.</p> + + <p>Organs, however little developed, if of use, should not be called + rudimentary; they cannot properly be said to be in an atrophied + condition; they may be called nascent, and may hereafter be developed to + any extent by natural selection. Rudimentary organs, on the other hand, + are essentially useless, as teeth which never cut through the gums; in a + still less developed condition, they would be of still less use. They + cannot, therefore, under their present condition, have been formed by + natural selection, which acts solely by the preservation of useful + modifications; they have been retained, as we shall see, by inheritance, + and relate to a former condition of their possessor. It is difficult to + know what are nascent organs; looking to the future, we cannot of course + tell how any part will be developed, and whether it is now nascent; + looking to the past, creatures with an organ in a nascent condition will + generally have been supplanted and exterminated by their successors with + the organ in a more perfect and developed condition. The wing of the + penguin is of high service, and acts as a fin; it may, therefore, + represent the nascent state of the wings of birds; not that I believe + this to be the case, it is more probably a reduced organ, modified for a + new function: the wing of the Apteryx is useless, and is truly + rudimentary. The mammary glands of the Ornithorhynchus may, perhaps, be + considered, in comparison with the udder of a cow, as in a nascent state. + The ovigerous frena of certain cirripedes, which are only slightly + developed and which have ceased to give attachment to the ova, are + nascent branchiæ.</p> + + <p>Rudimentary organs in the individuals of the same species are very + liable to vary in degree of development <!-- Page 453 --><span + class="pagenum"><a name="page453"></a>[453]</span>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 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 <!-- Page 454 --><span class="pagenum"><a + name="page454"></a>[454]</span>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 re-statement 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 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, <!-- Page 455 + --><span class="pagenum"><a name="page455"></a>[455]</span>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 be modified and used for another purpose. Or an organ + might be retained for one alone of its 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 <!-- Page 456 --><span class="pagenum"><a + name="page456"></a>[456]</span>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 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> </p> + + <p><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 <!-- Page 457 --><span class="pagenum"><a + name="page457"></a>[457]</span>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 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 <!-- Page 458 --><span + class="pagenum"><a name="page458"></a>[458]</span>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 <span + class="correction" title="text reads `inumerable'">innumerable</span> + species, genera, and families of organic beings, with which this world is + 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> + +<hr /> + +<p><!-- Page 459 --><span class="pagenum"><a name="page459"></a>[459]</span></p> + +<h3>CHAPTER XIV.</h3> + +<p class="cenhead"><span class="sc">Recapitulation and Conclusion.</span></p> + +<blockquote class="b1n"> + + <p>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> + +</blockquote> + + <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 serious 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. <!-- Page 460 --><span class="pagenum"><a + name="page460"></a>[460]</span></p> + + <p>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, 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 varieties which have been + experimentised on have been <!-- Page 461 --><span class="pagenum"><a + name="page461"></a>[461]</span>produced under domestication; and as + domestication (I do not mean mere confinement) 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 <!-- Page + 462 --><span class="pagenum"><a name="page462"></a>[462]</span>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 have been 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 + <!-- Page 463 --><span class="pagenum"><a + name="page463"></a>[463]</span>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 <!-- Page 464 --><span class="pagenum"><a + name="page464"></a>[464]</span>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 <!-- Page 465 --><span + class="pagenum"><a name="page465"></a>[465]</span>improved; 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 <!-- + Page 466 --><span class="pagenum"><a name="page466"></a>[466]</span>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 from a few created + forms with subsequent modification.</p> + + <p> </p> + + <p>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. + <!-- Page 467 --><span class="pagenum"><a + name="page467"></a>[467]</span></p> + + <p>Man does not actually produce variability; he only 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 rapid increase of many animals and plants + during a succession of peculiar seasons, or when naturalised in a new + country. 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 + <!-- Page 468 --><span class="pagenum"><a + name="page468"></a>[468]</span>shall decrease, or finally become extinct. + As the individuals 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 <!-- Page 469 --><span class="pagenum"><a + name="page469"></a>[469]</span>nature. But, besides such differences, all + naturalists 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 <!-- Page 470 --><span class="pagenum"><a + name="page470"></a>[470]</span>view we can understand how it is that in + each region 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 in their affinities 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 + 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 <!-- Page + 471 --><span class="pagenum"><a name="page471"></a>[471]</span>larger + groups tend to give birth to new and dominant 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 + truer, 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 <!-- Page 472 --><span class="pagenum"><a + name="page472"></a>[472]</span>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, with the great majority 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 <!-- Page 473 + --><span class="pagenum"><a name="page473"></a>[473]</span>conclusion + 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; <!-- + Page 474 --><span class="pagenum"><a name="page474"></a>[474]</span>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, <!-- Page 475 + --><span class="pagenum"><a name="page475"></a>[475]</span>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 <!-- Page 476 --><span + class="pagenum"><a name="page476"></a>[476]</span>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 <!-- Page 477 --><span class="pagenum"><a + name="page477"></a>[477]</span>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 <!-- Page + 478 --><span class="pagenum"><a name="page478"></a>[478]</span>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 <!-- Page 479 --><span class="pagenum"><a + name="page479"></a>[479]</span>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 <!-- Page 480 --><span class="pagenum"><a + name="page480"></a>[480]</span>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 better 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> </p> + + <p>I have now recapitulated the chief facts and considerations which have + thoroughly convinced me that species have been modified, during a long + course of descent, by the preservation or the natural selection of many + successive slight favourable variations. I cannot believe that a false + theory would explain, as it seems to me that the theory of natural + selection does explain, <!-- Page 481 --><span class="pagenum"><a + name="page481"></a>[481]</span>the several large classes of facts above + specified. I see no good reason why the views given in this volume should + shock the religious feelings of any one. A celebrated author and divine + has written to me that "he has gradually learnt to see that it is just as + noble a conception of the Deity to believe that He created a few original + forms capable of self-development into other and needful forms, as to + believe that He required a fresh act of creation to supply the voids + caused by the action of His laws."</p> + + <p>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 + 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 <!-- Page 482 --><span + class="pagenum"><a name="page482"></a>[482]</span>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 to hide our ignorance under such + expressions as the "plan of creation," "unity of design," &c., 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 <!-- Page 483 --><span + class="pagenum"><a name="page483"></a>[483]</span>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 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 <!-- Page 484 + --><span class="pagenum"><a name="page484"></a>[484]</span>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 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 by the Creator.</p> + + <p> </p> + + <p>When the views advanced by me in this volume, and by Mr. Wallace in + the Linnean Journal, or when analogous views on the origin of species are + generally <!-- Page 485 --><span class="pagenum"><a + name="page485"></a>[485]</span>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 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 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 <!-- + Page 486 --><span class="pagenum"><a name="page486"></a>[486]</span>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, &c., 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 + 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 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 <!-- + Page 487 --><span class="pagenum"><a + name="page487"></a>[487]</span>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 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 <!-- Page + 488 --><span class="pagenum"><a name="page488"></a>[488]</span>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 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 <!-- Page 489 + --><span class="pagenum"><a name="page489"></a>[489]</span>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 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 <!-- Page 490 + --><span class="pagenum"><a name="page490"></a>[490]</span>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; <span class="correction" + title="text reads `Inheritrnce'">Inheritance</span> which is almost + implied by reproduction; Variability from the indirect and direct action + of the external conditions 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 by the Creator 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> + +<hr /> + +<p><!-- Page 491 --><span class="pagenum"><a name="page491"></a>[491]</span></p> + +<h2>INDEX.</h2> + + <div class="poem"> + <div class="stanza"> + <p class="i8"><b>A.</b></p> + </div> + + <div class="stanza"> + <p>Aberrant groups, <a href="#page429">429</a>.</p> + <p>Abyssinia, plants of, <a href="#page375">375</a>.</p> + <p>Acclimatisation, <a href="#page139">139</a>.</p> + <p>Affinities of extinct species, <a href="#page329">329</a>.</p> + <p>—— of organic beings, <a href="#page411">411</a>.</p> + <p>Agassiz on Amblyopsis, <a href="#page139">139</a>.</p> + <p>—— on groups of species suddenly appearing, <a href="#page302">302</a>, <a href="#page305">305</a>.</p> + <p>—— on embryological succession, <a href="#page338">338</a>.</p> + <p>—— on the glacial period, <a href="#page366">366</a>.</p> + <p>—— on embryological characters, <a href="#page418">418</a>.</p> + <p>—— on the embryos of vertebrata, <a href="#page439">439</a>.</p> + <p>—— on parallelism of embryological development and geological succession, <a href="#page449">449</a>.</p> + <p>Algæ of New Zealand, <a href="#page376">376</a>.</p> + <p>Alligators, males, fighting, <a href="#page88">88</a>.</p> + <p>Amblyopsis, blind fish, <a href="#page139">139</a>.</p> + <p>America, North, productions allied to those of Europe, <a href="#page371">371</a>.</p> + <p>————, boulders and glaciers of, <a href="#page373">373</a>.</p> + <p>——, South, no modern formations on west coast, <a href="#page290">290</a>.</p> + <p>Ammonites, sudden extinction of, <a href="#page321">321</a>.</p> + <p>Anagallis, sterility of, <a href="#page247">247</a>.</p> + <p>Analogy of variations, <a href="#page159">159</a>.</p> + <p>Ancylus, <a href="#page386">386</a>.</p> + <p>Animals, not domesticated from being variable, <a href="#page17">17</a>.</p> + <p>——, domestic, descended from several stocks, <a href="#page19">19</a>.</p> + <p>————, acclimatisation of, <a href="#page141">141</a>.</p> + <p>—— of Australia, <a href="#page116">116</a>.</p> + <p>—— with thicker fur in cold climates, <a href="#page133">133</a>.</p> + <p>——, blind, in caves, <a href="#page137">137</a>.</p> + <p>——, extinct, of Australia, <a href="#page339">339</a>.</p> + <p>Anomma, <a href="#page240">240</a>.</p> + <p>Antarctic islands, ancient flora of, <a href="#page399">399</a>.</p> + <p>Antirrhinum, <a href="#page161">161</a>.</p> + <p>Ants attending aphides, <a href="#page210">210</a>.</p> + <p>——, slave-making instinct, <a href="#page219">219</a>.</p> + <p>——, neuter, structure of, <a href="#page236">236</a>.</p> + <p>Aphides, attended by ants, <a href="#page210">210</a>.</p> + <p>Aphis, development of, <a href="#page442">442</a>.</p> + <p>Apteryx, <a href="#page182">182</a>.</p> + <p>Arab horses, <a href="#page35">35</a>.</p> + <p>Aralo-Caspian Sea, <a href="#page339">339</a>.</p> + <p>Archaic, M. de, on the succession of species, <a href="#page325">325</a>.</p> + <p>Artichoke, Jerusalem, <a href="#page142">142</a>.</p> + <p>Ascension, plants of, <a href="#page389">389</a>.</p> + <p>Asclepias, pollen of, <a href="#page193">193</a>.</p> + <p>Asparagus, <a href="#page359">359</a>.</p> + <p>Aspicarpa, <a href="#page417">417</a>.</p> + <p>Asses, striped, <a href="#page163">163</a>.</p> + <p>Ateuchus, <a href="#page135">135</a>.</p> + <p>Audubon on habits of frigate-bird, <a href="#page185">185</a>.</p> + <p>—— on variation in birds'-nests, <a href="#page212">212</a>.</p> + <p>—— on heron eating seeds, <a href="#page387">387</a>.</p> + <p>Australia, animals of, <a href="#page116">116</a>.</p> + <p>——. dogs of, <a href="#page215">215</a>.</p> + <p>——, extinct animals of, <a href="#page339">339</a>.</p> + <p>——, European plants in, <a href="#page375">375</a>.</p> + <p>Azara on flies destroying cattle, <a href="#page72">72</a>.</p> + <p>Azores, flora of, <a href="#page363">363</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>B.</b></p> + </div> + + <div class="stanza"> + <p>Babington, Mr., on British plants, <a href="#page48">48</a>.</p> + <p>Balancement of growth, <a href="#page147">147</a>.</p> + <p>Bamboo with hooks, <a href="#page197">197</a>.</p> + <p>Barberry, flowers of, <a href="#page98">98</a>.</p> + <p>Barrande, M., on Silurian colonies, <a href="#page313">313</a>.</p> + <p>—— on the succession of species, <a href="#page325">325</a>.</p> + <p>—— on parallelism of palæozoic formations, <a href="#page328">328</a>.</p> + <p>—— on affinities of ancient species, <a href="#page330">330</a>.</p> + <p>Barriers, importance of, <a href="#page347">347</a>.</p> + <p>Batrachians on islands, <a href="#page393">393</a>.</p> + <p>Bats, how structure acquired, <a href="#page180">180</a>.</p> + <p>——, distribution of, <a href="#page394">394</a>.</p> + <p>Bear, catching water-insects, <a href="#page184">184</a>.</p> + <p>Bee, sting of, <a href="#page202">202</a>.</p> + <p>——, queen, killing rivals, <a href="#page202">202</a>.</p> + <p>Bees fertilising flowers, <a href="#page73">73</a>.</p> + <p>——, hive, not sucking the red clover, <a href="#page95">95</a>.</p> +<!-- Page 492 --><span class="pagenum"><a name="page492"></a>[492]</span> + <p>————, cell-making instinct, <a href="#page224">224</a>.</p> + <p>——, humble, cells of, <a href="#page225">225</a>.</p> + <p>——, parasitic, <a href="#page218">218</a>.</p> + <p>Beetles, wingless, in Madeira, <a href="#page135">135</a>.</p> + <p>—— with deficient tarsi, <a href="#page135">135</a>.</p> + <p>Bentham, Mr., on British plants, <a href="#page48">48</a>.</p> + <p>——, on classification, <a href="#page419">419</a>.</p> + <p>Berkeley, Mr., on seeds in salt-water, <a href="#page358">358</a>.</p> + <p>Bermuda, birds of, <a href="#page391">391</a>.</p> + <p>Birds acquiring fear, <a href="#page212">212</a>.</p> + <p>—— annually cross the Atlantic, <a href="#page364">364</a>.</p> + <p>——, colour of, on continents, <a href="#page132">132</a>.</p> + <p>——, footsteps and remains of, in secondary rocks, <a href="#page304">304</a>.</p> + <p>——, fossil, in caves of Brazil, <a href="#page339">339</a>.</p> + <p>—— of Madeira, Bermuda, and Galapagos, <a href="#page391">391</a>.</p> + <p>——, song of males, <a href="#page89">89</a>.</p> + <p>—— transporting seeds, <a href="#page361">361</a>.</p> + <p>——, waders, <a href="#page385">385</a>.</p> + <p>——, wingless, <a href="#page134">134</a>, <a href="#page182">182</a>.</p> + <p>——, with traces of embryonic teeth, <a href="#page450">450</a>.</p> + <p>Bizcacha, <a href="#page349">349</a>.</p> + <p>——, affinities of, <a href="#page429">429</a>.</p> + <p>Bladder for swimming in fish, <a href="#page190">190</a>.</p> + <p>Blindness of cave animals, <a href="#page137">137</a>.</p> + <p>Blyth, Mr., on distinctness of Indian cattle, <a href="#page18">18</a>.</p> + <p>——, on striped Hemionus, <a href="#page163">163</a>.</p> + <p>——, on crossed geese, <a href="#page254">254</a>.</p> + <p>Boar, shoulder-pad of, <a href="#page88">88</a>.</p> + <p>Borrow, Mr., on the Spanish pointer, <a href="#page35">35</a>.</p> + <p>Bory St. Vincent on Batrachians, <a href="#page393">393</a>.</p> + <p>Bosquet, M., on fossil Chthamalus, <a href="#page305">305</a>.</p> + <p>Boulders, erratic, on the Azores, <a href="#page363">363</a>.</p> + <p>Branchiæ, <a href="#page190">190</a>.</p> + <p>Brent, Mr., on house-tumblers, <a href="#page214">214</a>.</p> + <p>——, on hawks killing pigeons, <a href="#page362">362</a>.</p> + <p>Brewer, Dr., on American cuckoo, <a href="#page217">217</a>.</p> + <p>Britain, mammals of, <a href="#page396">396</a>.</p> + <p>Bronn on duration of specific forms, <a href="#page294">294</a>.</p> + <p>Brown, Robert, on classification, <a href="#page415">415</a>.</p> + <p>Buckman on variation in plants, <a href="#page10">10</a>.</p> + <p>Buzareingues on sterility of varieties, <a href="#page270">270</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>C.</b></p> + </div> + + <div class="stanza"> + <p>Cabbage, varieties of, crossed, <a href="#page99">99</a>.</p> + <p>Calceolaria, <a href="#page251">251</a>.</p> + <p>Canary-birds, sterility of hybrids, <a href="#page252">252</a>.</p> + <p>Cape de Verde islands, <a href="#page398">398</a>.</p> + <p>Cape of Good Hope, plants of, <a href="#page110">110</a>, <a href="#page375">375</a>.</p> + <p>Carrier-pigeons killed by hawks, <a href="#page362">362</a>.</p> + <p>Cassini on flowers of compositæ, <a href="#page145">145</a>.</p> + <p>Catasetum, <a href="#page424">424</a>.</p> + <p>Cats, with blue eyes, deaf, <a href="#page12">12</a>.</p> + <p>——, variation in habits of, <a href="#page91">91</a>.</p> + <p>—— curling tail when going to spring, <a href="#page201">201</a>.</p> + <p>Cattle destroying fir-trees, <a href="#page72">72</a>.</p> + <p>—— destroyed by flies in La Plata, <a href="#page72">72</a>.</p> + <p>——, breeds of, locally extinct, <a href="#page111">111</a>.</p> + <p>——, fertility of Indian and European breeds, <a href="#page254">254</a>.</p> + <p>Cave, inhabitants of, blind, <a href="#page137">137</a>.</p> + <p>Centres of creation, <a href="#page352">352</a>.</p> + <p>Cephalopodæ, development of, <a href="#page442">442</a>.</p> + <p>Cervulus, <a href="#page253">253</a>.</p> + <p>Cetacea, teeth and hair, <a href="#page144">144</a>.</p> + <p>Ceylon, plants of, <a href="#page375">375</a>.</p> + <p>Chalk formation, <a href="#page322">322</a>.</p> + <p>Characters, divergence of, <a href="#page111">111</a>.</p> + <p>——, sexual, variable, <a href="#page156">156</a>.</p> + <p>——, adaptive or analogical, <a href="#page426">426</a>.</p> + <p>Charlock, <a href="#page76">76</a>.</p> + <p>Checks to increase, <a href="#page67">67</a>.</p> + <p>—— ——, mutual, <a href="#page71">71</a>.</p> + <p>Chickens, instinctive tameness of, <a href="#page216">216</a>.</p> + <p>Chthamalinæ, <a href="#page289">289</a>.</p> + <p>Chthamalus, cretacean species of, <a href="#page305">305</a>.</p> + <p>Circumstances favourable to selection of domestic products, <a href="#page40">40</a>.</p> + <p>—— —— to natural selection, <a href="#page102">102</a>.</p> + <p>Cirripedes capable of crossing, <a href="#page101">101</a>.</p> + <p>——, carapace aborted, <a href="#page148">148</a>.</p> + <p>——, their ovigerous frena, <a href="#page192">192</a>.</p> + <p>——, fossil, <a href="#page304">304</a>.</p> + <p>——, larvæ of, <a href="#page440">440</a>.</p> + <p>Classification, <a href="#page413">413</a>.</p> + <p>Clift, Mr., on the succession of types, <a href="#page339">339</a>.</p> + <p>Climate, effects of, in checking increase of beings, <a href="#page68">68</a>.</p> + <p>——, adaptation of, to organisms, <a href="#page139">139</a>.</p> +<!-- Page 493 --><span class="pagenum"><a name="page493"></a>[493]</span> + <p>Cobites, intestine of, <a href="#page190">190</a>.</p> + <p>Cockroach, <a href="#page76">76</a>.</p> + <p>Collections, palæontological, poor, <a href="#page288">288</a>.</p> + <p>Colour, influenced by climate, <a href="#page132">132</a>.</p> + <p>——, in relation to attacks by flies, <a href="#page198">198</a>.</p> + <p>Columba livia, parent of domestic pigeons, <a href="#page23">23</a>.</p> + <p>Colymbetes, <a href="#page386">386</a>.</p> + <p>Compensation of growth, <a href="#page147">147</a>.</p> + <p>Compositæ, outer and inner florets of, <a href="#page144">144</a>.</p> + <p>——, male flowers of, <a href="#page451">451</a>.</p> + <p>Conclusion, general, <a href="#page480">480</a>.</p> + <p>Conditions, slight changes in, favourable to fertility, <a href="#page267">267</a>.</p> + <p>Coot, <a href="#page185">185</a>.</p> + <p>Coral-islands, seeds drifted to, <a href="#page361">361</a>.</p> + <p>—— reefs, indicating movements of earth, <a href="#page310">310</a>.</p> + <p>Corn-crake, <a href="#page186">186</a>.</p> + <p>Correlation of growth in domestic productions, <a href="#page11">11</a>.</p> + <p>—— of growth, <a href="#page143">143</a>, <a href="#page198">198</a>.</p> + <p>Cowslip, <a href="#page49">49</a>.</p> + <p>Creation, single centres of, <a href="#page352">352</a>.</p> + <p>Crinum, <a href="#page250">250</a>.</p> + <p>Crosses, reciprocal, <a href="#page258">258</a>.</p> + <p>Crossing of domestic animals, importance in altering breeds, <a href="#page20">20</a>.</p> + <p>——, advantages of, <a href="#page96">96</a>.</p> + <p>—— unfavourable to selection, <a href="#page102">102</a>.</p> + <p>Crustacea of New Zealand, <a href="#page376">376</a>.</p> + <p>Crustacean, blind, <a href="#page137">137</a>.</p> + <p>Cryptocerus, <a href="#page239">239</a>.</p> + <p>Ctenomys, blind, <a href="#page137">137</a>.</p> + <p>Cuckoo, instinct of, <a href="#page216">216</a>.</p> + <p>Currants, grafts of, <a href="#page262">262</a>.</p> + <p>Currents of sea, rate of, <a href="#page360">360</a>.</p> + <p>Cuvier on conditions of existence, <a href="#page206">206</a>.</p> + <p>—— on fossil monkeys, <a href="#page304">304</a>.</p> + <p>——, Fred., on instinct, <a href="#page208">208</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>D.</b></p> + </div> + + <div class="stanza"> + <p>Dana, Prof., on blind cave-animals, <a href="#page139">139</a>.</p> + <p>——, on relations of crustaceans of Japan, <a href="#page372">372</a>.</p> + <p>——, on crustaceans of New Zealand, <a href="#page376">376</a>.</p> + <p>De Candolle on struggle for existence, <a href="#page62">62</a>.</p> + <p>—— on umbelliferæ, <a href="#page146">146</a>.</p> + <p>—— on general affinities, <a href="#page430">430</a>.</p> + <p>——, Alph., on low plants, widely dispersed, <a href="#page406">406</a>.</p> + <p>——, ——, on widely-ranging plants being variable, <a href="#page53">53</a>.</p> + <p>——, ——, on naturalisation, <a href="#page115">115</a>.</p> + <p>——, ——, on winged seeds, <a href="#page146">146</a>.</p> + <p>——, ——, on Alpine species suddenly becoming rare, <a href="#page175">175</a>.</p> + <p>——, ——, on distribution of plants with large seeds, <a href="#page360">360</a>.</p> + <p>——, ——, on vegetation of Australia, <a href="#page379">379</a>.</p> + <p>——, ——, on fresh-water plants, <a href="#page386">386</a>.</p> + <p>——, ——, on insular plants, <a href="#page389">389</a>.</p> + <p>Degradation of coast-rocks, <a href="#page282">282</a>.</p> + <p>Denudation, rate of, <a href="#page285">285</a>.</p> + <p>—— of oldest rocks, <a href="#page308">308</a>.</p> + <p>Development of ancient forms, <a href="#page336">336</a>.</p> + <p>Devonian system, <a href="#page334">334</a>.</p> + <p>Dianthus, fertility of crosses, <a href="#page256">256</a>.</p> + <p>Dirt on feet of birds, <a href="#page362">362</a>.</p> + <p>Dispersal, means of, <a href="#page356">356</a>.</p> + <p>—— during glacial period, <a href="#page365">365</a>.</p> + <p>Distribution, geographical, <a href="#page346">346</a>.</p> + <p>——, means of, <a href="#page356">356</a>.</p> + <p>Disuse, effects of, under nature, <a href="#page134">134</a>.</p> + <p>Divergence of character, <a href="#page111">111</a>.</p> + <p>Division, physiological, of labour, <a href="#page115">115</a>.</p> + <p>Dogs, hairless, with imperfect teeth, <a href="#page12">12</a>.</p> + <p>—— descended from several wild stocks, <a href="#page18">18</a>.</p> + <p>——, domestic instincts of, <a href="#page213">213</a>.</p> + <p>——, inherited civilisation of, <a href="#page215">215</a>.</p> + <p>——, fertility of breeds together, <a href="#page254">254</a>.</p> + <p>——, —— of crosses, <a href="#page268">268</a>.</p> + <p>——, proportions of, when young, <a href="#page444">444</a>.</p> + <p>Domestication, variation under, <a href="#page7">7</a>.</p> + <p>Downing, Mr., on fruit-trees in America, <a href="#page85">85</a>.</p> + <p>Downs, North and South, <a href="#page286">286</a>.</p> + <p>Dragon-flies, intestines of, <a href="#page190">190</a>.</p> + <p>Drift-timber, <a href="#page360">360</a>.</p> + <p>Driver-ant, <a href="#page240">240</a>.</p> + <p>Drones killed by other bees, <a href="#page202">202</a>.</p> + <p>Duck, domestic, wings of, reduced, <a href="#page11">11</a>.</p> + <p>——, logger-headed, <a href="#page182">182</a>.</p> +<!-- Page 494 --><span class="pagenum"><a name="page494"></a>[494]</span> + <p>Duckweed, <a href="#page385">385</a>.</p> + <p>Dugong, affinities of, <a href="#page414">414</a>.</p> + <p>Dung-beetles with deficient tarsi, <a href="#page135">135</a>.</p> + <p>Dyticus, <a href="#page386">386</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>E.</b></p> + </div> + + <div class="stanza"> + <p>Earl, Mr. W., on the Malay Archipelago, <a href="#page395">395</a>.</p> + <p>Ears, drooping, in domestic animals, <a href="#page11">11</a>.</p> + <p>——, rudimentary, <a href="#page454">454</a>.</p> + <p>Earth, seeds in roots of trees, <a href="#page361">361</a>.</p> + <p>Eciton, <a href="#page238">238</a>.</p> + <p>Economy of organisation, <a href="#page147">147</a>.</p> + <p>Edentata, teeth and hair, <a href="#page144">144</a>.</p> + <p>——, fossil species of, <a href="#page339">339</a>.</p> + <p>Edwards, Milne, on physiological divisions of labour, <a href="#page115">115</a>.</p> + <p>——, on gradations of structure, <a href="#page194">194</a>.</p> + <p>——, on embryonical characters, <a href="#page418">418</a>.</p> + <p>Eggs, young birds escaping from, <a href="#page87">87</a>.</p> + <p>Electric organs, <a href="#page192">192</a>.</p> + <p>Elephant, rate of increase, <a href="#page64">64</a>.</p> + <p>—— of glacial period, <a href="#page141">141</a>.</p> + <p>Embryology, <a href="#page438">438</a>.</p> + <p>Existence, struggle for, <a href="#page60">60</a>.</p> + <p>——, conditions of, <a href="#page206">206</a>.</p> + <p>Extinction, as bearing on natural selection, <a href="#page109">109</a>.</p> + <p>—— of domestic varieties, <a href="#page111">111</a>,</p> + <p>——, <a href="#page317">317</a>.</p> + <p>Eye, structure of, <a href="#page187">187</a>.</p> + <p>——, correction for aberration, <a href="#page202">202</a>.</p> + <p>Eyes reduced in moles, <a href="#page137">137</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>F.</b></p> + </div> + + <div class="stanza"> + <p>Fabre, M. on parasitic sphex, <a href="#page218">218</a>.</p> + <p>Falconer, Dr., on naturalisation of plants in India, <a href="#page65">65</a>.</p> + <p>—— on fossil crocodile, <a href="#page313">313</a>.</p> + <p>—— on elephants and mastodons, <a href="#page334">334</a>.</p> + <p>—— and Cautley on mammals of sub-Himalayan beds, <a href="#page340">340</a>.</p> + <p>Falkland Island, wolf of, <a href="#page394">394</a>.</p> + <p>Faults, <a href="#page285">285</a>.</p> + <p>Faunas, marine, <a href="#page348">348</a>.</p> + <p>Fear, instinctive, in birds, <a href="#page212">212</a>.</p> + <p>Feet of bird, young molluscs adhering to, <a href="#page385">385</a>.</p> + <p>Fertility of hybrids, <a href="#page249">249</a>.</p> + <p>—— from slight changes in conditions, <a href="#page267">267</a>.</p> + <p>—— of crossed varieties, <a href="#page268">268</a>.</p> + <p>Fir-trees destroyed by cattle, <a href="#page72">72</a>.</p> + <p>—— ——, pollen of, <a href="#page203">203</a>.</p> + <p>Fish, flying, <a href="#page182">182</a>.</p> + <p>——, teleostean, sudden appearance of, <a href="#page305">305</a>.</p> + <p>—— eating seeds, <a href="#page362">362</a>, <a href="#page387">387</a>.</p> + <p>——, fresh-water, distribution of, <a href="#page384">384</a>.</p> + <p>Fishes, ganoid, now confined to fresh water, <a href="#page107">107</a>.</p> + <p>——, electric organs of, <a href="#page192">192</a>.</p> + <p>——, ganoid, living in fresh water, <a href="#page321">321</a>.</p> + <p>—— of southern hemisphere, <a href="#page376">376</a>.</p> + <p>Flight, powers of, how acquired, <a href="#page182">182</a>.</p> + <p>Flowers, structure of, in relation to crossing, <a href="#page97">97</a>.</p> + <p>—— of compositæ and umbelliferæ, <a href="#page144">144</a>.</p> + <p>Forbes, E., on colours of shells, <a href="#page132">132</a>.</p> + <p>—— on abrupt range of shells in depth, <a href="#page175">175</a>.</p> + <p>—— on poorness of palæontological collections, <a href="#page288">288</a>.</p> + <p>—— on continuous succession of genera, <a href="#page316">316</a>.</p> + <p>—— on continental extensions, <a href="#page357">357</a>.</p> + <p>—— on distribution during glacial period, <a href="#page366">366</a>.</p> + <p>—— on parallelism in time and space, <a href="#page409">409</a>.</p> + <p>Forests, changes in, in America, <a href="#page74">74</a>.</p> + <p>Formation, Devonian, <a href="#page334">334</a>.</p> + <p>Formations, thickness of, in Britain, <a href="#page284">284</a>.</p> + <p>——, intermittent, <a href="#page290">290</a>.</p> + <p>Formica rufescens, <a href="#page219">219</a>.</p> + <p>—— sanguinea, <a href="#page219">219</a>.</p> + <p>—— flava, neuter of, <a href="#page240">240</a>.</p> + <p>Frena, ovigerous, of cirripedes, <a href="#page192">192</a>.</p> + <p>Fresh-water productions, dispersal of, <a href="#page383">383</a>.</p> + <p>Fries on species in large genera being closely allied to other species, <a href="#page57">57</a>.</p> + <p>Frigate-bird, <a href="#page185">185</a>.</p> + <p>Frogs on islands, <a href="#page393">393</a>.</p> + <p>Fruit-trees, gradual improvement of, <a href="#page37">37</a>.</p> + <p>—— —— in United States, <a href="#page85">85</a>.</p> + <p>—— ——, varieties of, acclimatised in United States, <a href="#page142">142</a>.</p> +<!-- Page 495 --><span class="pagenum"><a name="page495"></a>[495]</span> + <p>Fuci, crossed, <a href="#page258">258</a>.</p> + <p>Fur, thicker in cold climates, <a href="#page133">133</a>.</p> + <p>Furze, <a href="#page439">439</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>G.</b></p> + </div> + + <div class="stanza"> + <p>Galapagos Archipelago, birds of, <a href="#page390">390</a>.</p> + <p>——, productions of, <a href="#page398">398</a>, <a href="#page400">400</a>.</p> + <p>Galeopithecus, <a href="#page181">181</a>.</p> + <p>Game, increase of, checked by vermin, <a href="#page68">68</a>.</p> + <p>Gärtner on sterility of hybrids, <a href="#page247">247</a>, <a href="#page255">255</a>.</p> + <p>——, on reciprocal crosses, <a href="#page258">258</a>.</p> + <p>——, on crossed maize and verbascum, <a href="#page270">270</a>.</p> + <p>——, on comparison of hybrids and mongrels, <a href="#page272">272</a>.</p> + <p>Geese, fertility when crossed, <a href="#page253">253</a>.</p> + <p>——, upland, <a href="#page185">185</a>.</p> + <p>Genealogy important in classification, <a href="#page425">425</a>.</p> + <p>Geoffroy St. Hilaire on balancement, <a href="#page147">147</a>.</p> + <p>—— —— on homologous organs, <a href="#page434">434</a>.</p> + <p>—— ——, Isidore, on variability of repeated parts, <a href="#page149">149</a>.</p> + <p>—— ——, on correlation in monstrosities, <a href="#page11">11</a>.</p> + <p>—— ——, on correlation, <a href="#page144">144</a>.</p> + <p>—— ——, on variable parts being often monstrous, <a href="#page155">155</a>.</p> + <p>Geographical distribution, <a href="#page346">346</a>.</p> + <p>Geography, ancient, <a href="#page487">487</a>.</p> + <p>Geology, future progress of, <a href="#page487">487</a>.</p> + <p>——, imperfection of the record, <a href="#page279">279</a>.</p> + <p>Giraffe, tail of, <a href="#page195">195</a>.</p> + <p>Glacial period, <a href="#page365">365</a>.</p> + <p>Gmelin on distribution, <a href="#page365">365</a>.</p> + <p>Gnathodon, fossil, <a href="#page368">368</a>.</p> + <p>Godwin-Austen, Mr., on the Malay Archipelago, <a href="#page300">300</a>.</p> + <p>Goethe on compensation of growth, <a href="#page147">147</a>.</p> + <p>Gooseberry, grafts of, <a href="#page262">262</a>.</p> + <p>Gould, Dr. A., on land-shells, <a href="#page397">397</a>.</p> + <p>——, Mr., on colours of birds, <a href="#page132">132</a>.</p> + <p>——, on birds of the Galapagos, <a href="#page398">398</a>.</p> + <p>——, on distribution of genera of birds, <a href="#page404">404</a>.</p> + <p>Gourds, crossed, <a href="#page270">270</a>.</p> + <p>Grafts, capacity of, <a href="#page261">261</a>.</p> + <p>Grasses, varieties of, <a href="#page113">113</a>.</p> + <p>Gray, Dr. Asa, on trees of United States, <a href="#page100">100</a>.</p> + <p>——, on naturalised plants in the United States, <a href="#page115">115</a>.</p> + <p>——, on rarity of intermediate varieties, <a href="#page176">176</a>.</p> + <p>——, on Alpine plants, <a href="#page365">365</a>.</p> + <p>——, Dr. J. E., on striped mule, <a href="#page165">165</a>.</p> + <p>Grebe, <a href="#page185">185</a>.</p> + <p>Groups, aberrant, <a href="#page429">429</a>.</p> + <p>Grouse, colours of, <a href="#page84">84</a>.</p> + <p>——, red, a doubtful species, <a href="#page49">49</a>.</p> + <p>Growth, compensation of, <a href="#page147">147</a>.</p> + <p>——, correlation of, in domestic products, <a href="#page11">11</a>.</p> + <p>——, correlation of, <a href="#page143">143</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>H.</b></p> + </div> + + <div class="stanza"> + <p>Habit, effect of, under domestication, <a href="#page11">11</a>.</p> + <p>——, effect of, under nature, <a href="#page134">134</a>.</p> + <p>——, diversified, of same species, <a href="#page183">183</a>.</p> + <p>Hair and teeth, correlated, <a href="#page144">144</a>.</p> + <p>Harcourt, Mr. E. V., on the birds of Madeira, <a href="#page391">391</a>.</p> + <p>Hartung, M. on boulders in the Azores, <a href="#page363">363</a>.</p> + <p>Hazel-nuts, <a href="#page359">359</a>.</p> + <p>Hearne on habits of bears, <a href="#page184">184</a>.</p> + <p>Heath, changes in vegetation, <a href="#page72">72</a>.</p> + <p>Heer, O., on plants of Madeira, <a href="#page107">107</a>.</p> + <p>Helix pomatia, <a href="#page397">397</a>.</p> + <p>Helosciadium, <a href="#page359">359</a>.</p> + <p>Hemionus, striped, <a href="#page163">163</a>.</p> + <p>Herbert, W., on struggle for existence, <a href="#page62">62</a>.</p> + <p>——, on sterility of hybrids, <a href="#page249">249</a>.</p> + <p>Hermaphrodites crossing, <a href="#page96">96</a>.</p> + <p>Heron eating seed, <a href="#page387">387</a>.</p> + <p>Heron, Sir R., on peacocks, <a href="#page89">89</a>.</p> + <p>Heusinger on white animals not poisoned by certain plants, <a href="#page12">12</a>.</p> + <p>Hewitt, Mr., on sterility of first crosses, <a href="#page264">264</a>.</p> + <p>Himalaya, glaciers of, <a href="#page373">373</a>.</p> + <p>——, plants of, <a href="#page375">375</a>.</p> + <p>Hippeastrum, <a href="#page250">250</a>.</p> + <p>Holly-trees, sexes of, <a href="#page93">93</a>.</p> + <p>Hollyhock, varieties of, crossed, <a href="#page271">271</a>.</p> + <p>Hooker, Dr., on trees of New Zealand, <a href="#page100">100</a>.</p> +<!-- Page 496 --><span class="pagenum"><a name="page496"></a>[496]</span> + <p>——, on acclimatisation of Himalayan trees, <a href="#page140">140</a>.</p> + <p>——, on flowers of umbelliferæ, <a href="#page145">145</a>.</p> + <p>——, on glaciers of Himalaya, <a href="#page373">373</a>.</p> + <p>——, on algæ of New Zealand, <a href="#page376">376</a>.</p> + <p>——, on vegetation at the base of the Himalaya, <a href="#page378">378</a>.</p> + <p>——, on plants of Tierra del Fuego, <a href="#page374">374</a>, <a href="#page378">378</a>.</p> + <p>——, on Australian plants, <a href="#page375">375</a>, <a href="#page399">399</a>.</p> + <p>——, on relations of flora of South America, <a href="#page379">379</a>.</p> + <p>——, on flora of the Antarctic lands, <a href="#page381">381</a>, <a href="#page399">399</a>.</p> + <p>——, on the plants of the Galapagos, <a href="#page392">392</a>, <a href="#page398">398</a>.</p> + <p>Hooks on bamboos, <a href="#page197">197</a>.</p> + <p>—— to seeds on islands, <a href="#page392">392</a>.</p> + <p>Horner, Mr., on the antiquity of Egyptians, <a href="#page18">18</a>.</p> + <p>Horns, rudimentary, <a href="#page454">454</a>.</p> + <p>Horse, fossil, in La Plata, <a href="#page318">318</a>.</p> + <p>Horses destroyed by flies in La Plata, <a href="#page72">72</a>.</p> + <p>——, striped, <a href="#page163">163</a>.</p> + <p>——, proportions of, when young, <a href="#page444">444</a>.</p> + <p>Horticulturists, selection applied by, <a href="#page32">32</a>.</p> + <p>Huber on cells of bees, <a href="#page230">230</a>.</p> + <p>——, P., on reason blended with instinct, <a href="#page208">208</a>.</p> + <p>——, on habitual nature of instincts, <a href="#page208">208</a>.</p> + <p>——, on slave-making ants, <a href="#page219">219</a>.</p> + <p>——, on Melipona domestica, <a href="#page225">225</a>.</p> + <p>Humble-bees, cells of, <a href="#page225">225</a>.</p> + <p>Hunter, J., on secondary sexual characters, <a href="#page150">150</a>.</p> + <p>Hutton, Captain, on crossed geese, <a href="#page254">254</a>.</p> + <p>Huxley, Prof., on structure of hermaphrodites, <a href="#page101">101</a>.</p> + <p>——, on embryological succession, <a href="#page338">338</a>.</p> + <p>——, on homologous organs, <a href="#page438">438</a>.</p> + <p>——, on the development of aphis, <a href="#page442">442</a>.</p> + <p>Hybrids and mongrels compared, <a href="#page272">272</a>.</p> + <p>Hybridism, <a href="#page245">245</a>.</p> + <p>Hydra, structure of, <a href="#page190">190</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>I.</b></p> + </div> + + <div class="stanza"> + <p>Ibla, <a href="#page148">148</a>.</p> + <p>Icebergs transporting seeds, <a href="#page363">363</a>.</p> + <p>Increase, rate of, <a href="#page63">63</a>.</p> + <p>Individuals, numbers favourable to selection, <a href="#page102">102</a>.</p> + <p>——, many, whether simultaneously created, <a href="#page355">355</a>.</p> + <p>Inheritance, laws of, <a href="#page12">12</a>.</p> + <p>—— at corresponding ages, <a href="#page14">14</a>, <a href="#page86">86</a>.</p> + <p>Insects, colour of, fitted for habitations, <a href="#page84">84</a>.</p> + <p>——, sea-side, colours of, <a href="#page132">132</a>.</p> + <p>——, blind, in caves, <a href="#page138">138</a>.</p> + <p>——, luminous, <a href="#page193">193</a>.</p> + <p>——, neuter, <a href="#page236">236</a>.</p> + <p>Instinct, <a href="#page207">207</a>.</p> + <p>Instincts, domestic, <a href="#page213">213</a>.</p> + <p>Intercrossing, advantages of, <a href="#page96">96</a>.</p> + <p>Islands, oceanic, <a href="#page388">388</a>.</p> + <p>Isolation favourable to selection, <a href="#page104">104</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>J.</b></p> + </div> + + <div class="stanza"> + <p>Japan, productions of, <a href="#page372">372</a>.</p> + <p>Java, plants of, <a href="#page375">375</a>.</p> + <p>Jones, Mr. J. M., on the birds of Bermuda, <a href="#page391">391</a>.</p> + <p>Jussieu on classification, <a href="#page417">417</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>K.</b></p> + </div> + + <div class="stanza"> + <p>Kentucky, caves of, <a href="#page137">137</a>.</p> + <p>Kerguelen-land, flora of, <a href="#page381">381</a>, <a href="#page399">399</a>.</p> + <p>Kidney-bean, acclimatisation of, <a href="#page142">142</a>.</p> + <p>Kidneys of birds, <a href="#page144">144</a>.</p> + <p>Kirby on tarsi deficient in beetles, <a href="#page135">135</a>.</p> + <p>Knight, Andrew, on cause of variation, <a href="#page7">7</a>.</p> + <p>Kölreuter on the barberry, <a href="#page98">98</a>.</p> + <p>—— on sterility of hybrids, <a href="#page246">246</a>.</p> + <p>—— on reciprocal crosses, <a href="#page258">258</a>.</p> + <p>—— on crossed varieties of nicotiana, <a href="#page271">271</a>.</p> + <p>—— on crossing male and hermaphrodite flowers, <a href="#page451">451</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>L.</b></p> + </div> + + <div class="stanza"> + <p>Lamarck on adaptive characters, <a href="#page426">426</a>.</p> + <p>Land-shells, distribution of, <a href="#page397">397</a>.</p> + <p>—— of Madeira, naturalised, <a href="#page403">403</a>.</p> + <p>Languages, classification of, <a href="#page422">422</a>.</p> + <p>Lapse, great, of time, <a href="#page282">282</a>.</p> +<!-- Page 497 --><span class="pagenum"><a name="page497"></a>[497]</span> + <p>Larvæ, <a href="#page440">440</a>.</p> + <p>Laurel, nectar secreted by the leaves,</p> + <p>Laws of variation, <a href="#page131">131</a>.</p> + <p>Leech, varieties of, <a href="#page76">76</a>.</p> + <p>Leguminosæ, nectar secreted by glands, <a href="#page92">92</a>.</p> + <p>Lepidosiren, <a href="#page107">107</a>, <a href="#page330">330</a>.</p> + <p>Life, struggle for, <a href="#page60">60</a>.</p> + <p>Lingula, Silurian, <a href="#page307">307</a>.</p> + <p>Linnæus, aphorism of, <a href="#page413">413</a>.</p> + <p>Lion, mane of, <a href="#page88">88</a>.</p> + <p>——, young of, striped, <a href="#page439">439</a>.</p> + <p>Lobelia fulgens, <a href="#page73">73</a>, <a href="#page98">98</a>.</p> + <p>Lobelia, sterility of crosses, <a href="#page250">250</a>.</p> + <p>Loess of the Rhine, <a href="#page384">384</a>.</p> + <p>Lowness of structure connected with variability, <a href="#page149">149</a>.</p> + <p>Lowness, related to wide distribution, <a href="#page406">406</a>.</p> + <p>Lubbock, Mr., on the nerves of coccus, <a href="#page46">46</a>.</p> + <p>Lucas, Dr. P., on inheritance, <a href="#page12">12</a>.</p> + <p>——, on resemblance of child to parent, <a href="#page275">275</a>.</p> + <p>Lund and Clausen on fossils of Brazil, <a href="#page339">339</a>.</p> + <p>Lyell, Sir C, on the struggle for existence, <a href="#page62">62</a>.</p> + <p>——, on modern changes of the earth, <a href="#page95">95</a>.</p> + <p>——, on measure of denudation, <a href="#page284">284</a>.</p> + <p>——, on a carboniferous land-shell, <a href="#page289">289</a>.</p> + <p>——, on strata beneath Silurian system, <a href="#page308">308</a>.</p> + <p>——, on the imperfection of the geological record, <a href="#page311">311</a>.</p> + <p>——, on the appearance of species, <a href="#page312">312</a>.</p> + <p>——, on Barrande's colonies, <a href="#page313">313</a>.</p> + <p>——, on tertiary formations of Europe and North America, <a href="#page323">323</a>.</p> + <p>——, on parallelism of tertiary formations, <a href="#page328">328</a>.</p> + <p>——, on transport of seeds by icebergs, <a href="#page363">363</a>.</p> + <p>——, on great alternations of climate, <a href="#page382">382</a>.</p> + <p>——, on the distribution of fresh-water shells, <a href="#page385">385</a>.</p> + <p>——, on land-shells of Madeira, <a href="#page402">402</a>.</p> + <p>Lyell and Dawson on fossilized trees in Nova Scotia, <a href="#page297">297</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>M.</b></p> + </div> + + <div class="stanza"> + <p>Macleay on analogical characters, <a href="#page426">426</a>.</p> + <p>Madeira, plants of, <a href="#page107">107</a>.</p> + <p>——, beetles of, wingless, <a href="#page135">135</a>.</p> + <p>——, fossil land-shells of, <a href="#page339">339</a>.</p> + <p>——, birds of, <a href="#page390">390</a>.</p> + <p>Magpie tame in Norway, <a href="#page212">212</a>.</p> + <p>Maize, crossed, <a href="#page270">270</a>.</p> + <p>Malay Archipelago compared with Europe, <a href="#page300">300</a>.</p> + <p>——, mammals of, <a href="#page395">395</a>.</p> + <p>Malpighiaceæ, <a href="#page417">417</a>.</p> + <p>Mammæ, rudimentary, <a href="#page451">451</a>.</p> + <p>Mammals, fossil, in secondary formation, <a href="#page304">304</a>.</p> + <p>——, insular, <a href="#page394">394</a>.</p> + <p>Man, origin of races of, <a href="#page199">199</a>.</p> + <p>Manatee, rudimentary nails of, <a href="#page454">454</a>.</p> + <p>Marsupials of Australia, <a href="#page116">116</a>.</p> + <p>——, fossil species of, <a href="#page339">339</a>.</p> + <p>Martens, M., experiment on seeds, <a href="#page360">360</a>.</p> + <p>Martin, Mr. W. C., on striped mules, <a href="#page165">165</a>.</p> + <p>Matteucci on the electric organs of rays, <a href="#page193">193</a>.</p> + <p>Matthiola, reciprocal crosses of, <a href="#page258">258</a>.</p> + <p>Means of dispersal, <a href="#page356">356</a>.</p> + <p>Melipona domestica, <a href="#page225">225</a>.</p> + <p>Metamorphism of oldest rocks, <a href="#page308">308</a>.</p> + <p>Mice destroying bees, <a href="#page74">74</a>.</p> + <p>——, acclimatisation of, <a href="#page141">141</a>.</p> + <p>Migration, bears on first appearance of fossils, <a href="#page297">297</a>.</p> + <p>Miller, Prof., on the cells of bees, <a href="#page226">226</a>.</p> + <p>Mirabilis, crosses of, <a href="#page258">258</a>.</p> + <p>Missel-thrush, <a href="#page76">76</a>.</p> + <p>Misseltoe, complex relations of, <a href="#page3">3</a>.</p> + <p>Mississippi, rate of deposition at mouth, <a href="#page284">284</a>.</p> + <p>Mocking-thrush of the Galapagos, <a href="#page402">402</a>.</p> + <p>Modification of species, how far applicable, <a href="#page483">483</a>.</p> + <p>Moles, blind, <a href="#page137">137</a>.</p> + <p>Mongrels, fertility and sterility of, <a href="#page268">268</a>.</p> + <p>—— and hybrids compared, <a href="#page272">272</a>.</p> +<!-- Page 498 --><span class="pagenum"><a name="page498"></a>[498]</span> + <p>Monkeys, fossil, <a href="#page304">304</a>.</p> + <p>Monocanthus, <a href="#page424">424</a>.</p> + <p>Mons, Van, on the origin of fruit-trees, <a href="#page29">29</a>.</p> + <p>Moquin-Tandon on sea-side plants, <a href="#page132">132</a>.</p> + <p>Morphology, <a href="#page433">433</a>.</p> + <p>Mozart, musical powers of, <a href="#page209">209</a>.</p> + <p>Mud, seeds in, <a href="#page386">386</a>.</p> + <p>Mules, striped, <a href="#page165">165</a>.</p> + <p>Müller, Dr. F., on Alpine Australian plants, <a href="#page375">375</a>.</p> + <p>Murchison, Sir R., on the formations of Russia, <a href="#page290">290</a>.</p> + <p>——, on azoic formations, <a href="#page308">308</a>.</p> + <p>——, on extinction, <a href="#page317">317</a>.</p> + <p>Mustela vison, <a href="#page179">179</a>.</p> + <p>Myanthus, <a href="#page424">424</a>.</p> + <p>Myrmecocystus, <a href="#page239">239</a>.</p> + <p>Myrmica, eyes of, <a href="#page240">240</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>N.</b></p> + </div> + + <div class="stanza"> + <p>Nails, rudimentary, <a href="#page454">454</a>.</p> + <p>Natural history, future progress of, <a href="#page485">485</a>.</p> + <p>—— selection, <a href="#page80">80</a>.</p> + <p>—— system, <a href="#page413">413</a>.</p> + <p>Naturalisation of forms distinct from the indigenous species, <a href="#page115">115</a>.</p> + <p>—— in New Zealand, <a href="#page201">201</a>.</p> + <p>Nautilus, Silurian, <a href="#page307">307</a>.</p> + <p>Nectar of plants, <a href="#page92">92</a>.</p> + <p>Nectaries, how formed, <a href="#page92">92</a>.</p> + <p>Nelumbium luteum, <a href="#page387">387</a>.</p> + <p>Nests, variation in, <a href="#page211">211</a>.</p> + <p>Neuter insects, <a href="#page236">236</a>.</p> + <p>Newman, Mr., on humble-bees, <a href="#page74">74</a>.</p> + <p>New Zealand, productions of, not perfect, <a href="#page201">201</a>.</p> + <p>——, naturalised products of, <a href="#page337">337</a>.</p> + <p>——, fossil birds of, <a href="#page339">339</a>.</p> + <p>——, glacial action in, <a href="#page373">373</a>.</p> + <p>——, crustaceans of, <a href="#page376">376</a>.</p> + <p>——, algæ of, <a href="#page376">376</a>.</p> + <p>——, number of plants of, <a href="#page389">389</a>.</p> + <p>——, flora of, <a href="#page399">399</a>.</p> + <p>Nicotiana, crossed varieties of, <a href="#page271">271</a>.</p> + <p>——, certain species very sterile, <a href="#page257">257</a>.</p> + <p>Noble, Mr., on fertility of Rhododendron, <a href="#page252">252</a>.</p> + <p>Nodules, phosphatic, in azoic rocks, <a href="#page308">308</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>O.</b></p> + </div> + + <div class="stanza"> + <p>Oak, varieties of, <a href="#page50">50</a>.</p> + <p>Onites apelles, <a href="#page135">135</a>.</p> + <p>Orchis, pollen of, <a href="#page193">193</a>.</p> + <p>Organs of extreme perfection, <a href="#page186">186</a>.</p> + <p>——, electric, of fishes, <a href="#page192">192</a>.</p> + <p>—— of little importance, <a href="#page194">194</a>.</p> + <p>——, homologous, <a href="#page434">434</a>.</p> + <p>——, rudiments of, and nascent, <a href="#page450">450</a>.</p> + <p>Ornithorhynchus, <a href="#page107">107</a>, <a href="#page416">416</a>.</p> + <p>Ostrich not capable of flight, <a href="#page134">134</a>.</p> + <p>——, habit of laying eggs together, <a href="#page218">218</a>.</p> + <p>——, American, two species of, <a href="#page349">349</a>.</p> + <p>Otter, habits of, how acquired, <a href="#page179">179</a>.</p> + <p>Ouzel, water, <a href="#page185">185</a>.</p> + <p>Owen, Prof., on birds not flying, <a href="#page134">134</a>.</p> + <p>——, on vegetative repetition, <a href="#page149">149</a>.</p> + <p>——, on variable length of arms in ourang-outang, <a href="#page150">150</a>.</p> + <p>——, on the swim-bladder of fishes, <a href="#page191">191</a>.</p> + <p>——, on electric organs, <a href="#page192">192</a>.</p> + <p>——, on fossil horse of La Plata, <a href="#page319">319</a>.</p> + <p>——, on relations of ruminants and pachyderms, <a href="#page329">329</a>.</p> + <p>——, on fossil birds of New Zealand, <a href="#page339">339</a>.</p> + <p>——, on succession of types, <a href="#page339">339</a>.</p> + <p>——, on affinities of the dugong, <a href="#page414">414</a>.</p> + <p>——, on homologous organs, <a href="#page434">434</a>.</p> + <p>——, on the metamorphosis of cephalopods and spiders, <a href="#page442">442</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>P.</b></p> + </div> + + <div class="stanza"> + <p>Pacific Ocean, faunas of, <a href="#page348">348</a>.</p> + <p>Paley on no organ formed to give pain, <a href="#page201">201</a>.</p> + <p>Pallas on the fertility of the wild stocks of domestic animals, <a href="#page254">254</a>.</p> + <p>Paraguay, cattle destroyed by flies, <a href="#page72">72</a>.</p> + <p>Parasites, <a href="#page217">217</a>.</p> + <p>Partridge, dirt on feet, <a href="#page363">363</a>.</p> + <p>Parts greatly developed, variable, <a href="#page150">150</a>.</p> + <p>——, degrees of utility of, <a href="#page201">201</a>.</p> + <p>Parus major, <a href="#page184">184</a>.</p> + <p>Passiflora, <a href="#page251">251</a>.</p> + <p>Peaches in United States, <a href="#page85">85</a>.</p> + <p>Pear, grafts of, <a href="#page262">262</a>.</p> +<!-- Page 499 --><span class="pagenum"><a name="page499"></a>[499]</span> + <p>Pelargonium, flowers of, <a href="#page145">145</a>.</p> + <p>——, sterility of, <a href="#page251">251</a>.</p> + <p>Pelvis of women, <a href="#page144">144</a>.</p> + <p>Peloria, <a href="#page145">145</a>.</p> + <p>Period, glacial, <a href="#page365">365</a>.</p> + <p>Petrels, habits of, <a href="#page184">184</a>.</p> + <p>Phasianus, fertility of hybrids, <a href="#page253">253</a>.</p> + <p>Pheasant, young, wild, <a href="#page216">216</a>.</p> + <p>Philippi on tertiary species in Sicily, <a href="#page312">312</a>.</p> + <p>Pictet, Prof., on groups of species suddenly appearing, <a href="#page302">302</a>, <a href="#page305">305</a>.</p> + <p>——, on rate of organic change, <a href="#page313">313</a>.</p> + <p>——, on continuous succession of genera, <a href="#page316">316</a>.</p> + <p>——, on close alliance of fossils in consecutive formations, <a href="#page335">335</a>.</p> + <p>——, on embryological succession, <a href="#page338">338</a>.</p> + <p>Pierce, Mr., on varieties of wolves, <a href="#page91">91</a>.</p> + <p>Pigeons with feathered feet and skin between toes, <a href="#page12">12</a>.</p> + <p>——, breeds described, and origin of, <a href="#page20">20</a>.</p> + <p>——, breeds of, how produced, <a href="#page39">39</a>, <a href="#page42">42</a>.</p> + <p>——, tumbler, not being able to get out of egg, <a href="#page87">87</a>.</p> + <p>——, reverting to blue colour, <a href="#page160">160</a>.</p> + <p>——, instinct of tumbling, <a href="#page214">214</a>.</p> + <p>——, carriers, killed by hawks, <a href="#page362">362</a>.</p> + <p>——, young of, <a href="#page445">445</a>.</p> + <p>Pistil, rudimentary, <a href="#page451">451</a>.</p> + <p>Plants, poisonous, not affecting certain coloured animals, <a href="#page12">12</a>.</p> + <p>——, selection applied to, <a href="#page32">32</a>.</p> + <p>——, gradual improvement of, <a href="#page37">37</a>.</p> + <p>—— not improved in barbarous countries, <a href="#page38">38</a>.</p> + <p>—— destroyed by insects, <a href="#page67">67</a>.</p> + <p>——, in midst of range, have to struggle with other plants, <a href="#page77">77</a>.</p> + <p>——, nectar of, <a href="#page92">92</a>.</p> + <p>——, fleshy, on sea-shores, <a href="#page132">132</a>.</p> + <p>——, fresh-water, distribution of, <a href="#page386">386</a>.</p> + <p>——, low in scale, widely distributed, <a href="#page406">406</a>.</p> + <p>Plumage, laws of change in sexes of birds, <a href="#page89">89</a>.</p> + <p>Plums in the United States, <a href="#page85">85</a>.</p> + <p>Pointer dog, origin of, <a href="#page35">35</a>.</p> + <p>——, habits of, <a href="#page213">213</a>.</p> + <p>Poison not affecting certain coloured animals, <a href="#page12">12</a>.</p> + <p>——, similar effect of, on animals and plants, <a href="#page484">484</a>.</p> + <p>Pollen of fir-trees, <a href="#page203">203</a>.</p> + <p>Poole, Col., on striped hemionus, <a href="#page163">163</a>.</p> + <p>Potamogeton, <a href="#page387">387</a>.</p> + <p>Prestwich, Mr., on English and French eocene formations, <a href="#page328">328</a>.</p> + <p>Primrose, <a href="#page49">49</a>.</p> + <p>——, sterility of, <a href="#page247">247</a>.</p> + <p>Primula, varieties of, <a href="#page49">49</a>.</p> + <p>Proteolepas, <a href="#page148">148</a>.</p> + <p>Proteus, <a href="#page139">139</a>.</p> + <p>Psychology, future progress of, <a href="#page489">489</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>Q.</b></p> + </div> + + <div class="stanza"> + <p>Quagga, striped, <a href="#page165">165</a>.</p> + <p>Quince, grafts of, <a href="#page262">262</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>R.</b></p> + </div> + + <div class="stanza"> + <p>Rabbit, disposition of young, <a href="#page215">215</a>.</p> + <p>Races, domestic, characters of, <a href="#page16">16</a>.</p> + <p>Race-horses, Arab, <a href="#page35">35</a>.</p> + <p>——, English, <a href="#page356">356</a>.</p> + <p>Ramond on plants of Pyrenees, <a href="#page368">368</a>.</p> + <p>Ramsay, Prof., on thickness of the British formations, <a href="#page284">284</a>.</p> + <p>——, on faults, <a href="#page285">285</a>.</p> + <p>Ratio of increase, <a href="#page63">63</a>.</p> + <p>Rats, supplanting each other, <a href="#page76">76</a>.</p> + <p>——, acclimatisation of, <a href="#page141">141</a>.</p> + <p>——, blind in cave, <a href="#page137">137</a>.</p> + <p>Rattle-snake, <a href="#page201">201</a>.</p> + <p>Reason and instinct, <a href="#page208">208</a>.</p> + <p>Recapitulation, general, <a href="#page459">459</a>.</p> + <p>Reciprocity of crosses, <a href="#page258">258</a>.</p> + <p>Record, geological, imperfect, <a href="#page279">279</a>.</p> + <p>Rengger on flies destroying cattle, <a href="#page72">72</a>.</p> + <p>Reproduction, rate of, <a href="#page63">63</a>.</p> + <p>Resemblance to parents in mongrels and hybrids, <a href="#page273">273</a>.</p> + <p>Reversion, law of inheritance, <a href="#page14">14</a>.</p> + <p>—— in pigeons to blue colour, <a href="#page160">160</a>.</p> + <p>Rhododendron, sterility of, <a href="#page251">251</a>.</p> + <p>Richard, Prof., on Aspicarpa, <a href="#page417">417</a>.</p> + <p>Richardson, Sir J., on structure of squirrels, <a href="#page180">180</a>.</p> + <p>——, on fishes of the southern hemisphere, <a href="#page376">376</a>.</p> + <p>Robinia, grafts of, <a href="#page262">262</a>.</p> +<!-- Page 500 --><span class="pagenum"><a name="page500"></a>[500]</span> + <p>Rodents, blind, <a href="#page137">137</a>.</p> + <p>Rudimentary organs, <a href="#page450">450</a>.</p> + <p>Rudiments important for classification, <a href="#page416">416</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>S.</b></p> + </div> + + <div class="stanza"> + <p>Sagaret on grafts, <a href="#page262">262</a>.</p> + <p>Salmons, males fighting, and hooked jaws of, <a href="#page88">88</a>.</p> + <p>Salt-water, how far injurious to seeds, <a href="#page358">358</a>.</p> + <p>Saurophagus sulphuratus, <a href="#page183">183</a>.</p> + <p>Schiödte on blind insects, <a href="#page138">138</a>.</p> + <p>Schlegel on snakes, <a href="#page144">144</a>.</p> + <p>Sea-water, how far injurious to seeds, <a href="#page358">358</a>.</p> + <p>Sebright, Sir J., on crossed animals, <a href="#page20">20</a>.</p> + <p>——, on selection of pigeons, <a href="#page31">31</a>.</p> + <p>Sedgwick, Prof., on groups of species suddenly appearing, <a href="#page302">302</a>.</p> + <p>Seedlings destroyed by insects, <a href="#page67">67</a>.</p> + <p>Seeds, nutriment in, <a href="#page77">77</a>.</p> + <p>——, winged, <a href="#page146">146</a>.</p> + <p>——, power of resisting salt-water, <a href="#page358">358</a>.</p> + <p>—— in crops and intestines of birds, <a href="#page361">361</a>.</p> + <p>—— eaten by fish, <a href="#page362">362</a>, <a href="#page387">387</a>.</p> + <p>—— in mud, <a href="#page386">386</a>.</p> + <p>——, hooked, on islands, <a href="#page392">392</a>.</p> + <p>Selection of domestic products, <a href="#page29">29</a>.</p> + <p>——, principle not of recent origin, <a href="#page33">33</a>.</p> + <p>——, unconscious, <a href="#page34">34</a>.</p> + <p>——, natural, <a href="#page80">80</a>.</p> + <p>——, sexual, <a href="#page87">87</a>.</p> + <p>——, natural, circumstances favourable to, <a href="#page102">102</a>.</p> + <p>Sexes, relations of, <a href="#page87">87</a>.</p> + <p>Sexual characters variable, <a href="#page156">156</a>.</p> + <p>—— selection, <a href="#page87">87</a>.</p> + <p>Sheep, Merino, their selection, <a href="#page31">31</a>.</p> + <p>——, two sub-breeds unintentionally produced, <a href="#page36">36</a>.</p> + <p>——, mountain, varieties of, <a href="#page76">76</a>.</p> + <p>Shells, colours of, <a href="#page132">132</a>.</p> + <p>——, littoral, seldom embedded, <a href="#page288">288</a>.</p> + <p>——, fresh-water, dispersal of, <a href="#page385">385</a></p> + <p>—— of Madeira, <a href="#page391">391</a>.</p> + <p>——, land, distribution of, <a href="#page397">397</a>.</p> + <p>Silene, fertility of crosses, <a href="#page257">257</a>.</p> + <p>Silliman, Prof., on blind rat, <a href="#page137">137</a>.</p> + <p>Skulls of young mammals, <a href="#page197">197</a>, <a href="#page436">436</a>.</p> + <p>Slave-making instinct, <a href="#page219">219</a>.</p> + <p>Smith, Col. Hamilton, on striped horses, <a href="#page164">164</a>.</p> + <p>——, Mr. Fred., on slave-making ants, <a href="#page219">219</a>.</p> + <p>——, on neuter ants, <a href="#page239">239</a>.</p> + <p>——, Mr., of Jordan Hill, on the degradation of coast-rocks, <a href="#page283">283</a>.</p> + <p>Snap-dragon, <a href="#page161">161</a>.</p> + <p>Somerville, Lord, on selection of sheep, <a href="#page31">31</a>.</p> + <p>Sorbus, grafts of, <a href="#page262">262</a>.</p> + <p>Spaniel, King Charles's breed, <a href="#page35">35</a>.</p> + <p>Species, polymorphic, <a href="#page46">46</a>.</p> + <p>——, common, variable, <a href="#page53">53</a>.</p> + <p>—— in large genera variable, <a href="#page54">54</a>.</p> + <p>——, groups of, suddenly appearing, <a href="#page302">302</a>, <a href="#page307">307</a>.</p> + <p>—— beneath Silurian formations, <a href="#page307">307</a>.</p> + <p>—— successively appearing, <a href="#page312">312</a>.</p> + <p>—— changing simultaneously throughout the world, <a href="#page322">322</a>.</p> + <p>Spencer, Lord, on increase in size of cattle, <a href="#page35">35</a>.</p> + <p>Sphex, parasitic, <a href="#page218">218</a>.</p> + <p>Spiders, development of, <a href="#page442">442</a>.</p> + <p>Spitz-dog crossed with fox, <a href="#page268">268</a>.</p> + <p>Sports in plants, <a href="#page9">9</a>.</p> + <p>Sprengel, C. C, on crossing, <a href="#page98">98</a>.</p> + <p>——, on ray-florets, <a href="#page145">145</a>.</p> + <p>Squirrels, gradations in structure, <a href="#page180">180</a>.</p> + <p>Staffordshire, heath, changes in, <a href="#page71">71</a>.</p> + <p>Stag-beetles, fighting, <a href="#page88">88</a>.</p> + <p>Sterility from changed conditions of life, <a href="#page9">9</a>.</p> + <p>—— of hybrids, <a href="#page246">246</a>.</p> + <p>—— ——, laws of, <a href="#page255">255</a>.</p> + <p>—— ——, causes of, <a href="#page263">263</a>.</p> + <p>—— from unfavourable conditions, <a href="#page265">265</a>.</p> + <p>—— of certain varieties, <a href="#page269">269</a>.</p> + <p>St. Helena, productions of, <a href="#page390">390</a>.</p> + <p>St. Hilaire, Aug., on classification, <a href="#page418">418</a>.</p> + <p>St. John, Mr., on habits of cats, <a href="#page91">91</a>.</p> + <p>Sting of bee, <a href="#page202">202</a>.</p> + <p>Stocks, aboriginal, of domestic animals, <a href="#page18">18</a>.</p> + <p>Strata, thickness of, in Britain, <a href="#page284">284</a>.</p> + <p>Stripes on horses, <a href="#page163">163</a>.</p> +<!-- Page 501 --><span class="pagenum"><a name="page501"></a>[501]</span> + <p>Structure, degrees of utility of, <a href="#page201">201</a>.</p> + <p>Struggle for existence, <a href="#page60">60</a>.</p> + <p>Succession, geological, <a href="#page312">312</a>.</p> + <p>Succession of types in same areas, <a href="#page338">338</a>.</p> + <p>Swallow, one species supplanting another, <a href="#page76">76</a>.</p> + <p>Swim-bladder, <a href="#page190">190</a>.</p> + <p>System, natural, <a href="#page413">413</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>T.</b></p> + </div> + + <div class="stanza"> + <p>Tail of giraffe, <a href="#page195">195</a>.</p> + <p>—— of aquatic animals, <a href="#page196">196</a>.</p> + <p>——, rudimentary, <a href="#page454">454</a>.</p> + <p>Tarsi deficient, <a href="#page135">135</a>.</p> + <p>Tausch on umbelliferous flowers, <a href="#page146">146</a>.</p> + <p>Teeth and hair correlated, <a href="#page144">144</a>.</p> + <p>——, embryonic, traces of, in birds, <a href="#page450">450</a>.</p> + <p>——, rudimentary, in embryonic calf, <a href="#page450">450</a>, <a href="#page480">480</a>.</p> + <p>Tegetmeier, Mr., on cells of bees, <a href="#page228">228</a>, <a href="#page233">233</a>.</p> + <p>Temminck on distribution aiding classification, <a href="#page419">419</a>.</p> + <p>Thouin on grafts, <a href="#page262">262</a>.</p> + <p>Thrush, aquatic species of, <a href="#page185">185</a>.</p> + <p>——, mocking, of the Galapagos, <a href="#page402">402</a>.</p> + <p>——, young of, spotted, <a href="#page439">439</a>.</p> + <p>——, nest of, <a href="#page243">243</a>.</p> + <p>Thuret, M., on crossed fuci, <a href="#page258">258</a>.</p> + <p>Thwaites, Mr., on acclimatisation, <a href="#page140">140</a>.</p> + <p>Tierra del Fuego, dogs of, <a href="#page215">215</a>.</p> + <p>——, plants of, <a href="#page374">374</a>, <a href="#page378">378</a>.</p> + <p>Timber-drift, <a href="#page360">360</a>.</p> + <p>Time, lapse of, <a href="#page282">282</a>.</p> + <p>Titmouse, <a href="#page184">184</a>.</p> + <p>Toads on islands, <a href="#page393">393</a>.</p> + <p>Tobacco, crossed varieties of, <a href="#page271">271</a>.</p> + <p>Tomes, Mr., on the distribution of bats, <a href="#page395">395</a>.</p> + <p>Transitions in varieties rare, <a href="#page172">172</a>.</p> + <p>Trees on islands belong to peculiar orders, <a href="#page392">392</a>.</p> + <p>—— with separated sexes, <a href="#page99">99</a>.</p> + <p>Trifolium pratense, <a href="#page73">73</a>, <a href="#page94">94</a>.</p> + <p>—— incarnatum, <a href="#page94">94</a>.</p> + <p>Trigonia, <a href="#page321">321</a>.</p> + <p>Trilobites, <a href="#page307">307</a>.</p> + <p>——, sudden extinction of, <a href="#page321">321</a>.</p> + <p>Troglodytes, <a href="#page243">243</a>.</p> + <p>Tucutucu, blind, <a href="#page137">137</a>.</p> + <p>Tumbler pigeons, habits of, hereditary, <a href="#page214">214</a>.</p> + <p>——, young of, <a href="#page446">446</a>.</p> + <p>Turkey-cock, brush of hair on breast, <a href="#page90">90</a>.</p> + <p>Turkey, naked skin on head, <a href="#page197">197</a>.</p> + <p>——, young, wild, <a href="#page216">216</a>.</p> + <p>Turnip and cabbage, analogous variations of, <a href="#page159">159</a>.</p> + <p>Type, unity of, <a href="#page206">206</a>.</p> + <p>Types, succession of, in same areas, <a href="#page339">339</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>U.</b></p> + </div> + + <div class="stanza"> + <p>Udders enlarged by use, <a href="#page11">11</a>.</p> + <p>——, rudimentary, <a href="#page451">451</a>.</p> + <p>Ulex, young leaves of, <a href="#page439">439</a>.</p> + <p>Umbelliferæ, outer and inner florets of, <a href="#page144">144</a>.</p> + <p>Unity of type, <a href="#page206">206</a>.</p> + <p>Use, effects of, under domestication, <a href="#page11">11</a>.</p> + <p>——, effects of, in a state of nature, <a href="#page134">134</a>.</p> + <p>Utility, how far important in the construction of each part, <a href="#page199">199</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>V.</b></p> + </div> + + <div class="stanza"> + <p>Valenciennes on fresh-water fish, <a href="#page384">384</a>.</p> + <p>Variability of mongrels and hybrids, <a href="#page274">274</a>.</p> + <p>Variation under domestication, <a href="#page7">7</a>.</p> + <p>—— caused by reproductive system being affected by conditions of life, <a href="#page8">8</a>.</p> + <p>—— under nature, <a href="#page44">44</a>.</p> + <p>——, laws of, <a href="#page131">131</a>.</p> + <p>Variations appear at corresponding ages, <a href="#page14">14</a>, <a href="#page86">86</a>.</p> + <p>——, analogous in distinct species, <a href="#page159">159</a>.</p> + <p>Varieties, natural, <a href="#page44">44</a>.</p> + <p>——, struggle between, <a href="#page75">75</a>.</p> + <p>——, domestic, extinction of, <a href="#page111">111</a>.</p> + <p>——, transitional, rarity of, <a href="#page172">172</a>.</p> + <p>——, when crossed, fertile, <a href="#page268">268</a>.</p> + <p>——, when crossed, sterile, <a href="#page269">269</a>.</p> + <p>——, classification of, <a href="#page423">423</a>.</p> + <p>Verbascum, sterility of, <a href="#page251">251</a>.</p> + <p>——, varieties of, crossed, <a href="#page271">271</a>.</p> + <p>Verneuil, M. de, on the succession of species, <a href="#page325">325</a>.</p> + <p>Viola tricolor, <a href="#page73">73</a>.</p> +<!-- Page 502 --><span class="pagenum"><a name="page502"></a>[502]</span> + <p>Volcanic islands, denudation of, <a href="#page285">285</a>.</p> + <p>Vulture, naked skin on head, <a href="#page197">197</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>W.</b></p> + </div> + + <div class="stanza"> + <p>Wading-birds, <a href="#page386">386</a>.</p> + <p>Wallace, Mr., on origin of species, <a href="#page2">2</a>.</p> + <p>——, on law of geographical distribution, <a href="#page355">355</a>.</p> + <p>——, on the Malay Archipelago, <a href="#page395">395</a>.</p> + <p>Wasp, sting of, <a href="#page202">202</a>.</p> + <p>Water, fresh, productions of, <a href="#page383">383</a>.</p> + <p>Water-hen, <a href="#page185">185</a>.</p> + <p>Waterhouse, Mr., on Australian marsupials, <a href="#page116">116</a>.</p> + <p>——, on greatly developed parts being variable, <a href="#page150">150</a>.</p> + <p>——, on the cells of bees, <a href="#page225">225</a>.</p> + <p>——, on general affinities, <a href="#page429">429</a>.</p> + <p>Water-ouzel, <a href="#page185">185</a>.</p> + <p>Watson, Mr. H. C, on range of varieties of British plants, <a href="#page58">58</a>.</p> + <p>——, on acclimatisation, <a href="#page140">140</a>.</p> + <p>——, on flora of Azores, <a href="#page363">363</a>.</p> + <p>——, on Alpine plants, <a href="#page368">368</a>, <a href="#page376">376</a>.</p> + <p>——, on rarity of intermediate varieties, <a href="#page176">176</a>.</p> + <p>Weald, denudation of, <a href="#page285">285</a>.</p> + <p>Web of feet in water-birds, <a href="#page185">185</a>.</p> + <p>West Indian islands, mammals of, <a href="#page396">396</a>.</p> + <p>Westwood on species in large genera being closely allied to others, <a href="#page57">57</a>.</p> + <p>—— on the tarsi of Engidæ, <a href="#page157">157</a>.</p> + <p>—— on the antennæ of hymenopterous insects, <a href="#page415">415</a>.</p> + <p>Wheat, varieties of, <a href="#page113">113</a>.</p> + <p>White Mountains, flora of, <a href="#page365">365</a>.</p> + <p>Wings, reduction of size, <a href="#page134">134</a>.</p> + <p>—— of insects homologous with branchiæ, <a href="#page191">191</a>.</p> + <p>——, rudimentary, in insects, <a href="#page450">450</a>.</p> + <p>Wolf crossed with dog, <a href="#page214">214</a>.</p> + <p>—— of Falkland Isles, <a href="#page394">394</a>.</p> + <p>Wollaston, Mr., on varieties of insects, <a href="#page48">48</a>.</p> + <p>——, on fossil varieties of land-shells in Madeira, <a href="#page52">52</a>.</p> + <p>——, on colours of insects on sea-shore, <a href="#page132">132</a>.</p> + <p>——, on wingless beetles, <a href="#page135">135</a>.</p> + <p>——, on rarity of intermediate varieties, <a href="#page176">176</a>.</p> + <p>——, on insular insects, <a href="#page389">389</a>.</p> + <p>——, on land-shells of Madeira, naturalised, <a href="#page402">402</a>.</p> + <p>Wolves, varieties of, <a href="#page90">90</a>.</p> + <p>Woodpecker, habits of, <a href="#page184">184</a>.</p> + <p>——, green colour of, <a href="#page197">197</a>.</p> + <p>Woodward, Mr., on the duration of specific forms, <a href="#page294">294</a>.</p> + <p>——, on the continuous succession of genera, <a href="#page316">316</a>.</p> + <p>——, on the succession of types, <a href="#page339">339</a>.</p> + <p>World, species changing simultaneously throughout, <a href="#page322">322</a>.</p> + <p>Wrens, nest of, <a href="#page243">243</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>Y.</b></p> + </div> + + <div class="stanza"> + <p>Youatt, Mr., on selection, <a href="#page31">31</a>.</p> + <p>——, on sub-breeds of sheep, <a href="#page36">36</a>.</p> + <p>——, on rudimentary horns in young cattle, <a href="#page454">454</a>.</p> + </div> + + <div class="stanza"> + <p class="i8"><b>Z.</b></p> + </div> + + <div class="stanza"> + <p>Zebra, stripes on, <a href="#page163">163</a>.</p> + </div> + </div> +<h3>THE END.</h3> + +<hr /> + +<p class="cenhead"><span class="scac">LONDON: PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET, AND CHARING CROSS.</span></p> + +<div style='display:block;margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK ORIGIN OF SPECIES ***</div> +<div style='display:block;margin:1em 0;'>This file should be named 22764-h.htm or 22764-h.zip</div> +<div style='display:block;margin:1em 0;'>This and all associated files of various formats will be found in https://www.gutenberg.org/2/2/7/6/22764/</div> +<div style='display:block; margin:1em 0'> +Updated editions will replace the previous one—the old editions will +be renamed. +</div> + +<div style='display:block; margin:1em 0'> +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. 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