G ; glen ida feed 7 Oe ay OR LP EE ae ft eae Lids fj teal gies Cornell University “* Library OF THE Hew Work State College of Agriculture a TRL LTE sits Sia alsa lag OO: Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www. archive.org/details/cu31924003039660 NATURAL SELECTION AND TROPICAL NATURE NATURAL SELECTION AND TROPICAL NATURE ESSAYS ON DESCRIPTIVE AND THEORETICAL BIOLOGY BY ALFRED RUSSEL WALLACE AUTHOR OF ‘THE MALAY ARCHIPELAGO,’ ‘7sLAND LIFE,’ ‘ DARWINISM,’ ETC. NEW EDITION WITH CORRECTIONS AND ADDITIONS London MACMILLAN AND CO. AND NEW YORK 1895 All rights reserved i ‘ fog % : \ G a Natura SELECTION, First Edition 1870. Reprinted 1875 TropicaL Nature. First Edition 1878 First published together 1891. Reprinted 1895 PREFACE THE present volume consists mainly of a reprint of two volumes of essays — Contributions to the Theory of Natural Selection, which appeared in 1870, with a second edition in 1871, and has now been many years out of print; and, Tropical Nature and Other Essays, which appeared in 1878. In preparing a new edition of these works to appear as a single volume I have thought it advisable to omit two essays—that on “The Malayan Papilionide” as being too technical for general readers, and that on “The Distribution of Animals as indicating Geographical Changes,” which con- tains nothing that is not more fully treated in my other works. Another essay — ‘“By-Paths in the Domain of Biology ”—has also been partly omitted, one portion of it forming a short chapter on “The Antiquity and Origin of Man,” while another portion has been incorporated in the chapter on “The Colours of Animals and Sexual Selection.” More than compensating for these omissions are two new chapters—“ The Antiquity of Man in North America” and “The Debt of Science to Darwin.” Many corrections and some important additions have been made to the text, the chief of which are indicated in the table given below ; and to facilitate reference the two original works have separate headings, and form Parts I. and II. of the present volume. vi PREFACE ALTERATIONS IN THE SECOND EDITION OF CONTRIBUTIONS, ETC. Ist Ep. 2p Ep, oT 221 =| 221 Additional facts as to birds peeing the . song of other species . 105 223 223A Mr. Spruce’s remarks on young "birds 2238 pairing with old j ‘ 107 228 228A Pouchet’s observations on a change in eat the nests of swallows . omitted 229 — Passage omitted about nest of Golden Crested Warbler, which had been in- serted on Rennie’s authority, but has not been confirmed by any later ob- servers. 261 261 Daines Barrington, on importance of pro- tection to the female bird . . ‘ 1388 372 Note A. : : ‘ ‘ 205 3728 | NoteB. . : : . i : 209 ADDITIONAL MATTER IN THE PRESENT VOLUME. NatTuRAL SELECTION. PAGES Additional facts by Leroy, Spalding, Lowne, and Dixon on the Nest-Building and other Instincts of Birds . : « 108-112 Dr. Abbott on Nesting of Baltimore Oriole ‘ . 114 aa Professor Jeitteles and Mr. Henry Reeks on Alteratiene in Mode of Nest-Building . 3 . F . 115 TRoPicaAL NATURE. Note on Dr. Shufeldt’s Investigations into the Affinities of Swifts and Humming-Birds . 3 s ; . 337 Tue ANTIQUITY oF Man in Nortu AMERICA. (Additional Chapter) ‘ ‘ ‘ : » 4388-449 Tur Desr or ScrencE To DARWIN. (Additional Chapter) . . 2 F ., . 450-475 PARKSTONE, Dorset, March 1891. CONTENTS NATURAL SELECTION I, On toe Law WHICH HAS REGULATED THE INTRODUCTION OF New SPEcIES Geographical Distribution dependent on Geologic Changes—A Law deduced from well-known Geographical and Geological facts—The Form of a true system of Classification determined by this Law—Geographical Distribution of Organisms—Geological Distribution of the Forms of Life— High Organisation of very ancient Animals consistent with this Law—Objections to Forbes’s eee of Sia gs ean Organs—Conclusion . Pages 8-19 II. On tHE TENDENCY OF VARIETIES TO DEPART INDEFINITELY FROM THE ORIGINAL TYPE Introductory Note—Instability of Varieties supposed to prove the per- manent distinctness of Species—The Struggle for Existence—The Law of Population of Species—The Abundance or Rarity of a Species dependent upon its more or less perfect Adaptation to the Conditions of Existence—Useful Variations will tend to Increase; useless or hurtful Variations to Diminish—Superior Varieties will ultimately Extirpate the Original Species—The Partial Reversion of Domesti- cated Varieties explained —Lamarck’s Hypothesis very different from that now advanced—Conclusion . 7 : . 20-33 III. Mimicry, AND OTHER PROTECTIVE RESEMBLANCES AMONG ANIMALS Test of true and false Theories—Importance of the Principle of Utility —Popular Theories of Colour in Animals—Importance of Conceal- ment as Influencing Colour—Special Modifications of Colour—Theory viii CONTENTS of Protective Colouring—Objection that Colour, as being dangerous, should not exist in Nature—Mimicry—Mimicry among Lepidoptera —Lepidoptera mimicking other Insects—Mimicry among Beetles— Beetles mimicking other Insects—Insects mimicking Species of other Orders—Cases of Mimicry among the Vertebrata—Mimicry among Snakes—Mimicry among Birds—Mimicry among Mammals—Objec- tions to Mr. Bates’s Theory of Mimicry—Mimicry by Female Insects only—Cause of the dull Colours of Female Birds—Use of the gaudy Colours of many Caterpillars—Summary—General deductions as to Colour in Nature—Conclusion . : . . Pages 34-90 IV. On Instryort in Man anp ANIMALS How Instinct may be best Studied—Definition of Instinct—Does Man possess Instincts ?—How Indians travel through unknown and track- less Forests : j , . 91-97 V. THe Puinosopuy or Birps’ Nests Instinct or Reason in the Construction of Birds’ Nests—Do Men build by Reason or by Imitation ?—Why does each Bird build a peculiar kind of Nest?—How do young Birds learn to build their first Nest? Do Birds sing by Instinct or by Imitation }—How young Birds may learn to build Nests—The Skill exhibited in Nest-building Ex- aggerated—Man’s Works mainly Imitative—Birds do Alter and Improve their Nests when altered Conditions require it—Conclusion 98-117 VI. A THEory or Brrps’ Nests Changed Conditions and persistent Habits as influencing Nidification— Classification of Nests—Sexual differences of Colour in Birds—The Law which connects the Colours of Female Birds with the mode of Nidification—What the Facts Teach us—Colour more variable than Structure or Habits, and therefore the Character which has generally been Modified—Exceptional cases confirmatory of the above Explana- tion—Real or apparent exceptions to the Law stated at page 124— Various modes of Protection of Animals—Females of some groups require and obtain more Protection than the Males—Conclusion 118-140 VII. Creation sy Law Laws from which the Origin of Species may be deduced—Mr. Darwin’s Metaphors liable to Misconception—A case of Orchid-structure ex- CONTENTS ix plained by Natural Selection—Adaptation brought about by General Laws—Beauty in Nature—How New Forms are produced by Varia- tion and Selection—The Objection that there are Limits to Variation —Objection to the Argument from Classification—The Times on Natural Selection—Intermediate or generalised Forms of Extinct Animals an indication of Transmutation or Development—Conclu- sion—A Demonstration of the Origin of Species by Natural Selection Pages 141-166 VIII. THe DeveL.opMent oF Human Racks UNDER THE Law or NatuRAL SELECTION Wide differences of Opinion as to Man’s Origin—Outline of the Theory of Natural Selection—Different Effects of Natural Selection on Animals and on Man—Infiuence of External Nature in the develop- ment of the Human Mind—Extinction of Lower Races—The Origin of the Races of Man—The Bearing of these Views on the Antiquity of Man—Their Bearing on the Dignity and Supremacy of Man— Their Bearing on the future Development of Man—Summary—Con- clusion , : 3 : f + 167-185 IX. Tae Liurts or Naturat SELECTION AS APPLIED TO Man What Natural Selection can Not do—The Brain of the Savage shown to be Larger than he Needs it to be—Size of Brain an important Element of Mental Power—Comparison of the Brains of Man and of Anthropoid Apes—Range of Intellectual Power in Man—Intellect of Savages and of Animals compared—The use of the Hairy Covering of Mammalia—The Constant Absence of Hair from certain parts of Man’s body a remarkable Phenomenon—Savage Man feels the want of this Hairy Covering—Man’s Naked Skin could not have been pro- duced by Natural Selection—Feet and Hands of Man considered as Difficulties on the Theory of Natural Selection—The Voice of Man— —The Origin of some of Man’s Mental Faculties, by the preservation of Useful Variations, not possible—Difficulty as to the Origin of the Moral Sense—Summary of the Argument as to the Insufficiency of Natural Selection to account for the Development of Man—The Origin of Consciousness—The Nature of Matter—Matter is Force —All Force is probably Will-foree—Conclusion . - 186-214 x CONTENTS TROPICAL NATURE AND OTHER ESSAYS I. THe CLIMATE AND PHysicaL ASPECTS OF THE EQuaTORIAL ZONE The three Climatal Zones of the Earth—Temperature of the Equatorial Zone—Causes of the Uniform High Temperature near the Equator— Influence of the Heat of the Soil—Influence of the Aqueous Vapour of the Atmosphere—Influence of Winds on the Temperature of the Equator—Heat due to the Condensation of Atmospheric Vapour— General Features of the Equatorial Climate—Uniformity of the Equatorial Climate in all Parts of the Globe—Effects of Vegetation on Climate—Short Twilight of the Equatorial Zone—The Aspect of the Equatorial Heavens—Intensity of Meteorological Phenomena at the Equator—Concluding Remarks 4 ao Pages 217-237 II. EquatoRIAL VEGETATION The Equatorial Forest-belt and its Causes—Geueral Features of the Equa- torial Forests—Characteristics of the Larger Forest-Trees—Flowering Trunks and their Probable Cause—Uses of Equatorial Forest-trees— The Climbing Plants of the Equatorial Forests—Palms—Uses of Palm- trees and their Products—Ferns—Ginger-worts and Wild Bananas— Arums—Screw-pines—Orchids—Bamboos— Uses of the Bamboo—Man- groves—Sensitive Plants—Comparative Scarcity of Flowers—Con- cluding Remarks on Tropical Vegetation . — . » 238-269 III. Ayiman Lire 1n tHe Trorrca, Forests Difficulties of the Subject—General Aspect of the Animal Life of Equa- torial Forests—Diurnal Lepidoptera or Butterflies—Peculiar Habits of Tropical Butterflies—Ants, Wasps, and Bees—Ants—Special Rela. tions between Ants and Vegetation—Wasps and Bees—Orthoptera and other Insects—Beetles—Wingless Insects—General Observations on Tropical Insects—Birds—Parrots—Pigeons—Picarie—Cuckoos— Trogons, Barbets, Toucans, and Hornbills—Passeres—Reptiles and Amphibia: Lizards—Snakes—Frogs and Toads—Mammalia: Mon- keys—Bats—Summary of the Aspects of Animal Life in the Tropics 270-311 CONTENTS xi IV. Humine-Birps: As Intusrratine tHe LUXURIANOE OF TRopicaL NATURE Structure—Colours and Ornaments—Descriptive Names—The Motions and Habits of Humming-birds—Display of Ornaments by the Male—Food —Nests—Geographical Distribution and Variation—Humming-birds of Juan Fernandez as illustrating Variation and Natural Selection— The Relations and Affinities of Humming-birds—How to Determine Doubtful Affinities—Resemblances of Swifts and Humming-birds— Differences between Sun-birds and Humming-birds—Conclusion : Pages 312-337 V. Tus Contours or ANIMALS AND SEXUAL SELECTION General Phenomena of Colour in the Organic World—Theory of Heat and Light as producing Colour—Changes of Colour in Animals produced by Coloured Light—Classification of Organic Colours—Protective Colours—Warning Colours—Sexual Colours—Normal Colours—The Nature of Colour—How Animal Colours are produced—Colour a Normal Product of Organisation—Theory of Protective Colours— Theory of Warning Colours—Imitative Warning Colours—The Theory of Mimicry—Theory of Sexual Colours—Colour as a means of Recognition—Colour proportionate to Integumentary Development— Selection by Females not a cause of Colour—Probable use of the Horns of Beetles—Cause of the greater Brilliancy of some Female Insects— Origin of the Ornamental Plumage of Male Birds— Theory of the Display of Ornaments by Males—Natural Selection as neutralising Sexual Selection—Greater Brilliancy of some Female Birds — Colour - development as illustrated by Humming - birds — —Theory of Normal Colours—Local causes of Colour-development— The influence of Locality on Colour in Butterflies and birds—Sense- perception influenced by Colour of the Integuments—Summary on Colour-development in Animals. < 7 . 888-394 VI. Tue Contours of PLANTS AND THE ORIGIN OF THE Conour-SENSE Source of Colouring Matter in Plants—Protective Coloration and Mimicry in Plants—Attractive Colours of Fruits—Protective Colours of Fruits —Attractive Colours of Flowers—Attractive Odours in Flowers— Attractive Grouping of Flowers—Why Alpine Flowers are so Beauti- ful—Why Allied Species of Flowers differ in Size and Beauty— Absence of Colour in Wind-fertilised Flowers—The same Theory of Colour applicable to Animals and Plants—Relation of the Colours xil CONTENTS of Flowers and their Geographical Distribution — Recent Views as to Direct Action of Light on the Colours of Flowers and Fruits— Concluding Remarks on the Importance of Colour in the Organic World—On the Origin of the Colour-sense: Supposed Increase of Colour-perception within the Historical Period—Concluding Remarks on the Colour-sense . . F . . Pages 395-415 VII. Tae AntTIQuITy AND ORIGIN or Man Indications of Man’s extreme Antiquity—Antiquity of Intellectual Man —Sculptures on Easter Island—North American Earthworks—The Great Pyramid—Conclusion . 7 . . 416-432 VIII. Tae Antiquity or Man rn NortoH AMERICA Ancient Shell Mounds—Man Coeval with Extinct Mammalia—Man in the Glacial Period—Paleolithic Implements in North America—The Auriferous Gravels of California—Fossil Remains under the Ancient Lava Beds—Works of Art in the Auriferous Gravels—Human Remains in the Auriferous Gravels—Concluding Remarks on the Antiquity of Man. . . . . + 438-449 IX. Tae Dest or Scrence to Darwin The Century before Darwin—The Voyage of the Beagle—The Journal of Researches—Studies of Domestic Animals—Studies of Cultivated and Wild Plants—Researches on the Cowslip, Primrose, and Loose- strife—The Struggle for Existence—Geographical Distribution and Dispersal of Organisms—The Descent of Man and later Works— Estimate of Darwin’s Life-Work . : . « 450-475 INDEX. : . < F : 5 é 476 ESSAYS ON NATURAL SELECTION ON THE LAW WHICH HAS REGULATED THE INTRODUCTION OF NEW SPEcIES! Geographical Distribution dependent on Geologic Changes Every naturalist who has directed his attention to the subject of the geographical distribution of animals and plants must have been interested in the singular facts which it presents. Many of these facts are quite different from what would have been anticipated, and have hitherto been considered as highly curious, but quite inexplicable. None of the explanations attempted from the time of Linneus are now considered at all satisfactory ; none of them have given a cause sufficient to account for the facts known at the time, or comprehensive enough to include all the new facts which have since been, and are daily being, added. Of late years, however, a great light has been thrown upon the subject by geological investi- gations, which have shown that the present state of the earth and of the organisms now inhabiting it is but the last stage of a long and uninterrupted series of changes which it has undergone, and consequently, that to endeavour to explain and account for its present condition without any reference to those changes (as has frequently been done) must lead to very imperfect and erroneous conclusions. The facts proved by geology are briefly these: That 1 This article, written at Sarawak in February 1855 and published in the Annals and Magazine of Natural History, September 1855, was intended to show that some form of evolution of one species from another was needed in order to explain the various classes of facts here indicated ; but at that time no means had been suggested by which the actual change of species could have been brought about. 4 NATURAL SELECTION 1 during an immense but unknown period the surface of the earth has undergone successive changes; land has sunk be- neath the ocean, while fresh land has risen up from it ; mountain chains have been elevated; islands have been formed into continents, and continents submerged till they have become islands ; and these changes have taken place, not once merely, but perhaps hundreds, perhaps thousands of times.— That all these operations have been more or less continuous but unequal in their progress, and during the whole series the organic life of the earth has undergone a corresponding alteration. This alteration also has been gradual, but complete; after a certain interval not a single species existing which had lived at the commencement of the period. This complete renewal of the forms of life also appears to have occurred several times.—That from the last of the geological epochs to the present or historical epoch, the change of organic life has been gradual: the first appear- ance of animals now existing can in many cases be. traced, their numbers gradually increasing in the more recent forma- tions, while other species continually die out and disappear, so that the present condition of the organic world is clearly derived by a natural process of gradual extinction and crea- tion of species from that of the latest geological periods. We may therefore safely infer a like gradation and natural sequence from one geological epoch to another. Now, taking this as a fair statement of the results of geological inquiry, we see that the present geographical dis- tribution of life upon the earth must be the result of all the previous changes, both of the surface of the earth itself and of its inhabitants. Many causes, no doubt, have operated of which we must ever remain in ignorance, and we may, there- fore, expect to find many details very difficult of explanation, and in attempting to give one, must allow ourselves to call into our service geological changes which it is highly probable may have occurred, though we have no direct evidence of their individual operation. The great increase of our knowledge within the last twenty years, both of the present and past history of the organic world, has accumulated a body of facts which should afford a sufficient foundation for a comprehensive law embracing and I THE INTRODUCTION OF NEW SPECIES 5 explaining them all, and giving a direction to new researches. It is about ten years since the idea of such a law suggested itself to the writer of this essay, and he has since taken every opportunity of testing it by all the newly-ascertained facts with which he has become acquainted, or has been able to observe himself. These have all served to convince him of the correctness of his hypothesis. Fully to enter into such a subject would occupy much space, and it is only in con- sequence of some views having been lately promulgated, he believes, in a wrong direction, that he now ventures to present his ideas to the public, with only such obvious illustrations of the arguments and results as occur to him in a place far removed from all means of reference and exact information. A Law deduced from well-known Geographical and Geological Facts The following propositions in Organic Geography and Geo- logy give the main facts on which the hypothesis is founded. GEOGRAPHY 1. Large groups, such as classes and orders, are generally spread over the whole earth, while smaller ones, such as families and genera, are frequently confined to one portion, often to a very limited district. 2. In widely distributed families the genera are often limited in range; in widely distributed genera well-marked groups of species are peculiar to each geographical district. 3. When a group is confined to one district, and is rich in species, it is almost invariably the case that the most closely allied species are found in the same locality or in closely adjoining localities, and that therefore the natural sequence of the species by affinity is also geographical. 4. In countries of a similar climate, but separated by a wide sea or lofty mountains, the families, genera, and species of the one are often represented by closely allied families, genera, and species peculiar to the other. GEOLOGY 5. The distribution of the organic world in time is very similar to its present distribution in space. 6 NATURAL SELECTION I 6. Most of the larger and some small groups extend through several geological periods. 7. In each period, however, there are peculiar groups, found nowhere else, and extending through one or several formations. 8. Species of one genus, or genera of one family occurring in the same geological time, are more closely allied than those: separated in time. 9. As, generally, in geography no species or genus occurs in two very distant localities without being also found in intermediate places, so in geology the life of a species or genus has not been interrupted. In other words, no group or species has come into existence twice. 10. The following law may be deduced from these facts : Every species has come into existence coincident both in space and time with a pre-existing closely allied species. This law agrees with, explains, and illustrates all the facts connected with the following branches of the subject: 1st, The system of natural affinities. 2d, The distribution of animals and plants in space. 3d, The same in time, including all the phenomena of representative groups, and those which Professor Forbes supposed to manifest polarity. 4th, The phenomena of rudimentary organs. We will briefly endeavour to show its bearing upon each of these. The Form of a true system of Classification determined by this Law Tf the law above enunciated be true, it follows that the natural series of affinities will also represent the order in which the several species came into existence, each one having had for its immediate antitype a closely allied species existing at the time of its origin. It is evidently possible that two or three distinct species may have had a common antitype, and that each of these may again have become the antitypes from which other closely allied species were created. The effect of this would be, that so long as each species has had but one new species formed on its model, the line of affinities will be simple, and may be represented by placing the several species in direct succession in a straight line. But if two or more species have been independently formed on the plan of a, I THE INTRODUCTION OF NEW SPECIES 7 common antitype, then the series of affinities will be com- pound, and can only be represented by a forked or many- branched line. Now, all attempts at a Natural classification and arrangement of organic beings show that both these plans have obtained in creation. Sometimes the series of affinities can be well represented for a space by a direct progression from species to species or from group to group, but it is generally found impossible so to continue. There constantly occur two or more modifications of an organ or modifications of two distinct organs, leading us on to two distinct series of species, which at length differ so much from each other as to form distinct genera or families. These are the parallel series or representative groups of naturalists, and they often occur in different countries, or are found fossil in different forma- tions. They are said to have an analogy to each other when they are so far removed from their common antitype as to differ in many important points of structure, while they still preserve a family resemblance. We thus see how difficult it is to determine in every case whether a given relation is an analogy or an affinity, for it is evident that as we go back along the parallel or divergent series, towards the common antitype, the analogy which existed between the two groups becomes an affinity. We are also made aware of the diffi- culty of arriving at a true classification, even in a small and perfect group; in the actual state of nature it is almost impossible, the species being so numerous and the modifica- tions of form and structure so varied, arising probably from the immense number of species which have served as anti- types for the existing species, and thus produced a compli- cated branching of the lines of affinity, as intricate as the twigs of a gnarled oak or the vascular system of the human body. Again, if we consider that we have only fragments of this vast system, the stem and main branches being repre- sented by extinct species of which we have no knowledge, while a vast mass of limbs and boughs and minute twigs and scattered leaves is what we have to place in order, so as to determine the true position which each originally occupied with regard to the others, the whole difficulty of the true Natural System of classification becomes apparent to us. We shall thus find ourselves obliged to reject all those 8 NATURAL SELECTION I systems of classification which arrange species or groups in circles, as well as those which fix a definite number for the divisions of each group. The latter class have been very generally rejected by naturalists, as contrary to nature, notwithstanding the ability with which they have been advocated ; but the circular system of affinities seems to have obtained a deeper hold, many eminent naturalists having to some extent adopted it. We have, however, never been able to find a case in which the circle has been closed by a direct and close affinity. In most cases a palpable analogy has been substituted, in others the affinity is very obscure or altogether doubtful. The complicated branching of the lines of affinities in extensive groups must also afford great facilities for giving a show of probability to any such purely artificial arrange- ments. Their death-blow was given by the admirable paper of the lamented Mr. Strickland, published in the Annals of Natural History, in which he so clearly showed the true synthetical method of discovering the Natural System. Geographical Distribution of Organisms If we now consider the geographical distribution of animals and plants upon the earth, we shall find all the facts beautifully in accordance with, and readily explained by, the present hypothesis. A country having species, genera, and whole families peculiar to it, will be the necessary result of its having been isolated for a long period, sufficient for many series of species to have been created on the type of pre- existing ones, which, as well as many of the earlier-formed species, have become extinct, and thus made the groups appear isolated. If in any case the antitype had an extensive range, two or more groups of species might have been formed, each varying from it in a different manner, and thus producing several representative or analogous groups. The Sylviade of Europe and the Sylvicolide of North America, the Heliconide of South America and the Euplceas of the East, the group of Trogons inhabiting Asia and that peculiar to South America, are examples that may be accounted for in this manner. Such phenomena as are exhibited by the Galapagos Islands, which contain little groups of plants and animals peculiar to themselves, but most nearly allied to those of South America, I THE INTRODUCTION OF NEW SPECIES 9 have not hitherto received any, even a conjectural explana- tion. The Galapagos are a volcanic group of high antiquity, and have probably never been more closely connected with the continent than they are at present. They must have been first peopled, like other newly-formed islands, by the action of winds and currents, and at a period sufficiently remote to have had the original species die out, and the modi- fied prototypes only remain. In the same way we can account for the separate islands having each their peculiar species, either on the supposition that the same original emigration peopled the whole of the islands with the same species from which differently modified prototypes were created, or that the islands were successively peopled from each other, but that new species have been created in each on the plan of the pre- existing ones. St. Helena is a similar case of a very ancient island having obtained an entirely peculiar, though limited, flora. On the other hand, no example is known of an island which can be proved geologically to be of very recent origin (late in the Tertiary, for instance), and yet possesses generic or family groups, or even many species peculiar to itself. When a range of mountains has attained a great eleva- tion, and has so remained during a long geological period, the species of the two sides at and near their bases will be often very different, representative species of some genera occurring, and even whole genera being peculiar to one side only, as is remarkably seen in the case of the Andes and Rocky Mountains. A similar phenomenon occurs when an island has been separated from a continent at a very early period. The shallow sea between the Peninsula of Malacca, Java, Sumatra, and Borneo was probably a continent or large island at an early epoch, and may have become submerged as the volcanic ranges of Java and Sumatra were elevated ; the organic results we see in the very considerable number of species of animals common to some or all of these countries, while at the same time a number of closely allied repre- sentative species exist peculiar to each, showing that a con- siderable period has elapsed since their separation. The facts of geographical distribution and of geology may thus mutu- ally explain each other in doubtful cases, should the prin- ciples here advocated be clearly established. 10 NATURAL SELECTION I In all those cases in which an island has been separated from a continent, or raised by volcanic or coralline action from the sea, or in which a mountain-chain has been elevated in a recent geological epoch, the phenomena of peculiar groups or even of single representative species will not exist. Our own island is an example of this, its separation from the continent being geologically very recent, and we have con- sequently scarcely a species which is peculiar to it; while the Alpine range, one of the most recent mountain elevations, separates faunas and floras which scarcely differ more than may be due to climate and latitude alone. The series of facts alluded to in Proposition (3), of closely allied species in rich groups being found geographically near each other, is most striking and important. Mr. Lovell Reeve has well exemplified it in his able and interesting paper on the Distribution of the Bulimi. It is also seen in the Humming-birds and Toucans, little groups of two or three closely allied species being often found in the same or closely adjoining districts, as we have had the good fortune of per- sonally verifying. Fishes give evidence of a similar kind: each great river has its peculiar genera, and in more extensive genera its groups of closely allied species. But it is the same throughout Nature; every class and order of animals will contribute similar facts. Hitherto no attempt has been made to explain these singular phenomena, or to show how they have arisen. Why are the genera of Palms and of Orchids in almost every case confined to one hemisphere ? Why are the closely allied species of brown-backed Trogons all found in the East, and the green-backed in the West? Why are the Macaws and the Cockatoos similarly restricted 1 Insects furnish a countless number of analogous examples— the Goliathi of Africa, the Ornithopteree of the Indian Islands, the Heliconide of South America, the Danaidz of the East, and in all the most closely allied species found in geographical proximity. The question forces itself upon every thinking mind, Why are these things so? They could not be as they are had no law regulated their creation and dispersion. The law here enunciated not merely ex- plains but necessitates the facts we see to exist, while the vast and long-continued geological changes of the earth I THE INTRODUCTION OF NEW SPECIES 11 readily account for the exceptions and apparent discrepancies that here and there occur. The writer’s object in putting forward his views in the present imperfect manner is to submit them to the test of other minds, and to be made aware of all the facts supposed to be inconsistent with them. As his hypothesis is one which claims acceptance solely as explaining and connecting facts which exist in nature, he expects facts alone to be brought to disprove it, not & priori arguments against its probability. Geological Distribution of the Forms of Life The phenomena of geological distribution are exactly analogous to those of geography. Closely allied species are found associated in the same beds, and the change from species to species appears to have been as gradual in time as in space. Geology, however, furnishes us with positive proof of the extinction and production of species, though it does not inform us how either has taken place. The extinction of species, however, offers but little difficulty, and the modus operandi has been well illustrated by Sir C. Lyell in his ad- mirable Principles. Geological changes, however gradual, must occasionally have modified external conditions to such an extent as to have rendered the existence of certain species impossible. The extinction would in most cases be effected by a gradual dying-out, but in some instances there might have been a sudden destruction of a species of limited range. To discover how the extinct species have from time to time been replaced by new ones down to the very latest geological period, is the most difficult, and at the same time the most interesting problem in the natural history of the earth. The present inquiry, which seeks to eliminate from known facts a law which has determined, to a certain degree, what species could and did appear at a given epoch, may, it is hoped, be considered as one step in the right direction towards a com- plete solution of it. High Organisation of very ancient Animals consistent with this Law Much discussion has of late years taken place on the question whether the succession of life upon the globe has 12 : NATURAL SELECTION I been from a lower to a higher degree of organisation. The admitted facts seem to show that there has been a general, but not a detailed progression. Mollusca and Radiata existed before Vertebrata, and the progression from Fishes to Reptiles and Mammalia, and also from the lower mammals to the higher, is indisputable. On the other hand, it is said that the Mollusca and Radiata of the very earliest periods were more highly organised than the great mass of those now existing, and that the very first fishes that have been dis- covered are by no means the lowest organised of the class. Now it is believed the present hypothesis will harmonise with all these facts, and in a great measure serve to explain them ; for though it may appear to some readers essentially a theory of progression, it is in reality only one of gradual change. It is, however, by no means difficult to show that a real pro- gression in the scale of organisation is perfectly consistent with all the appearances, and even with apparent retrogression, should such occur. ; Returning to the analogy of a branching tree, as the bestmode of representing the natural arrangement of species and their successive creation, let us suppose that at an early geological epoch any group (say a class of the Mollusca) has attained to a great richness of species and a high organisation. Now let this great branch of allied species, by geological mutations, be completely or partially destroyed. Subsequently a new branch springs from the same trunk—that is to say, new species are successively created, having for their antitypes the same lower organised species which had served as the anti- types for the former group, but which have survived the modified conditions which destroyed it. This new group being subject to these altered conditions, has modifications of structure and organisation given to it, and becomes the repre- sentative group of the former one in another geological form- ation. It may, however, happen, that though later in time, the new series of species may never attain to so high a degree of organisation as those preceding it, but in its turn become extinct, and give place to yet another modification from the same root, which may be of higher or lower organisation, more or less numerous in species, and more or less varied in form and structure, than either of those which preceded it I THE INTRODUCTION OF NEW SPECIES 13 Again, each of these groups may not have become totally extinct, but may have left a few species, the modified proto- types of which have existed in each succeeding period, a faint memorial of their former grandeur and luxuriance. Thus every case of apparent retrogression may be in reality a pro- gress, though an interrupted one: when some monarch of the forest loses a limb, it may be replaced by a feeble and sickly substitute. The foregoing remarks appear to apply to the case of the Mollusca, which, at a very early period, had reached a high organisation and a great development of forms and species in the testaceous Cephalopoda. In each succeed- ing age modified species and genera replaced the former ones which had become extinct, and as we approach the present era, but few and small representatives of the group remain, while the Gasteropods and Bivalves have acquired an immense preponderance. In the long series of changes the earth has undergone, the process of peopling it with organic beings has been continually going on, and whenever any of the higher groups have become nearly or quite extinct, the lower forms which have better resisted the modified physical conditions have served as the antitypes on which to found the new races. In this manner alone, it is believed, can the represent- ative groups at successive periods, and the risings and fallings in the scale of organisation, be in every case explained. Objections to Forbes’ Theory of Polarity The hypothesis of polarity, recently put forward by Pro- fessor Edward Forbes to account for the abundance of generic forms at a very early period and at present, while in the in- termediate epochs there is a gradual diminution and impover- ishment, till the minimum occurred at the confines of the Paleozoic and Secondary epochs, appears to us quite unneces- sary, as the facts may be readily accounted for on the principles already laid down. Between the Palwozoic and Neozoic periods of Professor Forbes there is scarcely a species in com- mon, and the greater parts of the genera and families also disappear, to be replaced by new ones. It is almost univer- sally admitted that such a change in the organic world must have occupied a vast period of time. Of this interval we have no record ; probably because the whole area of the early 14 NATURAL SELECTION 1 formations now exposed to our researches was elevated at the end of the Paleozoic period, and remained so through the interval required for the organic changes which resulted in the fauna and flora of the Secondary period. The records of this interval are buried beneath the ocean which covers three- fourths of the globe. Now it appears highly probable that a long period of quiescence or stability in the physical condi- tions of a district would be most favourable to the existence of organic life in the greatest abundance, both as regards individuals and also as to variety of species and generic group, just as we now find that the places best adapted to the rapid growth and increase of individuals also contain the greatest profusion of species and the greatest variety of forms,—the tropics in comparison with the temperate and arctic regions. On the other hand, it seems no less probable that a change in the physical conditions of a district, even small in amount if rapid, or even gradual if to a great amount, would be highly unfavourable to the existence of individuals, might cause the extinction of many species, and would probably be equally unfavourable to the creation of new ones. In this too we may find an analogy with the present state of our earth, for it has been shown to be the violent extremes and rapid changes of physical conditions, rather than the actual mean state in the temperate and frigid zones, which renders them less prolific than the tropical regions, as exemplified by the great distance beyond the tropics to which tropical forms penetrate when the climate is equable, and also by the rich- ness in species and forms of tropical mountain regions which principally differ from the temperate zone in the uniformity of their climate. However this may be, it seems a fair assumption that during a period of geological repose the new species which we know to have been created would have appeared, that the creations would then exceed in number the extinctions, and therefore the number of species would increase. In a period of geological activity, on the other hand, it seems probable that the extinctions might exceed the creations, and the number of species consequently diminish. That such effects did take place in connection with the causes to which we have imputed them, is shown in the case of the Coal formation, the faults and contortions of which show a period I THE INTRODUCTION OF NEW SPECIES 15 of great activity and violent convulsions, and it is in the formation immediately succeeding this that the poverty of forms of life is most apparent. We have then only to sup- pose a long period of somewhat similar action during the vast unknown interval at the termination of the Paleozoic period, and then a decreasing violence or rapidity through the Secondary period, to allow for the gradual repopulation of the earth with varied forms, and the whole of the facts are explained.t We thus have a clue to the increase of the forms of life during certain periods, and their decrease during others, without recourse to any causes but those we know to have existed, and to effects fairly deducible from them. The pre- cise manner in which the geological changes of the early formations were effected is so extremely obscure, that when we can explain important facts by a retardation at one time and an acceleration at another: of a process which we know from its nature and from observation to have been unequal,— a cause so simple may surely be preferred to one so obscure and hypothetical as polarity. I would also venture to suggest some reasons against the very nature of the theory of Professor Forbes. Our know- ledge of the organic world during any geological epoch is necessarily very imperfect. Looking at the vast numbers of species and groups that have been discovered by geologists, this may be doubted; but we should compare their numbers not merely with those that now exist upon the earth, but with a far larger amount. We have no reason for believing that the number of species on the earth at any former period was much less than at present ; at all events the aquatic portion, with which geologists have most acquaintance, was probably often as great or greater. Now we know that there have been many complete changes of species ; new sets of organisms have many times been introduced in place of old ones which have become extinct, so that the total amount which have existed on the earth from the earliest geological period must have borne about the same proportion to those now living, as the whole human race who have lived and died upon the 1 Professor Ramsay has since shown that a glacial epoch probably occurred at the time of the Permian formation, which will more satisfactorily account for the comparative poverty of species, 16 NATURAL SELECTION 1 earth to the population at the present time. Again, at each epoch, the whole earth was, no doubt, as now, more or less the theatre of life, and as the successive generations of each species died, their exuvie and preservable parts would be deposited over every portion of the then existing seas and oceans, which we have reason for supposing to have been more, rather than less, extensive than at present. In order then to understand our possible knowledge of the early world and its inhabitants, we must compare, not the area of the whole field of our geo- logical researches with the earth’s surface, but the area of the examined portion of each formation separately with the whole earth. For example, during the Silurian period all the earth was Silurian, and animals were living and dying and deposit: ing their remains more or less over the whole area of the globe, and they were probably (the species at least) nearly as varied in different latitudes and longitudes as at present. What proportion do the Silurian districts bear to the whole surface of the globe, land and sea (for far more extensive Silurian districts probably exist beneath the ocean than above it), and what portion of the known Silurian districts has been actually examined for fossils? Would the area of rock actually laid open to the eye be the thousandth or the ten- thousandth part of the earth’s surface? Ask the same question with regard to the Oolite or the Chalk, or even to particular beds of these when they differ considerably in their fossils, and you may then get some notion of how small a portion of the whole we know. But yet more important is the probability, nay, almost the certainty, that whole formations containing the records of vast geological periods are entirely buried beneath the ocean, and for ever beyond our reach. Most of the gaps in the geological series may thus be filled up, and vast numbers of unknown and unimaginable animals, which might help to elucidate the affinities of the numerous isolated groups which are a perpetual puzzle to the zoologist, may there be buried, till future revolutions may raise them in their turn above the waters, to afford materials for the study of whatever race of intelligent beings may then have succeeded us. These con- siderations must lead us to the conclusion that our knowledge of the whole series of the former inhabitants of the earth is I THE INTRODUCTION OF NEW SPECIES 17 necessarily most imperfect and fragmentary,—as much so as our knowledge of the present organic world would be, were we forced to make our collections and observations only in spots equally limited in area and in number with those actually laid open for the collection of fossils. Now, the hypothesis of Professor Forbes is essentially one that assumes to a great extent the completeness of our knowledge of the whole series of organic beings which have existed on the earth. This appears to be a fatal objection to it, inde- pendently of all other considerations. It may be said that the same objections exist against every theory on such a sub- ject, but this is not necessarily the case. The hypothesis put forward in this paper depends in no degree upon the com- pleteness of our knowledge of the former condition of the organic world, but takes what facts we have as fragments of a vast whole, and deduces from them something of the nature | and proportions of that whole which we can never know in detail. It is founded upon isolated groups of facts, recognises their isolation, and endeavours to deduce from them the nature of the intervening portions. Rudimentary Organs Another important series of facts, quite in accordance with, and even necessary deductions from, the law now developed, are those of rudimentary organs. That these really do exist, and in most cases have no special function in the animal economy, is admitted by the first authorities in comparative anatomy. The minute limbs hidden beneath the skin in many of the snake-like lizards, the anal hooks of the boa constrictor, the complete series of jointed finger-bones in the paddle of the Manatus and whale, are a few of the most familiarinstances. In botanya similar class of facts has beenlong recognised. Abortive stamens, rudimentary floral envelopes and undeveloped carpels, are of the most frequent occurrence. To every thoughtful naturalist the question must arise, What are these for? What have they to do with the great laws of creation? Do they not teach us something of the system of Nature? If each species has been created independently, and without any necessary relations with pre-existing species, what do these rudiments, these apparent imperfections mean ? Cc 18 NATURAL SELECTION I There must be a cause for them; they must be the necessary results of some great natural law. Now, if, as it has been endeavoured to be shown, the great law which has regulated the peopling of the earth with animal and vegetable life is, that every change shall be gradual; that no new creature shall be formed widely differing from anything before exist- ing; that in this, as in everything else in nature, there shall be gradation and harmony,—then these rudimentary organs are necessary, and are an essential part of the system of nature. Ere the higher Vertebrata were formed, for instance, many steps were required, and many organs had to undergo modifications from the rudimental condition in which only they had as yet existed. We still see remaining an antitypal sketch of a wing adapted for flight in the scaly flapper of the penguin, and limbs first concealed beneath the skin, and then weakly protruding from it, were the necessary gradations before others should be formed fully adapted for locomotion.1 Many more of these modifications should we behold, and more complete series of them, had we a view of all the forms which have ceased to live. The great gaps that exist between fishes, reptiles, birds, and mammals would then, no doubt, be softened down by intermediate groups, and the whole organic world would be seen to be an unbroken and harmonious system. Conclusion It has now been shown, though most briefly and imper- fectly, how the law that “ Every species has come into existence coincident both in time and space with a pre-existing closely allied species,” connects together and renders intelligible a vast number of independent and hitherto unexplained facts. The natural system of arrangement of organic beings, their geo- graphical distribution, their geological sequence, the pheno- mena of representative and substituted groups in all their modifications, and the most singular peculiarities of anatomical structure, are all explained and illustrated by it, in perfect accordance with the vast mass of facts which the researches of modern naturalists have brought together, and, it is believed, 1 The theory of Natural Selection has now taught us that these are not the steps by which limbs have been formed ; and that most rudimentary organs have been produced by abortion, owing to disuse, as explained by Mr. Darwin. I THE INTRODUCTION OF NEW SPECIES 19 not materially opposed to any of them. It also claims a superiority over previous hypotheses, on the ground that it not merely explains, but necessitates what exists. Granted the law, and many of the most important facts in Nature could not have been otherwise, but are almost as necessary deductions from it as are the elliptic orbits of the planets from the law of gravitation. INTRODUCTORY NOTE TO CHAPTER II IN PRESENT EDITION As this chapter sets forth the main features of a theory identical with that discovered by Mr. Darwin many years before but not then published, and as it has thus an historical interest, a few words of personal statement may be permissible. After writing the preceding paper the question of how changes of species could have been brought about was rarely out of my mind, but no satisfactory conclusion was reached till February 1858. At that time I was suffering from a rather severe attack of intermittent fever at Ternate in the Moluccas, and one day while lying on my bed during the cold fit, wrapped in blankets, though the thermometer was at 88° F., the problem again presented itself to me, and something led me to think of the “positive checks” described by Malthus in his “Essay on Population,” a work I had read several years before, and which had made a deep and permanent impression on my mind. These checks—war, disease, famine and the like—must, it occurred to me, act on animals as well as on man. Then I thought of the enormously rapid multi- plication of animals, causing these checks to be much more effective in them than in the case of man; and while ponder- ing vaguely on this fact there suddenly flashed upon me the idea of the survival of the fittest—that the individuals removed by these checks must be on the whole inferior to those that survived. In the two hours that elapsed before my ague fit was over I had thought out almost the whole of the theory, and the same evening I sketched the draft of my paper, and in the two succeeding evenings wrote it out in full, and sent it by the next post to Mr. Darwin. Up to this time the only letters I had received from him were those printed in the second volume of his Life and Letters, (vol. ii. pp. 95 and 108), INTRODUCTORY NOTE TO CHAPTER II 21 in which he speaks of its being the twentieth year since he “opened his first note-book on the question how and in what way do species and varieties differ from each other,” and after referring to oceanic islands, the means of distribution of land- shells, etc., added: “My work, on which I have now been at work more or less for twenty years, will not fix or setile any- thing ; but I hope it will aid by giving a large collection of facts, with one definite end.” The words I have italicised, and the whole tone of his letters, led me to conclude that he had arrived at no definite view as to the origin of species, and I fully anticipated that my theory would be new to him, because it seemed to me to settle a great deal. The imme- diate result of my paper was that Darwin was induced at once to prepare for publication his book on the Origin of Species in the condensed form in which it appeared, instead of waiting an indefinite number of years to complete a work on a much larger scale which he had partly written, but which in all probability would not have carried conviction to so many persons in so short a time. I feel much satisfaction in having thus aided in bringing about the publication of this celebrated book, and with the ample recognition by Darwin himself of my independent discovery of “natural selection.” (See Origin of Species, 6th ed., introduction, p. 1, and Life and Letters, vol. ii. chap. iv., pp. 115-129 and 145.) II ON THE TENDENCY OF VARIETIES TO DEPART INDEFINITELY FROM THE ORIGINAL TYPE Instability of Varieties supposed to prove the permanent distinctness of Species OnE of the strongest arguments which have been adduced to prove the original and permanent distinctness of species is, that varieties produced in a state of domesticity are more or less unstable, and often have a tendency, if left to them- selves, to return to the normal form of the parent species ; and this instability is considered to be a distinctive peculi- arity of all varieties, even of those occurring among wild animals in a state of nature, and to constitute a provision for preserving unchanged the originally created distinct species. In the absence or scarcity of facts and observations as to varieties occurring among wild animals, this argument has had great weight with naturalists, and has led to a very general and somewhat prejudiced belief in the stability of species. Equally general, however, is the belief in what are called “permanent or true varieties,”—races of animals which con- tinually propagate their like, but which differ so slightly (although constantly) from some other race, that the one is considered to be a variety of the other. Which is the variety and which the original species, there is generally no means of determining, except in those rare cases in which the one race has been known to produce an offspring unlike itself and resembling the other. This, however, would seem quite incompatible with the “permanent invariability of species,” II ON THE TENDENCY OF VARIETIES, ETC. 23 but the difficulty is overcome by assuming that such varieties have strict limits, and can never again vary further from the original type, although they may return to it, which, from the analogy of the domesticated animals, is considered to be highly probable, if not certainly proved. It will be observed that this argument rests entirely on the assumption that varieties occurring in a state of nature are in all respects analogous to or even identical with those of domestic animals, and are governed by the same laws as regards their permanence or further variation. But it is the object of the present paper to show that this assumption is altogether false, that there is a general principle in nature which will cause many varieties to survive the parent species, and to give rise to successive variations departing further and further from the original type, and which also produces, in domesticated animals, the tendency of varieties to return to the parent form. The Struggle for Ewistence The life of wild animals is a struggle for existence. The full exertion of all their faculties and all their energies is required to preserve their own existence and provide for that of their infant offspring. The possibility of procuring food during the least favourable seasons, and of escaping the attacks of their most dangerous enemies, are the primary conditions which determine the existence both of individuals and of entire species. These conditions will also determine the population of a species ; and by a careful consideration of all the circumstances we may be enabled to comprehend, and in some degree to explain, what at first sight appears so inexplicable—the excessive abundance of some species, while others closely allied to them are very rare. The Law of Population of Species The general proportion that must obtain between certain groups of animals is readily seen. Large animals cannot be so abundant as small ones; the carnivora must be less numerous than the herbivora; eagles and lions can never be so plentiful as pigeons and antelopes ; and the wild asses of the Tartarian deserts cannot equal in numbers the horses of 24 NATURAL SELECTION i —_s the more luxuriant prairies and pampas of America. The greater or less fecundity of an animal is often considered to be one of the chief causes of its abundance or scarcity ; but a consideration of the facts will show us that it really has little or nothing to do with the matter. Even the least prolific of animals would increase rapidly if unchecked, whereas it is evident that the animal population of the globe must be stationary, or perhaps, through the influence of man, decreasing. Fluctuations there may be ; but permanent increase, except in restricted localities, is almost impossible. For example, our own observation must convince us that birds do not go on increasing every year in a geometrical ratio, as they would do were there not some powerful check to their natural increase. Very few birds produce less than two young ones each year, while many have six, eight, or ten; four will certainly be below the average ; and if we suppose that each pair produce young only four times in their life, that will also be below the average, supposing them not to die either by violence or want of food. Yet at this rate how tremendous would be the increase in a few years from a single pair! A simple calculation will show that in fifteen years each pair of birds would have increased to nearly ten millions!! whereas we have no reason to believe that the number of the birds of any country increases at all in fifteen or in one hundred and fifty years. With such powers of increase the population must have reached its limits, and have become stationary, in a very few years after the origin of each species. It is evident, therefore, that each year an immense number of birds must perish—as many in fact as are born; and as on the lowest calculation the progeny are each year twice as numerous as their parents, it follows that, whatever be the average number of individuals existing in any given country, twice that number must perish annually,—a striking result, but one which seems at least highly probable, and is perhaps under rather than over the truth. It would therefore appear that, so far as the continuance of the species and the keeping up the average number of individuals are concerned, large broods are superfluous. On the average all above one become 1 This is under estimated. The number would really amount to more than two thousand millicus ! lL ON THE TENDENCY OF VARIETIES, ETC. 25 food for hawks and kites, wild cats or weasels, or perish of cold and hunger as winter comes on. This is strikingly proved by the case of particular species; for we find that their abundance in individuals bears no relation whatever to their fertility in producing offspring. Perhaps the most remarkable instance of an immense bird population is that of the passenger pigeon of the United States, which lays only one, or at most two eggs, and is said to rear generally but one young one. Why is this bird so extraordinarily abundant, while others producing two or three times as many young are much less plentiful? The explana- tion is not difficult. The food most congenial to this species, and on which it thrives best, is abundantly distributed over a very extensive region, offering such differences of soil and climate, that in one part or another of the area the supply never fails. The bird is capable of a very rapid and long- continued flight, so that it can pass without fatigue over the whole of the district it inhabits, and as soon as the supply of food begins to fail in one place is able to discover a fresh feeding-ground. This example strikingly shows us that the procuring a constant supply of wholesome food is almost the sole condition requisite for ensuring the rapid increase of a given species, since neither the limited fecundity nor the un- restrained attacks of birds of prey and of man are here sufficient to check it. In no other birds are these peculiar circumstances so strikingly combined. Either their food is more liable to failure, or they have not sufficient power of wing to search for it over an extensive area, or during some season of the year it becomes very scarce, and less wholesome substitutes have to be found; and thus, though more fertile in offspring, they can never increase beyond the supply of food in the least favourable seasons. Many birds can only exist by migrating, when their food becomes scarce, to regions possessing a milder, or at least a different climate, though, as these migrating birds are seldom excessively abundant, it is evident that the countries they visit are still deficient in a constant and abundant supply of wholesome food. Those whose organisation does not permit them to migrate when their food becomes periodically scarce, can never attain a large population. This is probably the 26 NATURAL SELECTION 1! reason why woodpeckers are scarce with us, while in the tropics they are among the most abundant of solitary birds. Thus the house sparrow is more abundant than the redbreast, because its: food is more constant and plentiful,—seeds of grasses being preserved during the winter, and our farm-yards and stubble-fields furnishing an almost inexhaustible supply. Why, as a general rule, are aquatic, and especially sea-birds, very numerous in individuals? Not because they are more prolific than others, generally the contrary ; but because their food never fails, the sea-shores and river-banks daily swarm- ing with a fresh supply of small mollusca and crustacea. Exactly the same laws will apply to mammals. Wild cats are prolific and have few enemies; why then are they never as abundant as rabbits? The only intelligible answer is, that their supply of food is more precarious. It appears evident, therefore, that so long as a country remains physically un- changed, the numbers of its animal population cannot materially increase. If one species does so, some others requiring the same kind of food must diminish in proportion. The numbers that die annually must be immense; and as the individual existence of each animal depends upon itself, those that die must be the weakest—the very young, the aged, and the diseased—while those that prolong their existence can only be the most perfect in health and vigour—those who are best able to obtain food regularly, and avoid their numerous enemies. It is, as we commenced by remarking, “a struggle for existence,” in which the weakest and least perfectly organised must always succumb. The Abundance or Rarity of a Species dependent upon its more or less perfect Adaptation to the Conditions of Existence It seems evident that what takes place among the indi- viduals of a species must also occur among the several allied species of a group,—viz., that those which are best adapted to obtain a regular supply of food, and to defend themselves against the attacks of their enemies and the vicissitudes of the seasons, must necessarily obtain and preserve a superiority in population ; while those species which, from some defect of power or organisation, are the least capable of counteracting the vicissitudes of food-supply, etc., must diminish in numbers, It ON THE TENDENCY OF VARIETIES, ETC. 27 and, in extreme cases, become altogether extinct. Between these extremes the species will present various degrees of capacity for ensuring the means of preserving life; and it is thus we account for the abundance or rarity of species. Our ignorance will generally prevent us from accurately tracing the effects to their causes; but could we become perfectly acquainted with the organisation and habits of the various species of animals, and could we measure the capacity of each for performing the different acts necessary to its safety and existence under all the varying circumstances by which it is surrounded, we might be able even to calculate the pro- portionate abundance of individuals which is the necessary result. If now we have succeeded in establishing these two points —Ist, that the animal population of a country is generally stationary, being kept down by a periodical deficiency of food, and other checks ; and, 2d, that the comparative abundance or scarcity of the individuals of the several species is entirely due to their organisation and resulting habits, which, rendering it more difficult to procure a regular supply of food and to provide for their personal safety1 in some cases than in others, can only be balanced by a difference in the population which have to exist in a given area— we shall be in a condition to proceed to the consideration of varieties, to which the preceding remarks have a direct and very important application. Useful Variations will tend to Increase ; useless or hurtful Variations to Diminish Most or perhaps all the variations from the typical form of a species must have some definite effect, however slight, on the habits or capacities of the individuals. Even a change of colour might, by rendering them more or less distinguishable, affect their safety; a greater or less development of hair might modify their habits. More important changes, such as an increase in the power or dimensions of the limbs or any of the external organs, would more or less affect their mode of procuring food or the range of country which they could in- 1 ‘And that of their offspring”’ should have been added. But it must be remembered that the writer had no opportunity of correcting the proofs of this paper. 28 NATURAL SELECTION iI habit. It is also evident that most changes would affect, either favourably or adversely, the powers of prolonging existence. An antelope with shorter or weaker legs must necessarily suffer more from the attacks of the feline carni- vora; the passenger pigeon with less powerful wings would sooner or later be affected in its powers of procuring a regular supply of food; and in both cases the result must necessarily be a diminution of the population of the modified species. If, on the other hand, any species should produce a variety having slightly increased powers of preserving existence, that variety must: inevitably in time acquire a superiority in numbers. These results must follow as surely as old age, in- temperance, or scarcity of food produce an increased mortality. In both cases there may be many individual exceptions: but on the average the rule will invariably be found to hold good. All varieties will therefore fall into two classes—those which under the same conditions would never reach the population of the parent species, and those which would in time obtain and keep a numerical superiority. Now, let some alteration of physical conditions occur in the district—a long period of drought, a destruction of vegetation by locusts, the irruption of some fresh carnivorous animal seeking “ pastures new ”— any change in fact tending to render existence more difficult to the species in question, and tasking its utmost powers to avoid complete extermination,—it is evident that, of all the individuals composing the species, those forming the least numerous and most feebly organised variety would suffer first, and, were the pressure severe, must soon become extinct. The same causes continuing in action, the parent species would next suffer, would gradually diminish in numbers, and with a recurrence of similar unfavourable conditions might also become extinct. The superior variety would then alone remain, and on a return to favourable circumstances would rapidly increase in numbers and occupy the place of the extinct species and variety. Superior Varieties will ultimately Eatirpate the original Species The variety would now have replaced the species, of which it would be a more perfectly developed and more highly organised form. It would be in all respects better adapted Ir ON THE TENDENCY OF VARIETIES, ETC. 29 to secure its safety, and to prolong its individual existence and that of the race. Such a variety could not return to the original form; for that form is an inferior one, and could never compete with it for existence. Granted, therefore, a “tendency ” to reproduce the original type of the species, still the variety must ever remain preponderant in numbers, and under adverse physical conditions again alone survive. But this new, improved, and populous race might itself, in course of time, give rise to new varieties, exhibiting several diverging modifications of form, any of which, tending to increase the facilities for preserving existence, must, by the same general law, in their turn become predominant. Here, then, we have progression and continued divergence deduced from the general laws which regulate the existence of animals in a state of nature, and from the undisputed fact that varieties do fre- quently occur. It is not, however, contended that this result would be invariable ; a change of physical conditions in the district might at times materially modify it, rendering the race which had been the most capable of supporting existence under the former conditions now the least so, and even causing the extinction of the newer and, for a time, superior race, while the old or parent species and its first inferior varieties continued to flourish. Variations in unimportant parts might also occur, having no perceptible effect on the life-preserving powers ; and the varieties so furnished might run a course parallel with the parent species, either giving rise to further variations or returning to the former type. All we argue for is, that certain varieties have a tendency to maintain their existence longer than the original species, and this tendency must make itself felt; for though the doctrine of chances or averages can never be trusted on a limited scale, yet, if applied to high numbers, the results come nearer to what theory demands, and, as we approach to an infinity of examples, become strictly accurate. Now the scale on which nature works is so vast—the numbers of individuals and the periods of time with which she deals approach so near to infinity—that any cause, however slight, and however liable to be veiled and counteracted by accidental circumstances, must in the end produce its full legitimate results. 30 NATURAL SELECTION II The Partial Reversion of Domesticated Varieties eaplained Let us now turn to domesticated animals, and inquire how varieties produced among them are affected by the principles here enunciated. The essential difference in the condition of wild and domestic animals is this,—that among the former, their well-being and very existence depend upon the full exercise and healthy condition of all their senses and physical powers, whereas, among the latter, these are only partially exercised, and in some cases are absolutely unused.