Geological Evidences of the Antiquity of Man/Chapter 21
ON THE ORIGIN OF SPECIES BY VARIATION AND NATURAL SELECTION.
MR. DARWIN'S THEORY OF THE ORIGIN OF SPECIES BY NATURAL SELECTION—MEMOIR BY MR. WALLACE—MANNER IN WHICH FAVOURED RACES PREVAIL IN THE STRUGGLE FOR EXISTENCE—FORMATION OF NEW RACES BY BREEDING—HYPOTHESES OF DEFINITE AND INDEFINITE MODIFIABILITY EQUALLY ARBITRARY—COMPETITION AND EXTINCTION OF RACES—PROGRESSION NOT A NECESSARY ACCOMPANIMENT OF VARIATION—DISTINCT CLASSES OF PHENOMENA WHICH NATURAL SELECTION EXPLAINS—UNITY OF TYPE, RUDIMENTARY ORGANS, GEOGRAPHICAL DISTRIBUTION, RELATION OF THE EXTINCT TO THE LIVING FAUNA AND FLORA, AND MUTUAL RELATIONS OF SUCCESSIVE GROUPS OF FOSSIL FORMS—LIGHT THROWN ON EMBRYOLOGICAL DEVELOPMENT BY NATURAL SELECTION—WHY LARGE GENERA HAVE MORE VARIABLE SPECIES THAN SMALL ONES—DR. HOOKER ON THE EVIDENCE AFFORDED BY THE VEGETABLE KINGDOM IN FAVOUR OF CREATION BY VARIATION—SEFSTRÖM ON ALTERNATE GENERATION—HOW FAR THE DOCTRINE OF INDEPENDENT CREATION IS OPPOSED TO THE LAWS NOW GOVERNING THE MIGRATION OF SPECIES.
FOR many years after the promulgation of Lamarck's doctrine of progressive development, geologists were much occupied with the question whether the past changes in the animate and inanimate world were brought about by sudden and paroxysmal action, or gradually and continuously, by causes differing neither in kind nor degree from those now in operation.
The anonymous author of 'The Vestiges of Creation' published in 1844 a treatise, written in a clear and attractive style, which made the English public familiar with the leading views of Lamarck on transmutation and progression, but brought no new facts or original line of argument to support those views, or to combat the principal objections which the scientific world entertained against them.
No decided step in this direction was made until the publication in 1858 of two papers, one by Mr. Darwin and another by Mr. Wallace, followed in 1859 by Mr. Darwin's celebrated work on 'The Origin of Species by Means of Natural Selection; or, the Preservation of favoured Races in the Struggle for Life.' The author of this treatise had for twenty previous years strongly inclined to believe that variation and the ordinary laws of reproduction were among the secondary causes always employed by the Author of nature, in the introduction from time to time of new species into the world, and he had devoted himself patiently to the collecting of facts, and making of experiments in zoology and botany, with a view of testing the soundness of the theory of transmutation. Part of the MS. of his projected work was read to Dr. Hooker as early as 1844, and some of the principal results were communicated to me on several occasions. Dr. Hooker and I had repeatedly urged him to publish without delay, but in vain, as he was always unwilling to interrupt the course of his investigations; until at length Mr. Alfred R. Wallace, who had been engaged for years in collecting and studying the animals of the East Indian archipelago, thought out, independently for himself, one of the most novel and important of Mr. Darwin's theories. This he embodied in an essay 'On the Tendency of Varieties to depart indefinitely from the original Type.' It was written at Ternate, in February 1858, and sent to Mr. Darwin, with a request that it might be shown to me if thought sufficiently novel and interesting. Dr. Hooker and I were of opinion that it should be immediately printed, and we succeeded in persuading Mr. Darwin to allow one of the MS. chapters of his 'Origin of Species,' entitled 'On the Tendency of Species to form Varieties, and on the Perpetuation of Species and Varieties by natural Means of Selection,' to appear at the same time.
By reference to these memoirs it will be seen that both writers begin by applying to the animal and vegetable worlds the Malthusian doctrine of population, or its tendency to increase in a geometrical ratio, while food can only be made to augment even locally in an arithmetical one. There being, therefore, no room or means of subsistence for a large proportion of the plants and animals which are born into the world, a great number must annually perish. Hence there is a constant struggle for existence among the individuals which represent each species, and the vast majority can never reach the adult state, to say nothing of the multitudes of ova and seeds, which are never hatched or allowed to germinate. Of birds it is estimated that the number of those which die every year equals the aggregate number by which the species to which they respectively belong is on the average permanently represented.
The trial of strength, which must decide what individuals are to survive and what to succumb, occurs in the season when the means of subsistence are fewest, or enemies most numerous, or when the individuals are enfeebled by climate or other causes; and it is then that those varieties which have any, even the slightest, advantage over others come off victorious. They may often owe their safety to what would seem to a casual observer a trifling difference, such as a darker or lighter shade of colour rendering them less visible to a species which preys upon them, or sometimes to attributes more obviously advantageous, such as greater cunning, or superior powers of flight or swiftness of foot. These peculiar qualities and faculties, bodily and instinctive, may enable them to outlive their less favoured rivals, and being transmitted by the force of inheritance to their offspring, will constitute new races, or what Mr. Darwin calls 'incipient species.' If one variety, being in other respects just equal to its competitors, happens to be more prolific, some of its offspring will stand a greater chance of being among those which will escape destruction, and their descendants, being in like manner very fertile, will continue to multiply at the expense of all less prolific varieties.
As breeders of domestic animals, when they choose certain varieties in preference to others to breed from, speak technically of their method as that of 'selecting,' Mr. Darwin calls the combination of natural causes, which may enable certain varieties of wild animals or plants to prevail over others of the same species, 'natural selection.'
A breeder finds that a new race of cattle with short horns or without horns may be formed, in the course of several generations, by choosing varieties having the most stunted horns as his stock from which to breed; so nature, by altering, in the course of ages, the conditions of life, the geographical features of a country, its climate, the associated plants and animals, and, consequently, the food and enemies of a species and its mode of life, may be said, by this means, to select certain varieties best adapted for the new state of things. Such new races may often supplant the original type from which they have diverged, although that type may have been perpetuated without modification for countless anterior ages in the same region, so long as it was in harmony with the surrounding conditions then prevailing.
Lamarck, when speculating on the origin of the long neck of the giraffe, imagined that quadruped to have stretched himself up in order to reach the boughs of lofty trees, until by continued efforts, and longing to reach higher, he obtained an elongated neck. Mr. Darwin and Mr. Wallace simply suppose that, in a season of scarcity, a longer-necked variety, having the advantage in this respect over most of the herd, as being able to browse on foliage out of their reach, survived them, and transmitted its peculiarity of cervical conformation to its successors.
By the multiplying of slight modifications in the course of thousands of generations, and by the handing down of the newly-acquired peculiarities by inheritance, a greater and greater divergence from the original standard is supposed to be effected, until what may be called a new species, or, in a greater lapse of time, a new genus, will be the result.
Every naturalist admits that there is a general tendency in animals and plants to vary; but it is usually taken for granted, though we have no means of proving the assumption to be true, that there are certain limits beyond which each species cannot pass under any circumstances, or in any number of generations. Mr. Darwin and Mr. Wallace say that the opposite hypothesis, which assumes that every species is capable of varying indefinitely from its original type, is not a whit more arbitrary, and has this manifest claim to be preferred, that it will account for a multitude of phenomena which the ordinary theory is incapable of explaining.
We have no right, they say, to assume, should we find that a variable species can no longer be made to vary in a certain direction, that it has reached the utmost limit to which it might, under more favourable conditions, or if more time were allowed, be made to diverge from the parent type.
Hybridisation is not considered by Mr. Darwin as a cause of new species, but rather as tending to keep variation within bounds. Varieties which are nearly allied cross readily with each other, and with the parent stock, and such crossing tends to keep the species true to its type, while forms which are less nearly related, although they may intermarry, produce no mule offspring capable of perpetuating their kind.
The competition of races and species, observes Mr. Darwin, is always most severe between those which, are most closely allied and which fill nearly the same place in the economy of nature. Hence, when the conditions of existence are modified, the original stock runs great risk of being superseded by some one of its modified offshoots. The new race or species may not be absolutely superior in the sum of its powers and endowments to the parent stock, and may even be more simple in structure and of a lower grade of intelligence, as well as of organisation, provided, on the whole, it happens to have some slight advantage over its rivals. Progression, therefore, is not a necessary accompaniment of variation and natural selection, though, when a higher organisation happens to be coincident with superior fitness to new conditions, the new species will have greater power and a greater chance of permanently maintaining and extending its ground. One of the principal claims of Mr. Darwin's theory to acceptance is, that it enables us to dispense with a law of progression as a necessary accompaniment of variation. It will account equally well for what is called degradation, or a retrograde movement towards a simpler structure, and does not require Lamarck's continual creation of monads; for this was a necessary part of his system, in order to explain how, after the progressive power had been at work for myriads of ages, there were as many beings of the simplest structure in existence as ever.
Mr. Darwin labours to show, and with no small success, that all true classification in zoology and botany is, in fact, genealogical, and that community of descent is the hidden bond which naturalists have been unconsciously seeking, while they often imagined that they were looking for some unknown plan of creation.
As the 'Origin of Species,' is in itself a condensed abstract of a much larger work not yet published, I could not easily give an analysis of its contents within narrower limits than those of the original, but it may be useful to enumerate briefly some of the principal classes of phenomena on which the theory of 'Natural Selection' is believed by its author to throw light.
In the first place, it would explain, says Mr. Darwin, the unity of type which runs through the whole organic world, and why there is sometimes a fundamental agreement in structure in the same class of beings which is quite independent of their habits of life, for such structure, derived by inheritance from a remote progenitor, has been modified, in the course of ages, in different ways, according to the conditions of existence. It would also explain why all living and extinct beings are united, by complex radiating and circuitous lines of affinity with one another, into one grand system; also, there having been a continued extinction of old races and species in progress, and a formation of new ones by variation, why in some genera which are largely represented, or to which a great many species belong, many of these are closely but unequally related; also, why there are distinct geographical provinces of species of animals and plants, for, after long isolation by physical barriers, each fauna and flora, by varying continually, must become distinct from its ancestral type, and from the new forms assumed by other descendants which have diverged from the same stock.
The theory of indefinite modification would also explain why rudimentary organs are so useful in classification, being the remnants preserved by inheritance of organs which the present species once used—as in the case of the rudiments of eyes in insects and reptiles inhabiting dark caverns, or of the wings of birds and beetles which have lost all power of flight. In such cases the affinities of species are often more readily discerned by reference to these imperfect structures than by others of much more physiological importance to the individuals themselves.
The same hypothesis would explain why there are no mammalia in islands far from continents, except bats, which can reach them by flying; and also why the birds, insects, plants, and other inhabitants of islands, even when specifically unlike, usually agree generically with those of the nearest continent, it being assumed that the original stock of such species came by migration from the nearest land.
Variation and natural selection would also afford a key to a multitude of geological facts otherwise wholly unaccounted for, as, for example, why there is generally an intimate connection between the living animals and plants of each great division of the globe and the extinct fauna and flora of the post-tertiary or tertiary formations of the same region; as, for example, in North America, where we not only find among the living mollusca peculiar forms foreign to Europe, such as Gnathodon and Fulgur (a subgenus of Pyrula), but meet also with extinct species of those same genera in the tertiary fauna of the same part of the world. In like manner, among the mammalia we find in Australia not only living kangaroos and wombats, but fossil individuals of extinct species of the same genera. So also there are recent and fossil sloths, armadilloes, and other edentata in South America, and living and extinct species of elephant, rhinoceros, tiger, and bear in the great Europeo-Asiatic continent. The theory of the origin of new species by variation will also explain why a species which has once died out never reappears, and why the fossil fauna and flora recede farther and farther from the living type in proportion as we trace it back to remoter ages. It would also account for the fact, that when we have to intercalate a new set of fossiliferous strata between two groups previously known, the newly discovered fossils serve to fill up gaps between specific or generic types previously familiar to us, supplying often the missing links of the chain, which, if transmutation is accepted, must once have been continuous.
One of the most original speculations in Mr. Darwin's work is derived from the fact that, in the breeding of animals, it is often observed that at whatever age any variation first appears in the parent, it tends to reappear at a corresponding age in the offspring. Hence the young individuals of two races which have sprung from the same parent stock are usually more like each other than the adults. Thus the puppies of the greyhound and bull-dog are much more nearly alike in their proportions than the grown-up dogs, and in like manner the foals of the cart and racehorse than the adult individuals. For the same reason we may understand why the species of the same genus, or genera of the same family, resemble each other more nearly in their embryonic than in their more fully developed state, or how it is that in the eyes of most naturalists the structure of the embryo is even more important in classification than that of the adult, 'for the embryo is the animal in its less modified state, and in so far it reveals the structure of its progenitor. In two groups of 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, however much the structure of the adult may have been modified.'
If then there had been a system of progressive development, the successive changes through which the embryo of a species of a high class, a mammifer, for example, now passes, may be expected to present us with a picture of the stages through which, in the course of ages, that class of animals has successively passed in advancing from a lower to a higher grade. Hence the embryonic states exhibited one after the other by the human individual bear a certain amount of resemblance to those of the fish, reptile, and bird before assuming those of the highest division of the vertebrata.
Mr. Darwin, after making a laborious analysis of many floras, found that those genera which are represented by a large number of species contain a greater number of variable species, relatively speaking, than the smaller genera, or those less numerously represented. This fact he adduces in support of his opinion that varieties are incipient species, for he observes that the existence of the larger genera implies, in the period immediately preceding our own, that the manufacturing of species has been active, in which case we ought generally to find the same forces still in full activity, more especially as we have every reason to believe the process by which new species are produced is a slow one.
Dr. Hooker tells us that he was long disposed to doubt this result, as he was acquainted with so many variable small genera, but after examining Mr. Darwin's data, he was compelled to acquiesce in his generalisation.
It is one of those conclusions, to verify which requires the investigation of many thousands of species, and to which exceptions may easily be adduced, both in the animal and vegetable kingdoms, so that it will be long before we can expect it to be thoroughly tested, and, if true, fairly appreciated. Among the most striking exceptions will be some genera still large, but which are beginning to decrease, the conditions which were favourable to their former predominance having already begun to change. To many, this doctrine of Natural Selection, or 'the preservation of favoured races in the struggle for life,' seems so simple, when once clearly stated, and so consonant with known facts and received principles, that they have difficulty in conceiving how it can constitute a great step in the progress of science. Such is often the case with important discoveries, but in order to assure ourselves that the doctrine was by no means obvious, we have only to refer back to the writings of skilful naturalists who attempted in the earlier part of the nineteenth century, to theorise on this subject, before the invention of this new method of explaining how certain forms are supplanted by new ones, and in what manner these last are selected out of innumerable varieties, and rendered permanent.
Dr. Hooker, on the Theory of 'Creation by Variation' as applied to the Vegetable Kingdom.
Of Dr. Hooker, whom I have often cited in this chapter, Mr. Darwin has spoken in the Introduction to his 'Origin of Species,' as one 'who had, for fifteen years, aided him in every possible way, by his large stores of knowledge, and his excellent judgement.' This distinguished botanist published his 'Introductory Essay to the Flora of Australia' in 1859, the year after the memoir on 'Natural Selection' was communicated to the Linnæan Society, and a few months before the appearance of the 'Origin of Species.'
Having, in the course of his extensive travels, studied the botany of arctic, temperate, and tropical regions, and written on the flora of India, which he had examined at all heights above the sea, from the plains of Bengal to the limits of perpetual snow in the Himalaya, and having specially devoted his attention to 'geographical varieties,' or those changes of character which plants exhibit, when traced over wide areas and seen under new conditions; being also practically versed in the description and classification of new plants, from various parts of the world, and having been called upon carefully to consider the claims of thousands of varieties to rank as species, no one was better qualified by observation and reflection to give an authoritative opinion on the question, whether the present vegetation of the globe is or is not in accordance with the theory which Mr. Darwin has proposed. We cannot but feel, therefore, deeply interested when we find him making the following declaration: 'The mutual relations of the plants of each great botanical province, and, in fact, of the world generally, is just such as would have resulted if variation had gone on operating throughout indefinite periods, in the same manner as we see it act in a limited number of centuries, so as gradually to give rise in the course of time, to the most widely divergent forms.'
In the same Essay, this author remarks, 'The element of mutability pervades the whole Vegetable Kingdom; no class, nor order, nor genus of more than a few species claims absolute exemption from it, whilst the grand total of unstable forms, generally assumed to be species, probably exceeds that of the stable.' Yet he contends that species are neither visionary, nor even arbitrary creations of the naturalist, but realities, though they may not remain true for ever (p. 11). The majority of them, he remarks, are so far constant, 'within the range of our experience,' and their forms and characters so faithfully handed down, through thousands of generations, that they admit of being treated as if they were permanent and immutable. But the range of 'our experience' is so limited, that it will not account for a single fact in the present geographical distribution, or origin of any one species of plant, nor for the amount of variation it has undergone, nor will it indicate the time when it first appeared, nor the form it had when created.'
To what an extent the limits of species are indefinable, is evinced, he says, by the singular fact that, among those botanists who believe them to be immutable, the number of flowering plants is by some assumed to be 80,000, and by others over 150,000. The general limitation of species to certain areas, suggests the idea that each of them, with all their varieties, have sprung from a common parent, and have spread in various directions from a common centre. The frequency also of the grouping of genera within certain geographical limits, is in favour of the same law, although the migration of species may sometimes cause apparent exceptions to the rule, and make the same types appear to have originated independently at different spots.
Certain genera of plants, which like the brambles, roses, and willows in Europe, consist of a continuous series of varieties, between the terms of which no intermediate forms can be intercalated, may be supposed to be on the increase, and therefore undergoing much variation; whereas genera which present no such perplexing gradations, may be those which have been losing species and varieties by extinction. The annihilation of the intermediate forms which once existed, makes it an easy task to distinguish those which remain.
It had usually been supposed by the advocates of the immutability of species, that domesticated races, if allowed to run wild, always revert to their parent type. Mr. Wallace had said in reply, that a domesticated species, if it loses the protection of man, can only stand its ground in a wild state by resuming those habits, and recovering those attributes which it may have lost when under domestication. If these faculties are so much enfeebled as to be irrecoverable, it will perish; if not, and if it can adapt itself to the surrounding conditions, it will revert to the state in which man first found it; for in one, two, or three thousand years, which may have elapsed since it was originally tamed, there will not have been time for such geographical, climatal, and organic changes, as would only be suited to a new race, or a new and allied species.
But in regard to plants, Dr. Hooker questions the fact of reversion. According to him, species in general do not readily vary, but when they once begin to do so, the new varieties, as every horticulturist knows, show a great inclination to go on departing more and more from the old stock. As the best marked varieties of a wild species occur on the confines of the area which it inhabits, so the best marked varieties of a cultivated plant, are those last produced by the gardener. Cabbages, for example, wall fruit, and cerealia, show no disposition, when neglected, to assume the characters of the wild states of these plants. Hence the difficulty of determining what are the true parent species of most of our cultivated plants. Thus the finer kinds of apples, if grown from seed, degenerate and become crabs, but in so doing they do not revert to the original wild crab-apple, but become crab states of the varieties to which they belong.
It would lead me into too long a digression, were I to attempt to give a fuller analysis of this admirable essay; but I may add, that none of the observations are more in point, as bearing on the doctrine of what Hooker terms 'creation by variation,' than the great extent to which the internal characters and properties of plants, or their physiological constitution are capable of being modified, while they exhibit externally, no visible departure from the normal form. Thus, in one region a species may possess peculiar medicinal qualities which it wants in another, or it may be hardier and better able to resist cold. The average range in altitude, says Hooker, of each species of flowering plant in the Himalayan Mountains, whether in the tropical, temperate, or Alpine region, is 4,000 feet, which is equivalent to twelve degrees of isothermals of latitude. If an individual of any of these species be taken from the upper limits of its range and carried to England, it is found to be better able to stand our climate than those from the lower or warmer stations. When several of these internal or physiological modifications are accompanied by variation in size, habits of growth, colour of the flowers, and other external characters, and these are found to be constant in successive generations, botanists may well begin to differ in opinion as to whether they ought to regard them as distinct species or not.
Hitherto, no rival hypothesis has been proposed as a substitute for the doctrine of transmutation; for 'independent creation,' as it is often termed, or the direct intervention of the Supreme Cause, must simply be considered as an avowal that we deem the question to lie beyond the domain of science.
The discovery by Sefström of alternate generation enlarges our views of the range of metamorphosis through which a species may pass, so that some of its stages (as when a Sertularia and a Medusa interchange) deviate so far from others as to have been referred by able zoologists to distinct genera, or even families. But in all these cases the organism, after running through a certain cycle of change, returns to the exact point from which it set out, and no new form or species is thereby introduced into the world. The only secondary cause, therefore, which has, as yet, been even conjecturally brought forward, to explain how, in the ordinary course of nature, a new specific form may be generated is, as Lamarck declared, 'variation,' and this has been rendered a far more probable hypothesis by the way in which Natural Selection is shown to give intensity and permanency to certain varieties.
When I formerly advocated the doctrine that species were primordial creations, and not derivative, I endeavoured to explain the manner of their geographical distribution, and the affinity of living forms to the fossil types nearest akin to them in the tertiary strata of the same part of the globe, by supposing that the creative power, which originally adapts certain types to aquatic and others to terrestrial conditions, has, at successive geological epochs introduced new forms best suited to each area and climate, so as to fill the places of those which may have died out.
In that case, although the new species would differ from the old (for these would not be revived, having been already proved by the fact of their extinction, to be incapable of holding their ground), still, they would resemble their predecessors generically. For, as Mr. Darwin states in regard to new races, those of a dominant type inherit the advantages which made their parent species flourish in the same country, and they likewise partake in those general advantages which made the genus to which the parent species belonged, a large genus in its own country.
We might, therefore, by parity of reasoning, have anticipated that the creative power, adapting the new types to the new combination of organic and inorganic conditions of a given region, such as its soil, climate, and inhabitants, would introduce new modifications of the old types,—marsupials, for example in Australia, new sloths and armadilloes in South America, new heaths at the Cape, new roses in the northern, and new camelias in the southern hemisphere. But to this line of argument Mr. Darwin and Dr. Hooker reply, that when animals or plants migrate into new countries, whether assisted by man, or without his aid, the most successful colonisers appertain by no means to those types which are most allied to the old indigenous species. On the contrary, it more frequently happens that members of genera, orders, or even classes, distinct and foreign to the invaded country, make their way most rapidly, and become dominant at the expense of the endemic species. Such is the case with the placental quadrupeds in Australia, and with horses and many foreign plants in the pampas of South America, and numberless instances in the United States and elsewhere, which might easily be enumerated. Hence, the transmutationists infer that, the reason why these foreign types, so peculiarly fitted for these regions have never before been developed there, is simply that they were excluded by natural barriers. But these barriers of sea, or desert, or mountain, could never have been of the least avail, had the creative force acted independently of material laws, or had it not pleased the Author of Nature that the origin of new species should be governed by some secondary causes analogous to those which we see preside over the appearance of new varieties, which never appear except as the offspring of a parent stock very closely resembling them.
- See Proceedings of Linnean Society, 1858.
- Origin of Species, p. 121.
- Origin, p. 498.
- Darwin, Origin, &c., p. 448.
- Origin of Species, ch. ii. p. 56.
- Introductory Essay on Flora of Australia, p. vi.
- Introductory Essay, &c., sold separately. Lovell Reeve, London, 1859.
- Hooker, Introductory Essay, Flora of Australia.
- Ibid. p. 13.
- Introductory Essay, Flora of Australia, p. ix.