Darwinism (Wallace)/Chapter VI
DIFFICULTIES AND OBJECTIONS
Difficulty as to smallness of variations—As to the right variations occurring when required—The beginnings of important organs—The mammary glands—The eyes of flatfish—Origin of the eye—Useless or non-adaptive characters—Recent extension of the region of utility in plants—The same in animals—Uses of tails—Of the horns of deer—Of the scale-ornamentation of reptiles—Instability of non-adaptive characters—Delboeuf's law—No "specific" character proved to be useless—The swamping effects of intercrossing—Isolation as preventing intercrossing—Gulick on the effects of isolation—Cases in which isolation is ineffective.
In the present chapter I propose to discuss the more obvious and often repeated objections to Darwin's theory, and to show how far they affect its character as a true and sufficient explanation of the origin of species. The more recondite difficulties, affecting such fundamental questions as the causes and laws of variability, will be left for a future chapter, after we have become better acquainted with the applications of the theory to the more important adaptations and correlations of animal and plant life.
One of the earliest and most often repeated objections was, that it was difficult "to imagine a reason why variations tending in an infinitesimal degree in any special direction should be preserved," or to believe that the complex adaptation of living organisms could have been produced "by infinitesimal beginnings." Now this term "infinitesimal," used by a well-known early critic of the Origin of Species, was never made use of by Darwin himself, who spoke only of variations being "slight," and of the "small amount" of the variations that might be selected. Even in using these terms he undoubtedly afforded grounds for the objection above made, that such small and slight variations could be of no real use, and would not determine the survival of the individuals possessing them. We have seen, however, in our third chapter, that even Darwin's terms were hardly justified; and that the variability of many important species is of considerable amount, and may very often be properly described as large. As this is found to be the case both in animals and plants, and in all their chief groups and subdivisions, and also to apply to all the separate parts and organs that have been compared, we must take it as proved that the average amount of variability presents no difficulty whatever in the way of the action of natural selection. It may be here mentioned that, up to the time of the preparation of the last edition of The Origin of Species, Darwin had not seen the work of Mr. J.A. Allen of Harvard University (then only just published), which gave us the first body of accurate comparisons and measurements demonstrating this large amount of variability. Since then evidence of this nature has been accumulating, and we are, therefore, now in a far better position to appreciate the facilities for natural selection, in this respect, than was Mr. Darwin himself.
Another objection of a similar nature is, that the chances are immensely against the right variation or combination of variations occurring just when required; and further, that no variation can be perpetuated that is not accompanied by several concomitant variations of dependent parts—greater length of a wing in a bird, for example, would be of little use if unaccompanied by increased volume or contractility of the muscles which move it. This objection seemed a very strong one so long as it was supposed that variations occurred singly and at considerable intervals; but it ceases to have any weight now we know that they occur simultaneously in various parts of the organism, and also in a large proportion of the individuals which make up the species. A considerable number of individuals will, therefore, every year possess the required combination of characters; and it may also be considered probable that when the two characters are such that they always act together, there will be such a correlation between them that they will frequently vary together. But there is another consideration that seems to show that this coincident variation is not essential. All animals in a state of nature are kept, by the constant struggle for existence and the survival of the fittest, in such a state of perfect health and usually superabundant vigour, that in all ordinary circumstances they possess a surplus power in every important organ—a surplus only drawn upon in cases of the direst necessity when their very existence is at stake. It follows, therefore, that any additional power given to one of the component parts of an organ must be useful—an increase, for example, either in the wing muscles or in the form or length of the wing might give some increased powers of flight; and thus alternate variations—in one generation in the muscles, in another generation in the wing itself—might be as effective in permanently improving the powers of flight as coincident variations at longer intervals. On either supposition, however, this objection appears to have little weight if we take into consideration the large amount of coincident variability that has been shown to exist.
The Beginnings of Important Organs.
We now come to an objection which has perhaps been more frequently urged than any other, and which Darwin himself felt to have much weight—the first beginnings of important organs, such, for example, as wings, eyes, mammary glands, and numerous other structures. It is urged, that it is almost impossible to conceive how the first rudiments of these could have been of any use, and, if not of use they could not have been preserved and further developed by natural selection.
Now, the first remark to be made on objections of this nature is, that they are really outside the question of the origin of all existing species from allied species not very far removed from them, which is all that Darwin undertook to prove by means of his theory. Organs and structures such as those above mentioned all date back to a very remote past, when the world and its inhabitants were both very different from what they are now. To ask of a new theory that it shall reveal to us exactly what took place in remote geological epochs, and how it took place, is unreasonable. The most that should be asked is, that some probable or possible mode of origination should be pointed out in some at least of these difficult cases, and this Mr. Darwin has done. One or two of these may be briefly given here, but the whole series should be carefully read by any one who wishes to see how many curious facts and observations have been required in order to elucidate them; whence we may conclude that further knowledge will probably throw light on any difficulties that still remain.
In the case of the mammary glands Mr. Darwin remarks that it is admitted that the ancestral mammals were allied to the marsupials. Now in the very earliest mammals, almost before they really deserved that name, the young may have been nourished by a fluid secreted by the interior surface of the marsupial sack, as is believed to be the case with the fish (Hippocampus) whose eggs are hatched within a somewhat similar sack. This being the case, those individuals which secreted a more nutritious fluid, and those whose young were able to obtain and swallow a more constant supply by suction, would be more likely to live and come to a healthy maturity, and would therefore be preserved by natural selection.
In another case which has been adduced as one of special difficulty, a more complete explanation is given. Soles, turbots, and other flatfish are, as is well known, unsymmetrical. They live and move on their sides, the under side being usually differently coloured from that which is kept uppermost. Now the eyes of these fish are curiously distorted in order that both eyes may be on the upper side, where alone they would be of any use. It was objected by Mr. Mivart that a sudden transformation of the eye from one side to the other was inconceivable, while, if the transit were gradual the first step could be of no use, since this would not remove the eye from the lower side. But, as Mr. Darwin shows by reference to the researches of Malm and others, the young of these fish are quite symmetrical, and during their growth exhibit to us the whole process of change. This begins by the fish (owing to the increasing depth of the body) being unable to maintain the vertical position, so that it falls on one side. It then twists the lower eye as much as possible towards the upper side; and, the whole bony structure of the head being at this time soft and flexible, the constant repetition of this effort causes the eye gradually to move round the head till it comes to the upper side. Now if we suppose this process, which in the young is completed in a few days or weeks, to have been spread over thousands of generations during the development of these fish, those usually surviving whose eyes retained more and more of the position into which the young fish tried to twist them, the change becomes intelligible; though it still remains one of the most extraordinary cases of degeneration, by which symmetry—which is so universal a characteristic of the higher animals—is lost, in order that the creature may be adapted to a new mode of life, whereby it is enabled the better to escape danger and continue its existence.
The most difficult case of all, that of the eye—the thought of which even to the last, Mr. Darwin says, "gave him a cold shiver"—is nevertheless shown to be not unintelligible; granting of course the sensitiveness to light of some forms of nervous tissue. For he shows that there are, in several of the lower animals, rudiments of eyes, consisting merely of pigment cells covered with a translucent skin, which may possibly serve to distinguish light from darkness, but nothing more. Then we have an optic nerve and pigment cells; then we find a hollow filled with gelatinous substance of a convex form—the first rudiment of a lens. Many of the succeeding steps are lost, as would necessarily be the case, owing to the great advantage of each modification which gave increased distinctness of vision, the creatures possessing it inevitably surviving, while those below them became extinct. But we can well understand how, after the first step was taken, every variation tending to more complete vision would be preserved till we reached the perfect eye of birds and mammals. Even this, as we know, is not absolutely, but only relatively, perfect. Neither the chromatic nor the spherical aberration is absolutely corrected; while long-and short-sightedness, and the various diseases and imperfections to which the eye is liable, may be looked upon as relics of the imperfect condition from which the eye has been raised by variation and natural selection.
These few examples of difficulties as to the origin of remarkable or complex organs must suffice here; but the reader who wishes further information on the matter may study carefully the whole of the sixth and seventh chapters of the last edition of The Origin of Species, in which these and many other cases are discussed in considerable detail.
Useless or non-adaptive Characters.
Many naturalists seem to be of opinion that a considerable number of the characters which distinguish species are of no service whatever to their possessors, and therefore cannot have been produced or increased by natural selection. Professors Bronn and Broca have urged this objection on the continent. In America, Dr. Cope, the well-known palaeontologist, has long since put forth the same objection, declaring that non-adaptive characters are as numerous as those which are adaptive; but he differs completely from most who hold the same general opinion in considering that they occur chiefly "in the characters of the classes, orders, families, and other higher groups;" and the objection, therefore, is quite distinct from that in which it is urged that "specific characters" are mostly useless. More recently, Professor G.J. Romanes has urged this difficulty in his paper on "Physiological Selection" (Journ. Linn. Soc., vol. xix. pp. 338, 344). He says that the characters "which serve to distinguish allied species are frequently, if not usually, of a kind with which natural selection can have had nothing to do," being without any utilitarian significance. Again he speaks of "the enormous number," and further on of "the innumerable multitude" of specific peculiarities which are useless; and he finally declares that the question needs no further arguing, "because in the later editions of his works Mr. Darwin freely acknowledges that a large proportion of specific distinctions must be conceded to be useless to the species presenting them."
I have looked in vain in Mr. Darwin's works to find any such acknowledgment, and I think Mr. Romanes has not sufficiently distinguished between "useless characters" and "useless specific distinctions." On referring to all the passages indicated by him I find that, in regard to specific characters, Mr. Darwin is very cautious in admitting inutility. His most pronounced "admissions" on this question are the following: "But when, from the nature of the organism and of the conditions, modifications have been induced which are unimportant for the welfare of the species, they may be, and apparently often have been, transmitted in nearly the same state to numerous, otherwise modified, descendants" (Origin, p. 175). The words I have here italicised clearly show that such characters are usually not "specific," in the sense that they are such as distinguish species from each other, but are found in numerous allied species. Again: "Thus a large yet undefined extension may safely be given to the direct and indirect results of natural selection; but I now admit, after reading the essay of Nägeli on plants, and the remarks by various authors with respect to animals, more especially those recently made by Professor Broca, that in the earlier editions of my Origin of Species I perhaps attributed too much to the action of natural selection or the survival of the fittest. I have altered the fifth edition of the Origin so as to confine my remarks to adaptive changes of structure, but I am convinced, from the light gained during even the last few years, that very many structures which now appear to us useless, will hereafter be proved to be useful, and will therefore come within the range of natural selection. Nevertheless I did not formerly consider sufficiently the existence of structures which, as far as we can at present judge, are neither beneficial nor injurious; and this I believe to be one of the greatest oversights as yet detected in my work." Now it is to be remarked that neither in these passages nor in any of the other less distinct expressions of opinion on this question, does Darwin ever admit that "specific characters"—that is, the particular characters which serve to distinguish one species from another—are ever useless, much less that "a large proportion of them" are so, as Mr. Romanes makes him "freely acknowledge." On the other hand, in the passage which I have italicised he strongly expresses his view that much of what we suppose to be useless is due to our ignorance; and as I hold myself that, as regards many of the supposed useless characters, this is the true explanation, it may be well to give a brief sketch of the progress of knowledge in transferring characters from the one category to the other.
We have only to go back a single generation, and not even the most acute botanist could have suggested a reasonable use, for each species of plant, of the infinitely varied forms, sizes, and colours of the flowers, the shapes and arrangement of the leaves, and the numerous other external characters of the whole plant. But since Mr. Darwin showed that plants gained both in vigour and in fertility by being crossed with other individuals of the same species, and that this crossing was usually effected by insects which, in search of nectar or pollen, carried the pollen from one plant to the flowers of another plant, almost every detail is found to have a purpose and a use. The shape, the size, and the colour of the petals, even the streaks and spots with which they are adorned, the position in which they stand, the movements of the stamens and pistil at various times, especially at the period of, and just after, fertilisation, have been proved to be strictly adaptive in so many cases that botanists now believe that all the external characters of flowers either are or have been of use to the species.
It has also been shown, by Kerner and other botanists, that another set of characteristics have relation to the prevention of ants, slugs, and other animals from reaching the flowers, because these creatures would devour or injure them without effecting fertilisation. The spines, hairs, or sticky glands on the stem or flower-stalk, the curious hairs or processes shutting up the flower, or sometimes even the extreme smoothness and polish of the outside of the petals so that few insects can hang to the part, have been shown to be related to the possible intrusion of these "unbidden guests." And, still more recently, attempts have been made by Grant Allen and Sir John Lubbock to account for the innumerable forms, textures, and groupings of leaves, by their relation to the needs of the plants themselves; and there can be little doubt that these attempts will be ultimately successful. Again, just as flowers have been adapted to secure fertilisation or cross-fertilisation, fruits have been developed to assist in the dispersal of seeds; and their forms, sizes, juices, and colours can be shown to be specially adapted to secure such dispersal by the agency of birds and mammals; while the same end is secured in other cases by downy seeds to be wafted through the air, or by hooked or sticky seed-vessels to be carried away, attached to skin, wool, or feathers.
Here, then, we have an enormous extension of the region of utility in the vegetable kingdom, and one, moreover, which includes almost all the specific characters of plants. For the species of plants are usually characterised either by differences in the form, size, and colour of the flowers, or of the fruits; or, by peculiarities in the shape, size, dentation, or arrangement of the leaves; or by peculiarities in the spines, hairs, or down with which various parts of the plant are clothed. In the case of plants it must certainly be admitted that "specific" characters are pre-eminently adaptive; and though there may be some which are not so, yet all those referred to by Darwin as having been adduced by various botanists as useless, either pertain to genera or higher groups, or are found in some plants of a species only—that is, are individual variations not specific characters.
In the case of animals, the most recent wide extension of the sphere of utility has been in the matter of their colours and markings. It was of course always known that certain creatures gained protection by their resemblance to their normal surroundings, as in the case of white arctic animals, the yellow or brown tints of those living in deserts, and the green hues of many birds and insects surrounded by tropical vegetation. But of late years these cases have been greatly increased both in number and variety, especially in regard to those which closely imitate special objects among which they live; and there are other kinds of coloration which long appeared to have no use. Large numbers of animals, more especially insects, are gaudily coloured, either with vivid hues or with striking patterns, so as to be very easily seen. Now it has been found, that in almost all these cases the creatures possess some special quality which prevents their being attacked by the enemies of their kind whenever the peculiarity is known; and the brilliant or conspicuous colours or markings serve as a warning or signal flag against attack. Large numbers of insects thus coloured are nauseous and inedible; others, like wasps and bees, have stings; others are too hard to be eaten by small birds; while snakes with poisonous fangs often have some characteristic either of rattle, hood, or unusual colour, which indicates that they had better be left alone.
But there is yet another form of coloration, which consists in special markings—bands, spots, or patches of white, or of bright colour, which vary in every species, and are often concealed when the creature is at rest but displayed when in motion,—as in the case of the bands and spots so frequent on the wings and tails of birds. Now these specific markings are believed, with good reason, to serve the purpose of enabling each species to be quickly recognised, even at a distance, by its fellows, especially the parents by their young and the two sexes by each other; and this recognition must often be an important factor in securing the safety of individuals, and therefore the wellbeing and continuance of the species. These interesting peculiarities will be more fully described in a future chapter, but they are briefly referred to here in order to show that the most common of all the characters by which species are distinguished from each other—their colours and markings—can be shown to be adaptive or utilitarian in their nature.
But besides colour there are almost always some structural characters which distinguish species from species, and, as regards many of these also, an adaptive character can be often discerned. In birds, for instance, we have differences in the size or shape of the bill or the feet, in the length of the wing or the tail, and in the proportions of the several feathers of which these organs are composed. All these differences in the organs on which the very existence of birds depends, which determine the character of flight, facility for running or climbing, for inhabiting chiefly the ground or trees, and the kind of food that can be most easily obtained for themselves and their offspring, must surely be in the highest degree utilitarian; although in each individual case we, in our ignorance of the minutiae of their life-history, may be quite unable to see the use. In mammalia specific differences other than colour usually consist in the length or shape of the ears and tail, in the proportions of the limbs, or in the length and quality of the hair on different parts of the body. As regards the ears and tail, one of the objections by Professor Bronn relates to this very point. He states that the length of these organs differ in the various species of hares and of mice, and he considers that this difference can be of no service whatever to their possessors. But to this objection Darwin replies, that it has been shown by Dr. Schöbl that the ears of mice "are supplied in an extraordinary manner with nerves, so that they no doubt serve as tactile organs." Hence, when we consider the life of mice, either nocturnal or seeking their food in dark and confined places, the length of the ears may be in each case adapted to the particular habits and surroundings of the species. Again, the tail, in the larger mammals, often serves the purpose of driving off flies and other insects from the body; and when we consider in how many parts of the world flies are injurious or even fatal to large mammals, we see that the peculiar characteristics of this organ may in each case have been adapted to its requirements in the particular area where the species was developed. The tail is also believed to have some use as a balancing organ, which assists an animal to turn easily and rapidly, much as our arms are used when running; while in whole groups it is a prehensile organ, and has become modified in accordance with the habits and needs of each species. In the case of mice it is thus used by the young. Darwin informs us that the late Professor Henslow kept some harvest-mice in confinement, and observed that they frequently curled their tails round the branches of a bush placed in the cage, and thus aided themselves in climbing; while Dr. Günther has actually seen a mouse suspend itself by the tail (Origin, p. 189).
Again, Mr. Lawson Tait has called attention to the use of the tail in the cat, squirrel, yak, and many other animals as a means of preserving the heat of the body during the nocturnal and the winter sleep. He says, that in cold weather animals with long or bushy tails will be found lying curled up, with their tails carefully laid over their feet like a rug, and with their noses buried in the fur of the tail, which is thus used exactly in the same way and for the same purpose as we use respirators.
Another illustration is furnished by the horns of deer which, especially when very large, have been supposed to be a danger to the animal in passing rapidly through dense thickets. But Sir James Hector states, that the wapiti, in North America, throws back its head, thus placing the horns along the sides of the back, and is then enabled to rush through the thickest forest with great rapidity. The brow-antlers protect the face and eyes, while the widely spreading horns prevent injury to the neck or flanks. Thus an organ which was certainly developed as a sexual weapon, has been so guided and modified during its increase in size as to be of use in other ways. A similar use of the antlers of deer has been observed in India.
The various classes of facts now referred to serve to show us that, in the case of the two higher groups—mammalia and birds—almost all the characters by which species are distinguished from each other are, or may be, adaptive. It is these two classes of animals which have been most studied and whose life-histories are supposed to be most fully known, yet even here the assertion of inutility, by an eminent naturalist, in the case of two important organs, has been sufficiently met by minute details either in the anatomy or in the habits of the groups referred to. Such a fact as this, together with the extensive series of characters already enumerated which have been of late years transferred from the "useless" to the "useful" class, should convince us, that the assertion of "inutility" in the case of any organ or peculiarity which is not a rudiment or a correlation, is not, and can never be, the statement of a fact, but merely an expression of our ignorance of its purpose or origin.
Instability of Non-adaptive Characters.
One very weighty objection to the theory that specific characters can ever be wholly useless (or wholly unconnected with useful organs by correlation of growth) appears to have been overlooked by those who have maintained the frequency of such characters, and that is, their almost necessary instability. Darwin has remarked on the extreme variability of secondary sexual characters—such as the horns, crests, plumes, etc., which are found in males only,—the reason being, that, although of some use, they are not of such direct and vital importance as those adaptive characters on which the wellbeing and very existence of the animals depend. But in the case of wholly useless structures, which are not rudiments of once useful organs, we cannot see what there is to ensure any amount of constancy or stability. One of the cases on which Mr. Romanes lays great stress in his paper on "Physiological Selection" (Journ. Linn. Soc., vol. xix. p. 384) is that of the fleshy appendages on the corners of the jaw of Normandy pigs and of some other breeds. But it is expressly stated that they are not constant; they appear "frequently," or "occasionally," they are "not strictly inherited, for they occur or fail in animals of the same litter;" and they are not always symmetrical, sometimes appearing on one side of the face alone. Now whatever may be the cause or explanation of these anomalous appendages they cannot be classed with "specific characters," the most essential features of which are, that they are symmetrical, that they are inherited, and that they are constant. Admitting that this peculiar appendage is (as Mr. Romanes says rather confidently, "we happen to know it to be") wholly useless and meaningless, the fact would be rather an argument against specific characters being also meaningless, because the latter never have the characteristics which this particular variation possesses.
These useless or non-adaptive characters are, apparently, of the same nature as the "sports" that arise in our domestic productions, but which, as Mr. Darwin says, without the aid of selection would soon disappear; while some of them may be correlations with other characters which are or have been useful. Some of these correlations are very curious. Mr. Tegetmeier informed Mr. Darwin that the young of white, yellow, or dun-coloured pigeons are born almost naked, whereas other coloured pigeons are born well clothed with down. Now, if this difference occurred between wild species of different colours, it might be said that the nakedness of the young could not be of any use. But the colour with which it is correlated might, as has been shown, be useful in many ways. The skin and its various appendages, as horns, hoofs, hair, feathers, and teeth, are homologous parts, and are subject to very strange correlations of growth. In Paraguay, horses with curled hair occur, and these always have hoofs exactly like those of a mule, while the hair of the mane and tail is much shorter than usual. Now, if any one of these characters were useful, the others correlated with it might be themselves useless, but would still be tolerably constant because dependent on a useful organ. So the tusks and the bristles of the boar are correlated and vary in development together, and the former only may be useful, or both may be useful in unequal degrees.
The difficulty as to how individual differences or sports can become fixed and perpetuated, if altogether useless, is evaded by those who hold that such characters are exceedingly common. Mr. Romanes says that, upon his theory of physiological selection, "it is quite intelligible that when a varietal form is differentiated from its parent form by the bar of sterility, any little meaningless peculiarities of structure or of instinct should at first be allowed to arise, and that they should then be allowed to perpetuate themselves by heredity," until they are finally eliminated by disuse. But this is entirely begging the question. Do meaningless peculiarities, which we admit often arise as spontaneous variations, ever perpetuate themselves in all the individuals constituting a variety or race, without selection either human or natural? Such characters present themselves as unstable variations, and as such they remain, unless preserved and accumulated by selection; and they can therefore never become "specific" characters unless they are strictly correlated with some useful and important peculiarities.
As bearing upon this question we may refer to what is termed Delboeuf's law, which has been thus briefly stated by Mr. Murphy in his work on Habit and Intelligence, p. 241.
"If, in any species, a number of individuals, bearing a ratio not infinitely small to the entire number of births, are in every generation born with a particular variation which is neither beneficial nor injurious, and if it is not counteracted by reversion, then the proportion of the new variety to the original form will increase till it approaches indefinitely near to equality."
It is not impossible that some definite varieties, such as the melanic form of the jaguar and the bridled variety of the guillemot are due to this cause; but from their very nature such varieties are unstable, and are continually reproduced in varying proportions from the parent forms. They can, therefore, never constitute species unless the variation in question becomes beneficial, when it will be fixed by natural selection. Darwin, it is true, says—"There can be little doubt that the tendency to vary in the same manner has often been so strong that all the individuals of the same species have been similarly modified without the aid of any form of selection." But no proof whatever is offered of this statement, and it is so entirely opposed to all we know of the facts of variation as given by Darwin himself, that the important word "all" is probably an oversight.
On the whole, then, I submit, not only has it not been proved that an "enormous number of specific peculiarities" are useless, and that, as a logical result, natural selection is "not a theory of the origin of species," but only of the origin of adaptations which are usually common to many species, or, more commonly, to genera and families; but, I urge further, it has not even been proved that any truly "specific" characters—those which either singly or in combination distinguish each species from its nearest allies—are entirely unadaptive, useless, and meaningless; while a great body of facts on the one hand, and some weighty arguments on the other, alike prove that specific characters have been, and could only have been, developed and fixed by natural selection because of their utility. We may admit, that among the great number of variations and sports which continually arise many are altogether useless without being hurtful; but no cause or influence has been adduced adequate to render such characters fixed and constant throughout the vast number of individuals which constitute any of the more dominant species.
The Swamping Effects of Intercrossing.
This supposed insuperable difficulty was first advanced in an article in the North British Review in 1867, and much attention has been attracted to it by the acknowledgment of Mr. Darwin that it proved to him that "single variations," or what are usually termed "sports," could very rarely, if ever, be perpetuated in a state of nature, as he had at first thought might occasionally be the case. But he had always considered that the chief part, and latterly the whole, of the materials with which natural selection works, was afforded by individual variations, or that amount of ever fluctuating variability which exists in all organisms and in all their parts. Other writers have urged the same objection, even as against individual variability, apparently in total ignorance of its amount and range; and quite recently Professor G.J. Romanes has adduced it as one of the difficulties which can alone be overcome by his theory of physiological selection. He urges, that the same variation does not occur simultaneously in a number of individuals inhabiting the same area, and that it is mere assumption to say it does; while he admits that "if the assumption were granted there would be an end of the present difficulty; for if a sufficient number of individuals were thus simultaneously and similarly modified, there need be no longer any danger of the variety becoming swamped by intercrossing." I must again refer my readers to my third chapter for the proof that such simultaneous variability is not an assumption but a fact; but, even admitting this to be proved, the problem is not altogether solved, and there is so much misconception regarding variation, and the actual process of the origin of new species is so obscure, that some further discussion and elucidation of the subject are desirable.
In one of the preliminary chapters of Mr. Seebohm's recent work on the Charadriidae, he discusses the differentiation of species; and he expresses a rather widespread view among naturalists when, speaking of the swamping effects of intercrossing, he adds: "This is unquestionably a very grave difficulty, to my mind an absolutely fatal one, to the theory of accidental variation." And in another passage he says: "The simultaneous appearance, and its repetition in successive generations, of a beneficial variation, in a large number of individuals in the same locality, cannot possibly be ascribed to chance." These remarks appear to me to exhibit an entire misconception of the facts of variation as they actually occur, and as they have been utilised by natural selection in the modification of species. I have already shown that every part of the organism, in common species, does vary to a very considerable amount, in a large number of individuals, and in the same locality; the only point that remains to be discussed is, whether any or most of these variations are "beneficial." But every one of these variations consists either in increase or diminution of size or power of the organ or faculty that varies; they can all be divided into a more effective and a less effective group—that is, into one that is more beneficial or less beneficial. If less size of body would be beneficial, then, as half the variations in size are above and half below the mean or existing standard of the species, there would be ample beneficial variations; if a darker colour or a longer beak or wing were required, there are always a considerable number of individuals darker and lighter in colour than the average, with longer or with shorter beaks and wings, and thus the beneficial variation must always be present. And so with every other part, organ, function, or habit; because, as variation, so far as we know, is and always must be in the two directions of excess and defect in relation to the mean amount, whichever kind of variation is wanted is always present in some degree, and thus the difficulty as to "beneficial" variations occurring, as if they were a special and rare class, falls to the ground. No doubt some organs may vary in three or perhaps more directions, as in the length, breadth, thickness, or curvature of the bill. But these may be taken as separate variations, each of which again occurs as "more" or "less"; and thus the "right" or "beneficial" or "useful" variation must always be present so long as any variation at all occurs; and it has not yet been proved that in any large or dominant species, or in any part, organ, or faculty of such species, there is no variation. And even were such a case found it would prove nothing, so long as in numerous other species variation was shown to exist; because we know that great numbers of species and groups throughout all geological time have died out, leaving no descendants; and the obvious and sufficient explanation of this fact is, that they did not vary enough at the time when variation was required to bring them into harmony with changed conditions. The objection as to the "right" or "beneficial" variation occurring when required, seems therefore to have no weight in view of the actual facts of variation.
Isolation to prevent Intercrossing.
Most writers on the subject consider the isolation of a portion of a species a very important factor in the formation of new species, while others maintain it to be absolutely essential. This latter view has arisen from an exaggerated opinion as to the power of intercrossing to keep down any variety or incipient species, and merge it in the parent stock. But it is evident that this can only occur with varieties which are not useful, or which, if useful, occur in very small numbers; and from this kind of variations it is clear that new species do not arise. Complete isolation, as in an oceanic island, will no doubt enable natural selection to act more rapidly, for several reasons. In the first place, the absence of competition will for some time allow the new immigrants to increase rapidly till they reach the limits of subsistence. They will then struggle among themselves, and by survival of the fittest will quickly become adapted to the new conditions of their environment. Organs which they formerly needed, to defend themselves against, or to escape from, enemies, being no longer required, would be encumbrances to be got rid of, while the power of appropriating and digesting new and varied food would rise in importance. Thus we may explain the origin of so many flightless and rather bulky birds in oceanic islands, as the dodo, the cassowary, and the extinct moas. Again, while this process was going on, the complete isolation would prevent its being checked by the immigration of new competitors or enemies, which would be very likely to occur in a continuous area; while, of course, any intercrossing with the original unmodified stock would be absolutely prevented. If, now, before this change has gone very far, the variety spreads into adjacent but rather distant islands, the somewhat different conditions in each may lead to the development of distinct forms constituting what are termed representative species; and these we find in the separate islands of the Galapagos, the West Indies, and other ancient groups of islands.
But such cases as these will only lead to the production of a few peculiar species, descended from the original settlers which happened to reach the islands; whereas, in wide areas, and in continents, we have variation and adaptation on a much larger scale; and, whenever important physical changes demand them, with even greater rapidity. The far greater complexity of the environment, together with the occurrence of variations in constitution and habits, will often allow of effective isolation, even here, producing all the results of actual physical isolation. As we have already explained, one of the most frequent modes in which natural selection acts is, by adapting some individuals of a species to a somewhat different mode of life, whereby they are able to seize upon unappropriated places in nature, and in so doing they become practically isolated from their parent form. Let us suppose, for example, that one portion of a species usually living in forests ranges into the open plains, and finding abundance of food remains there permanently. So long as the struggle for existence is not exceptionally severe, these two portions of the species may remain almost unchanged; but suppose some fresh enemies are attracted to the plains by the presence of these new immigrants, then variation and natural selection would lead to the preservation of those individuals best able to cope with the difficulty, and thus the open country form would become modified into a marked variety or into a distinct species; and there would evidently be little chance of this modification being checked by intercrossing with the parent form which remained in the forest.
Another mode of isolation is brought about by the variety—either owing to habits, climate, or constitutional change—breeding at a slightly different time from the parent species. This is known to produce complete isolation in the case of many varieties of plants. Yet another mode of isolation is brought about by changes of colour, and by the fact that in a wild state animals of similar colours prefer to keep together and refuse to pair with individuals of another colour. The probable reason and utility of this habit will be explained in another chapter, but the fact is well illustrated by the cattle which have run wild in the Falkland Islands. These are of several different colours, but each colour keeps in a separate herd, often restricted to one part of the island; and one of these varieties—the mouse-coloured—is said to breed a month earlier than the others; so that if this variety inhabited a larger area it might very soon be established as a distinct race or species. Of course where the change of habits or of station is still greater, as when a terrestrial animal becomes sub-aquatic, or when aquatic animals come to live in tree-tops, as with the frogs and Crustacea described at p. 118, the danger of intercrossing is reduced to a minimum.
Several writers, however, not content with the indirect effects of isolation here indicated, maintain that it is in itself a cause of modification, and ultimately of the origination of new species. This was the keynote of Mr. Vernon Wollaston's essay on "Variation of Species," published in 1856, and it is adopted by the Rev. J.G. Gulick in his paper on "Diversity of Evolution under one Set of External Conditions" (Journ. Linn. Soc. Zool., vol. xi. p. 496). The idea seems to be that there is an inherent tendency to variation in certain divergent lines, and that when one portion of a species is isolated, even though under identical conditions, that tendency sets up a divergence which carries that portion farther and farther away from the original species. This view is held to be supported by the case of the land shells of the Sandwich Islands, which certainly present some very remarkable phenomena. In this comparatively small area there are about 300 species of land shells, almost all of which belong to one family (or sub-family), the Achatinellidae, found nowhere else in the world. The interesting point is the extreme restriction of the species and varieties. The average range of each species is only five or six miles, while some are restricted to but one or two square miles, and only a very few range over a whole island. The forest region that extends over one of the mountain-ranges of the island of Oahu, is about forty miles in length and five or six miles in breadth; and this small territory furnishes about 175 species, represented by 700 or 800 varieties. Mr. Gulick states, that the vegetation of the different valleys on the same side of this range is much the same, yet each has a molluscan fauna differing in some degree from that of any other. "We frequently find a genus represented in several successive valleys by allied species, sometimes feeding on the same, sometimes on different plants. In every such case the valleys that are nearest to each other furnish the most nearly allied forms; and a full set of the varieties of each species presents a minute gradation of forms between the more divergent types found in the more widely separated localities." He urges, that these constant differences cannot be attributed to natural selection, because they occur in different valleys on the same side of the mountain, where food, climate, and enemies are the same; and also, because there is no greater difference in passing from the rainy to the dry side of the mountains than in passing from one valley to another on the same side an equal distance apart. In a very lengthy paper, presented to the Linnean Society last year, on "Divergent Evolution through Cumulative Segregation," Mr. Gulick endeavours to work out his views into a complete theory, the main point of which may perhaps be indicated by the following passage: "No two portions of a species possess exactly the same average character, and the initial differences are for ever reacting on the environment and on each other in such a way as to ensure increasing divergence in each successive generation as long as the individuals of the two groups are kept from intercrossing."
It need hardly be said that the views of Mr. Darwin and myself are inconsistent with the notion that, if the environment were absolutely similar for the two isolated portions of the species, any such necessary and constant divergence would take place. It is an error to assume that what seem to us identical conditions are really identical to such small and delicate organisms as these land molluscs, of whose needs and difficulties at each successive stage of their existence, from the freshly-laid egg up to the adult animal, we are so profoundly ignorant. The exact proportions of the various species of plants, the numbers of each kind of insect or of bird, the peculiarities of more or less exposure to sunshine or to wind at certain critical epochs, and other slight differences which to us are absolutely immaterial and unrecognisable, may be of the highest significance to these humble creatures, and be quite sufficient to require some slight adjustments of size, form, or colour, which natural selection will bring about. All we know of the facts of variation leads us to believe that, without this action of natural selection, there would be produced over the whole area a series of inconstant varieties mingled together, not a distinct segregation of forms each confined to its own limited area.
Mr. Darwin has shown that, in the distribution and modification of species, the biological is of more importance than the physical environment, the struggle with other organisms being often more severe than that with the forces of nature. This is particularly evident in the case of plants, many of which, when protected from competition, thrive in a soil, climate, and atmosphere widely different from those of their native habitat. Thus, many alpine plants only found near perpetual snow thrive well in our gardens at the level of the sea; as do the tritomas from the sultry plains of South Africa, the yuccas from the arid hills of Texas and Mexico, and the fuchsias from the damp and dreary shores of the Straits of Magellan. It has been well said that plants do not live where they like, but where they can; and the same remark will apply to the animal world. Horses and cattle run wild and thrive both in North and South America; rabbits, once confined to the south of Europe, have established themselves in our own country and in Australia; while the domestic fowl, a native of tropical India, thrives well in every part of the temperate zone.
If, then, we admit that when one portion of a species is separated from the rest, there will necessarily be a slight difference in the average characters of the two portions, it does not follow that this difference has much if any effect upon the characteristics that are developed by a long period of isolation. In the first place, the difference itself will necessarily be very slight unless there is an exceptional amount of variability in the species; and in the next place, if the average characters of the species are the expression of its exact adaptation to its whole environment, then, given a precisely similar environment, and the isolated portion will inevitably be brought back to the same average of characters. But, as a matter of fact, it is impossible that the environment of the isolated portion can be exactly like that of the bulk of the species. It cannot be so physically, since no two separated areas can be absolutely alike in climate and soil; and even if these are the same, the geographical features, size, contour, and relation to winds, seas, and rivers, would certainly differ. Biologically, the differences are sure to be considerable. The isolated portion of a species will almost always be in a much smaller area than that occupied by the species as a whole, hence it is at once in a different position as regards its own kind. The proportions of all the other species of animals and plants are also sure to differ in the two areas, and some species will almost always be absent in the smaller which are present in the larger country. These differences will act and react on the isolated portion of the species. The struggle for existence will differ in its severity and in its incidence from that which affects the bulk of the species. The absence of some one insect or other creature inimical to the young animal or plant may cause a vast difference in its conditions of existence, and may necessitate a modification of its external or internal characters in quite a different direction from that which happened to be present in the average of the individuals which were first isolated.
On the whole, then, we conclude that, while isolation is an important factor in effecting some modification of species, it is so, not on account of any effect produced, or influence exerted by isolation per se, but because it is always and necessarily accompanied by a change of environment, both physical and biological. Natural selection will then begin to act in adapting the isolated portion to its new conditions, and will do this the more quickly and the more effectually because of the isolation. We have, however, seen reason to believe that geographical or local isolation is by no means essential to the differentiation of species, because the same result is brought about by the incipient species acquiring different habits or frequenting a different station; and also by the fact that different varieties of the same species are known to prefer to pair with their like, and thus to bring about a physiological isolation of the most effective kind. This part of the subject will be again referred to when the very difficult problems presented by hybridity are discussed.
Cases in which Isolation is Ineffective.
One objection to the views of those who, like Mr. Gulick, believe isolation itself to be a cause of modification of species deserves attention, namely, the entire absence of change where, if this were a vera causa, we should expect to find it. In Ireland we have an excellent test case, for we know that it has been separated from Britain since the end of the glacial epoch, certainly many thousand years. Yet hardly one of its mammals, reptiles, or land molluscs has undergone the slightest change, even although there is certainly a distinct difference in the environment both inorganic and organic. That changes have not occurred through natural selection, is perhaps due to the less severe struggle for existence owing to the smaller number of competing species; but, if isolation itself were an efficient cause, acting continuously and cumulatively, it is incredible that a decided change should not have been produced in thousands of years. That no such change has occurred in this, and many other cases of isolation, seems to prove that it is not in itself a cause of modification.
There yet remain a number of difficulties and objections relating to the question of hybridity, which are so important as to require a separate chapter for their adequate discussion.
- See Origin of Species, pp. 176-198.
- See Kerner's Flowers and their Unbidden Guests for numerous other structures and peculiarities of plants which are shown to be adaptive and useful.
- Nature, vol. xx. p. 603.
- Nature, vol. xxxviii. p. 328.
- A very remarkable illustration of function in an apparently useless ornament is given by Semper. He says, "It is known that the skin of reptiles encloses the body with scales. These scales are distinguished by very various sculpturings, highly characteristic of the different species. Irrespective of their systematic significance they appear to be of no value in the life of the animal; indeed, they are viewed as ornamental without regard to the fact that they are microscopic and much too delicate to be visible to other animals of their own species. It might, therefore, seem hopeless to show the necessity for their existence on Darwinian principles, and to prove that they are physiologically active organs. Nevertheless, recent investigations on this point have furnished evidence that this is possible.
"It is known that many reptiles, and above all the snakes, cast off the whole skin at once, whereas human beings do so by degrees. If by any accident they are prevented doing so, they infallibly die, because the old skin has grown so tough and hard that it hinders the increase in volume which is inseparable from the growth of the animal. The casting of the skin is induced by the formation on the surface of the inner epidermis, of a layer of very fine and equally distributed hairs, which evidently serve the purpose of mechanically raising the old skin by their rigidity and position. These hairs then may be designated as casting hairs. That they are destined and calculated for this end is evident to me from the fact established by Dr. Braun, that the casting of the shells of the river crayfish is induced in exactly the same manner by the formation of a coating of hairs which mechanically loosens the old skin or shell from the new. Now the researches of Braun and Cartier have shown that these casting hairs—which serve the same purpose in two groups of animals so far apart in the systematic scale—after the casting, are partly transformed into the concentric stripes, sharp spikes, ridges, or warts which ornament the outer edges of the skin-scales of reptiles or the carapace of crabs." [The Natural Conditions of Existance as they affect Animal Life, p. 19.] Professor Semper adds that this example, with many others that might be quoted, shows that we need not abandon the hope of explaining morphological characters on Darwinian principles, although their nature is often difficult to understand.
During a recent discussion of this question in the pages of Nature, Mr. St. George Mivart adduces several examples of what he deems useless specific characters. Among them are the aborted index finger of the lemurine Potto, and the thumbless hands of Colobus and Ateles, the "life-saving action" of either of which he thinks incredible. These cases suggest two remarks. In the first place, they involve generic, not specific, characters; and the three genera adduced are somewhat isolated, implying considerable antiquity and the extinction of many allied forms. This is important, because it affords ample time for great changes of conditions since the structures in question originated; and without a knowledge of these changes we can never safely assert that any detail of structure could not have been useful. In the second place, all three are cases of aborted or rudimentary organs; and these are admitted to be explained by non-use, leading to diminution of size, a further reduction being brought about by the action of the principle of economy of growth. But, when so reduced, the rudiment might be inconvenient or even hurtful, and then natural selection would aid in its complete abortion; in other words, the abortion of the part would be useful, and would therefore be subject to the law of survival of the fittest. The genera Ateles and Colobus are two of the most purely arboreal types of monkeys, and it is not difficult to conceive that the constant use of the elongated fingers for climbing from tree to tree, and catching on to branches while making great leaps, might require all the nervous energy and muscular growth to be directed to the fingers, the small thumb remaining useless. The case of the Potto is more difficult, both because it is, presumably, a more ancient type, and its actual life-history and habits are completely unknown. These cases are, therefore, not at all to the point as proving that positive specific characters—not mere rudiments characterising whole genera—are in any case useless.
Mr. Mivart further objects to the alleged rigidity of the action of natural selection, because wounded or malformed animals have been found which had evidently lived a considerable time in their imperfect condition. But this simply proves that they were living under a temporarily favourable environment, and that the real struggle for existence, in their case, had not yet taken place. We must surely admit that, when the pinch came, and when perfectly formed stoats were dying for want of food, the one-footed animal, referred to by Mr. Mivart, would be among the first to succumb; and the same remark will apply to his abnormally toothed hares and rheumatic monkeys, which might, nevertheless, get on very well under favourable conditions. The struggle for existence, under which all animals and plants have been developed, is intermittent, and exceedingly irregular in its incidence and severity. It is most severe and fatal to the young; but when an animal has once reached maturity, and especially when it has gained experience by several years of an eventful existence, it may be able to maintain itself under conditions which would be fatal to a young and inexperienced creature of the same species. The examples adduced by Mr. Mivart do not, therefore, in any way impugn the hardness of nature as a taskmaster, or the extreme severity of the recurring struggle for existence.[See Nature, vol. xxxix. p. 127.]
- Origin of Species, p. 72.
- Darwin's latest expression of opinion on this question is interesting, since it shows that he was inclined to return to his earlier view of the general, or universal, utility of specific characters. In a letter to Semper (30th Nov. 1878) he writes: "As our knowledge advances, very slight differences, considered by systematists as of no importance in structure, are continually found to be functionally important; and I have been especially struck with this fact in the case of plants, to which my observations have, of late years, been confined. Therefore it seems to me rather rash to consider slight differences between representative species, for instance, those inhabiting the different islands of the same archipelago, as of no functional importance, and as not in any way due to natural selection" (Life of Darwin, vol. iii. p. 161).
- See Variation of Animals and Plants, vol. i. p. 86.
- Journal of the Linnean Society, Zoology, vol. xx. p. 215.
- In Mr. Gulick's last paper (Journal of Linn. Soc. Zool., vol. xx. pp. 189-274) he discusses the various forms of isolation above referred to, under no less than thirty-eight different divisions and subdivisions, with an elaborate terminology, and he argues that these will frequently bring about divergent evolution without any change in the environment or any action of natural selection. The discussion of the problem here given will, I believe, sufficiently expose the fallacy of his contention; but his illustration of the varied and often recondite modes by which practical isolation may be brought about, may help to remove one of the popular difficulties in the way of the action of natural selection in the origination of species.