Wallace’s Theory Criticised
Wallace’s view that the dull plumage of the hen bird is due to her greater need of protection is based on the assumption that the hen bird alone takes part in incubation.
Is this assumption a correct one?
It certainly is not in all cases. As D. Dewar has stated in Birds of the Plains, the showy white cock Paradise Fly-catcher (Terpsiphone paradisi) sits in broad daylight on the open nest quite as much as the hen does. And this may prove to be true of many other species of birds. Again, the cocks of the various species of Indian sunbirds are brightly coloured while the hens are dull brown. In these species the hen alone sits on the eggs, but, as the nest is well covered-in, the hen might display all the colours of the rainbow without being visible to passing birds. Moreover, as D. Dewar pointed out in a paper read before the Royal Society of Arts (Journal, vol. lvii., p. 104), although, in most species of Indian dove, the sexes show little or no dissimilarity, there is one species (Œnopopelia tranquebarica) which exhibits considerable sexual dimorphism. But the nesting habits of this peculiar species are in all respects similar to those of the other species of dove. Why then the marked dissimilarity of the sexes?
Another objection to the theory of Wallace is that urged by J. T. Cunningham (Archiv für Entwicklungsmechanik der Organismen, vol. xxvi., p. 378), namely, that the secondary sexual characters in those species which possess them show an entire absence of uniformity in nature and position. “Why,” asks Cunningham, “should the male constitution of the stag show itself in bony excrescences of the skull, in the peacock in excessive growth of the other end of the body? Why should the larynx be modified in one mammal, the teeth in another, the nose in another? Why is the male newt distinguished by a dorsal fin, the male frog by a swelling on the fore foot?”
Another objection to the explanation of sexual dimorphism suggested by Wallace, is that in many species of bird, as, for example, the house sparrow and the green paroquets of India, the external differences between the sexes are so slight that it is unreasonable to believe that they are the result of natural selection. It seems impossible to hold that the Rose-ringed Paroquet (Palæornis torquatus)—a species which nests in holes—would have become extinct if the hens had developed the narrow rose-coloured collar that characterises the cocks.
Darwin pointed out that while Wallace’s hypothesis might appear plausible if applied to colour, it can scarcely be said to explain the origin of such structures as the musical apparatus of certain male insects, or the larger size of the larynx in some birds and mammals. We thus see that suggestions offered by Wallace, although they contain a modicum of truth, fail to explain the phenomena of sexual dimorphism.
The fairest possible criticism of these views is that of Darwin:—
“It will have been seen that I cannot follow Mr Wallace in the belief that dull colours, when confined to the females, have been in most cases specially gained for the sake of protection. There can, however, be no doubt, as formerly remarked, that both sexes of many birds have had their colours modified, so as to escape the notice of their enemies; or in some instances, so as to approach their prey unobserved, just as owls have had their plumage rendered soft, that their flight may not be overheard” (The Descent of Man, p. 745).
The Theory of Thomson and Geddes
Thomson and Geddes have attempted to explain sexual dimorphism on the hypothesis that males are essentially dissipators of energy, while females tend to conserve energy. They point out that the spermatozoon is a small intensely active body, which dissipates its energy in motion, while the ovum is a large inert body—the result of the female tendency to conserve energy and to build up material. The various ornaments and excrescences which appear in male organisms are the result of this male tendency to dissipate energy. In the spermatozoon the dissipated energy appears in the form of active movement; in the adult organism it takes the shape of plumes and other ornaments, of song and contests for the females.
This theory, however, does not explain what we might call the haphazard nature of sexual dimorphism. If sexual dissimilarity is due to the tendency of the male to dissipate energy, why do we see very marked dimorphism in one species, and no dimorphism in a very nearly allied species? Why are the males larger than the females in some species, and smaller in other species? Again, how is it that in certain species of birds—the quails of the genus Turnix, the Painted Snipe (Rhynchæa), and the Phalaropes—it is the female who possesses the more showy plumage? Moreover, this theory, equally with that of Wallace, does not explain why the excrescences which characterise the male appear in various parts of the body in different species.
Stolzmann’s Theory
Stolzmann has made an ingenious attempt to explain why in birds the cock is so frequently more conspicuously coloured than the hen. He asserts that among birds the males are more numerous than the females, and that this preponderance is not advantageous to the species. Those males which have not managed to secure a mate are apt to persecute the females while sitting on the eggs, to the detriment of these latter. Natural selection, says Stolzmann, is concerned with the well-being of the species rather than of the individual. Hence anything that would tend to lessen the number of males would be a good thing for the species, so that a peculiarity, such as bright plumage, which renders the males conspicuous, or ornamental plumes, which cause their flight to be slow, and so leads to their destruction, will be seized upon and perpetuated by natural selection. He points out that the cock of one species of hummingbird—Loddigesia mirabilis—has not only longer tail feathers, but a shorter wing than the female, and must, in consequence, find it comparatively difficult to obtain food, and be more liable to fall a victim to birds of prey than the hen. Stolzmann further suggests that the excessive pugnacity of male birds at the breeding season may lead to the destruction of some individuals, and so prove of advantage to the species.
Several objections seem to present themselves to this most ingenious theory.
In the first place, there does not appear to be any satisfactory evidence to show that more cocks than hens are born.
We may grant that a superfluity of cocks is injurious to any species, since the unmated ones are likely to persecute the hens; we may also grant that many cocks are handicapped in the struggle for existence by the excessive growth of certain of their feathers, but we fail to see how this excessive development has been caused by natural selection in the manner suggested by Stolzmann. Although it may be advantageous to the species for the cocks to be showy, natural selection can perpetuate this only by weeding out the least conspicuous of the cocks. But it is the more gaudy ones, those, according to Stolzmann, whose presence is beneficial to the species, which will be eliminated by natural selection. So that, in this case, that force will act in a manner contrary to the interests of the species, if Stolzmann’s idea is a correct one.
The theory in question would therefore seem to be untenable. Nevertheless there is doubtless some truth in the notion that too many males spoil the species. Thus, excessive showiness and high mortality among the males may be beneficial to the species. But we must not forget that the more beneficial it is, the stronger must be the tendency of natural selection to eliminate the males that possess the desired peculiarity.
Neo-Lamarckian Explanation
Cunningham’s Theory
J. T. Cunningham makes an attempt to explain the phenomena of sexual dimorphism on Neo-Lamarckian principles. His theory is set forth in a paper entitled The Heredity of Secondary Sexual Characters in relation to Hormones, which was read before the Zoological Society of London, and published in full in the Archiv für Entwicklungsmechanik der Organismen. “The significant correlation of male sexual characters,” he writes, “is not with any general or essential property of the male sex, such as katabolism (or the tendency to dissipate energy, as we have called it), but with certain habits and functions confined to one sex, but differing in different animals. . . . In those animals which possess such (i.e. secondary sexual) characters, the parts of the soma (i.e. the body) affected differ as much as they can differ; any part of the soma may show a sexual difference: teeth in one mammal, skull in another; feathers of the tail in one bird, those of the neck in another, and so on. But in all cases such unisexual characters correspond to their functions or use in habits and instincts which are associated, but only indirectly, with sexual production. These habits are as diverse and as irregular in their distribution as the characters. The cocks of common fowls and of the Phasianidæ generally are polygamous, fight with each other for the possession of the females, and take no part in incubation or care of the young, and they differ from the hens in their enlarged brilliant plumage, spurs on the legs, and combs, wattles, or other excrescences on the head. In the Columbidæ per contra the males are not polygamous, but pair for life, the males do not fight, and share equally with the females in parental duties.
“Corresponding with this contrast of sexual habits is the contrast of sexual dimorphism, which is virtually absent in the Columbidæ.
“I think, then, the only scientific explanation is that the difference of habits is the cause of the sexual dimorphism, and that the special sexual habits which occur in some species but not in others are the causes of the sexual characters. . . . The habits in question always involve certain definite stimulations applied to those parts of the body whose modification constitutes the somatic sexual characters. The stimulations are confined, as the characters are confined, to one sex, to one period of life, to one season of the year, to those animals which have the characters, to those parts of the body which are modified.” Mr Cunningham believes that these stimulations cause hypertrophy or excessive growth of the part affected, and that this peculiarity is transmitted to the offspring. And thus he supposes all the ornaments and excrescences of the males of various species to have arisen.
As evidence in favour of his view, he points out that these excrescences are, in many species, not only functionless but absolutely injurious, as in the case of the comb and wattles of the jungle cock and his domestic descendants, which merely serve as a handle for enemies to seize.
Cunningham asserts that the only objection to his theory is the dogma that acquired characters cannot be inherited. This assertion is, however, not correct. It is, indeed, a very serious objection that all the evidence available seems to show that acquired characters are not inherited, but this is by no means the only difficulty.
Before mentioning these further objections, let us say a word on the subject of the inheritance of acquired characters. Mr Cunningham himself compares the formation of a splint or spavin in a horse as the result of special strain, to the acquisition of secondary sexual characters. Unfortunately for Cunningham’s theory, but fortunately for mankind in general, spavined horses and mares do not beget spavined offspring. If, then, spavin is not inherited, is it not unreasonable to assert that the thickening of the bone that develops on the head of a butting animal is inherited?
Another objection to Cunningham’s theory is that many birds which show off their plumage most vigorously possess no ornamental plumes. As Howard has recorded, many of our dull-coloured British warblers show off in the same manner as bright-coloured birds do. If the exercise has caused the development and inheritance of plumes in some species, why not in the others?
Again, Cunningham is not correct in saying that sexual dimorphism is “virtually absent” in the Columbidæ. Few birds display so striking a sexual dimorphism as the Orange Dove (Chrysœna victor) of Fiji, in which the male is bright orange and the hen green. We have already cited the case of the curious sexually dimorphic red turtle-dove. Now, the courting attitudes and actions of this species are precisely the same as those of other allied turtle-doves; why, then, have these exercises caused only one species to become sexually dimorphic?
Existing Theories not Satisfactory
Our survey of the more important attempts which have been made to explain the phenomena of sexual dimorphism leads to the conclusion that these still require elucidation. We have weighed each theory in the balance and found it wanting.
The outstanding feature of sexual dissimilarity is the apparently haphazard manner of its occurrence.
We have already alluded to the case of the doves in India. In that country four species are widely distributed—namely, the Spotted Dove (Turtur suratensis), the Ring or Collared Dove (Turtur risorius), the Little Brown Dove (Turtur cambayensis), and the Red Turtle-dove (Œnopopelia tranqebarica). The habits of all these four species appear to be identical, nevertheless in the first three the sexes show little or no dissimilarity in outward appearance, while in the last the sexual dimorphism is so great that the cock and hen were formerly thought to belong to different species.
Another very curious case is that of the South American geese of the genus Chloëphaga, in which some species, as the familiar Upland or Magellan Goose of our parks (C. magellanica), have the sexes utterly unlike, while in others, as the Ruddy-headed Goose (C. rubidiceps), they are quite similar to each other.
The ducks furnish us with another very good example of the apparently haphazard nature of sexual dimorphism. In the Common Mallard or Wild Duck (Anas boscas) the cock is far more showily coloured than the hen, but in all the species most nearly allied to it the males are as inconspicuous as the females, e.g. in the Indian Spotted-bill (Anas pœcilorhyncha), the Australian Grey Duck (A. superciliosa), the African Yellow Bill (Anas undulata), and the American Dusky Duck (A. obscura). As the dusky duck inhabits North America, where the mallard is also found, the case is particularly striking.
Among mammals the lion and the tiger and the sable and roan antelopes (Hippotragus niger and H. equinus) furnish familiar examples of nearly-related species, in one of which the sexes are alike and in the other dissimilar in appearance.
Hormones
Another important point to be borne in mind is the intimate correlation that exists between the reproductive organs and the general appearance of the organism, more especially of the secondary sexual characters. These last, in most cases, do not show themselves until the maturity of the sexual organs. The well-known effects of castration illustrate this connection. Again, females in which the reproductive organs have ceased to be functional often assume male characters.
It has lately been proved by experiment that, in many cases at any rate, the development of the ornaments, etc., characteristic of the sexes is due to the secretion by the sexual cells of what are known as hormones—that is to say, secretions which excite development of the secondary sexual characters. The tendency to produce the external characteristics of the sex to which an organism belongs is inherited, but the actual development thereof is in many cases dependent on the secretion of these hormones. Accordingly, if a male individual be completely castrated it ceases to develop the external characters of its sex. The evidence upon which the doctrine of hormones is based is admirably summarised in the above-quoted paper by Cunningham. Into this evidence we cannot go. It must suffice that the doctrine is quite in accordance with all the observed results of castration.
It is worthy of notice that the various features which characterise the sexes in sexually dimorphic animals are not associated with any particular organ or parts of the body, nor do they necessarily affect the same part in allied species. “We cannot say,” writes J. T. Cunningham, “that any part of the soma (i.e. the body tissue) is specially sexual more than another part, except that such differences between the sexes are usually external. They usually affect the skin, and especially epidermic appendages, and the superficial parts of the skeleton, or whole limbs and appendages; or the difference may be one of size of the whole soma. In mammals and birds the male is often the larger, sometimes very much so, but there are cases in which the female is larger. There is no general rule.”
Another important point is, that females, although they themselves show no trace of the male character, are capable of transmitting it to their progeny. This can be proved by crossing a hen pheasant with a cock barn-door-fowl; the male offspring of the union display the plumes so characteristic of the cock pheasant. These cannot have been derived from the barn-door-fowl father; they must have come from the dull-coloured hen pheasant.
In this connection we may mention the curious fact recorded by Bonhote, on page 245 of the Proceedings of the Fourth International Ornithological Congress, that in the case of ducks descended from crosses between the pintail, the mallard, and the spotbill, the drakes in full breeding plumage showed a mixture of pintail and mallard characteristics, while, in their non-breeding plumage, the colouring of the spotbill is predominant.
Eye-colour, Comb, and Spurs
An important point, and one which does not seem to have been pointed out by any zoologist, is that eye-colour, comb, and spurs in birds and horns in mammals do not stand in the same relation to the sexual organs as do the other external characteristics. For example, the castrated Nilgai (Boselaphus tragocamelus) acquires horns, but not the characteristic male colour. In the common Indian Francolin Partridge (Francolinus pondicerianius), the cock differs from the hen only in the possession of spurs. The same applies to the various species of Snow Cock (Tetraogallus). There is a breed of game-cocks which display plumage like that of the hen, but such birds have the comb and spurs developed as in normally feathered cocks.
The white eye of the white-eyed Pochard Drake (Nyroca africana), and the yellow eye of the cock Golden Pheasant (Chrysolophus pictus), which are purely male characters, show themselves earlier than the male plumage. Occasionally a hen golden pheasant assumes the plumage of the cock, but she never acquires the yellow eye.
Many birds when kept in captivity lose some of the beauty of their plumage, and this is usually attributed to the sexual organs becoming impaired and reacting on the somatic tissue. But this explanation cannot in all cases be the correct one, because the linnet, although losing the male plumage in captivity, lives long and well in a cage and breeds readily with hen canaries.
Another curious fact is that the male plumage sometimes appears pathologically in hen birds, more especially in those which have become sterile from age or disease. This phenomenon occurs comparatively frequently in the gold pheasant, and more rarely in the common pheasant, the fowl, and the duck.
Phenomena such as these seem to suggest that in some cases the bright colours of the male may be pathological, that the hormones which the male sexual cells secrete may exercise an injurious effect on the somatic or body tissues. Decay is known to be accompanied by the production of brightly coloured pigment in the case of leaves. Finn suggests that the white plumage which the cock paradise fly-catcher assumes in the fourth year of his existence may be a livery of decay, a sign of senility.
The Four Kinds of Mutations
It is our belief that sexual dimorphism arises frequently, if not invariably, as a mutation. Mutations may be of four different kinds.
Those which appear only, or especially, in conjunction with the male organs, for example, whiteness in domesticated geese allowed to breed indiscriminately.
Those which appear only, or especially, in conjunction with the female organs; mutations of this description appear to be very rare, but it may be noted that in fowls allowed to breed indiscriminately, as in India, completely black hens are common, but completely black cocks are rarely, if ever, seen. This indicates an association between blackness and femininity.
Those which appear in the same manner in both sexes. The great majority of mutations appear to be of this kind.
Lastly, those that appear in both sexes but take a different form in the case of the two sexes; thus in cats a mutation has given rise to sandy males and tortoise-shell females. The mutation which has produced the black-winged peacock shows itself in the form of a black wing in the cock, while it causes the plumage of the hen to be grizzly white.
We shall deal with the phenomenon of correlation at some length in the next chapter. It is a subject to which sufficient attention has not been paid. Even as certain characters are correlated in certain species, so in some cases are certain characters correlated with sex.
Why this should be so we are not in a position to say; this, however, does not affect the indisputable fact that such correlation does exist.
Physicians in the course of their practice sometimes come across very curious cases of correlation in human beings.
Unilateral Transmission
“It is,” writes Thomson (Heredity, p. 290), “an interesting fact that an abnormal element in the inheritance may find expression in the males only or in the females only. If we could understand this we should be nearer understanding what sex really means.
“Hæmophilia, or a tendency to bleeding, is a heritable abnormality, partly associated with weakness in the blood-vessels, which do not contract as they should and are apt to break, and partly connected with a lack of coagulating power in the blood. It is usually confined to males. But as it passes from a father through a daughter to a grandson, and so on, it must be a latent part of the germinal inheritance of the females, though for some obscure physiological reason it fails to find expression in them, or has its expression quite disguised. Colour-blindness or Daltonism has been recorded (Horner) through the males only of seven generations. Dejerine cites another case (fide Appenzeller) in which all the males in a family history had cataract through four generations. There are other instances of what is sometimes awkwardly called the unilateral transmission of abnormal qualities. Edward Lambert, born in 1717, is said to have been covered with ‘spines.’ His children showed the same peculiarity, which began to be manifest from the sixth to the ninth month after birth. One of his children grew up and handed on the peculiarity to another generation. Indeed, it is said to have persisted for five generations, and in the males only—unilateral transmission.”
In our view, these abnormalities are of such a kind that they are only possible in connection with the male organ; in other words, they are mutations of the first of the four kinds cited above—those which appear only in connection with the male organ.
It is a curious fact that the general rule in nature seems to be that the male is ahead of the female in the course of evolution. The sexes may be alike at a given period in the life-history of the species. Presently a mutation appears which is confined to the male alone; thus arises the phenomenon of sexual dimorphism. The next step in the evolution of the species is frequently a mutation on the part of the female which brings her once again into line with the male, and so the sexual dimorphism disappears, for a time at any rate. A good example of this is furnished by the sparrows; in the common sparrow of a large part of Africa (Passer swainsoni) both sexes are very plain, like the hen of the house-sparrow; in this species (P. domesticus) as every one knows, the cock, though by no means brilliant, is noticeably handsomer than his mate; while in the Tree-sparrow (P. montanus) both sexes have a plumage of masculine type, much like that of the cock house-sparrow.
If we consider in conjunction with one another the various facts we have cited above, we begin to grasp the nature of the phenomena of sexual dimorphism.
Let us consider an imaginary case of a defenceless little bird which builds an open nest. Let us suppose that it is inconspicuously plumaged. Now suppose that a mutation of the first kind shows itself, a mutation which affects the cock only and makes him more conspicuous. Let us further suppose that the cock does not share in the duties of incubation. It is quite possible that, in spite of this apparently unfavourable mutation, the species may survive, for, as we have seen, it does not affect the hen, and she, since she alone incubates, stands the most in need of protective colouring. Moreover, as Stolzmann has suggested, the species can possibly afford to lose a few males. But suppose that both cock and hen share in the duties of incubation, it is then quite likely that the mutation will cause the species to become extinct, by the elimination of all the males. Or, let us suppose that the mutation in the direction of showy plumage affects both sexes, then in such a case the species will almost certainly become extinct. If, however, the hypothetical species nested in holes in trees, it is quite possible that it might survive notwithstanding its showy plumage.
Greater Value of Females
Whether, as Wallace suggests, the hen does most of the incubating, and is exposed to special danger when sitting on her eggs in an open nest, or, as Stolzmann urges, it is of advantage to the species that there should not be too many males, the result is the same, that the species can afford to allow the cock to be more gaily attired than the hen. In either case the colouration of the cock becomes a matter of comparatively little importance to the species, and this, coupled with the fact that the male tends to mutate more readily than the female, will explain why, in most species which exhibit sexual dimorphism, it is the cocks that are the more conspicuous. In certain species the cocks alone incubate, and these then become more important than the females to the race, so that they have not been permitted to become showy, while the hens have been allowed more freedom in this respect. The extreme variability of the Ruff (Pavoncella pugnax) in breeding plumage points to the fact that his colour is a matter of comparative indifference to the species; in consequence plenty of latitude is allowed to his tendency to vary.
Our view, then, is that evolution proceeds by mutations, which may be large or small.
The mutation is the result of a rearrangement in part or parts of the fertilised egg, and this rearrangement shows itself in the adult organism as a change in one or more of its characteristics. The mutation may be correlated with only one of the sexual organs, and when this is the case, it gives rise to the phenomenon of sexual dimorphism. The appearance in the adult of certain, if not of all, characteristics is affected by causes other than the nature of the biological molecules from which they are derived. The tendency to develop in a certain direction is there, but something else, such as the secretion of hormones from the sexual cells, is frequently necessary to enable a given tendency to fully develop itself. Thus it is that castration often affects the bodily appearance of those animals operated on. When a mutation appears, natural selection decides whether or not it shall persist.
CHAPTER VIII
THE FACTORS OF EVOLUTION
Variation along definite lines and Natural Selection are undoubtedly important factors of evolution—Whether or not sexual selection is a factor we are not yet in a position to decide—Modus operandi of Natural Selection—Correlation an important factor—Examples of correlation—Correlation is a subject that requires close study—Isolation a factor in evolution—Discriminate isolation—Indiscriminate isolation—Is the latter a factor?—Romanes’ views—Criticism of these—Indiscriminate isolation shown to be a factor—Summary of the methods in which new species arise—Natural Selection does not make species—It merely decides which of certain ready-made forms shall survive—Natural Selection compared to a competitive examination and to a medical board—We are yet in darkness as to the fundamental causes of the Origin of Species—In experiment and observation rather than speculation lies the hope of discovering the nature of these causes.
We have so far considered three factors of evolution. The first of these is the tendency of organisms to vary along definite lines. This is a most important factor, because, unless variation occurs in any given direction, there can be no evolution in that direction. Variations are the materials upon which the other factors, or causes, of evolution work. The second great factor is natural selection. Natural selection may be compared to a builder, and variations to his materials. The kind of building that a builder can construct depends very largely on the material supplied to him. The Forth Bridge could not have been built had those who constructed it had no material given them but bricks and mortar. Wallaceians regard natural selection as a builder who is supplied with every kind of building material—stone, bricks, wood, iron, aluminium, in any quantities he may desire. They therefore regard natural selection as the one and only cause which determines evolution. This, however, is a wrong idea. Natural selection should rather be likened to a builder who is supplied with a limited variety of building materials, so that considerable restrictions are imposed on his building operations. The doors, windows, fireplaces, etc., are supplied to him ready-made. He merely selects which of these he will use for each building.
The third factor of evolution which we have considered is sexual selection. As we have seen, sufficient attention has not been paid to this subject, so that we are not yet in a position to say how much, if any, influence it has exercised on the course of evolution.
The Struggle for Existence
In addition to these three factors, there are, we believe, some others. Before proceeding to a consideration of these, it is important to study carefully the modus operandi of natural selection, or, in other words, the nature of the struggle for existence, as many of the statements contained in recent books on evolution seem to us to be based upon a mistaken conception of this important factor.
As usual, Darwin’s disciples have failed to improve upon the account he gave of the nature of the struggle for existence. This is set forth in Chapter III. of the Origin of Species.
“The causes,” writes Darwin (new edition, p. 83), “which check the natural tendency of each species to increase in number are most obscure. Look at the most vigorous species; by as much as it swarms in numbers, by so much will it tend to increase still further. We know not exactly what the checks are even in a single instance.” This is perfectly true. Nevertheless elaborate theories of protective and warning colouration and mimicry have been built up on the tacit assumption that the checks to the multiplication of all, or nearly all, species are the creatures which prey upon them. Possibly no Wallaceian asserts this in so many words, but it is a logical deduction from the excessive prominence each one gives to the various theories of animal colouration; for, if the chief foes of an organism are not the creatures which prey upon it, how can the particular shade and pattern of its coat be of such paramount importance to it?
Checks on Increase
We shall endeavour to show that there are checks on the increase of a species far more potent than the devastation caused by those creatures which feed upon it. Let us, however, first briefly set forth some of the checks on the multiplication of organisms which Darwin mentions in the Origin of Species.
“Eggs, or very young animals,” he says, “seem generally to suffer the most, but this is not invariably the case.” This is, as we have already insisted, a most important point to be borne in mind, especially when considering the various current theories of animal colouration. When once the average animal has become adult its chances of survival are enormously increased.
A second check mentioned by Darwin is the limitation of food supply. “The amount of food for each species,” he writes (p. 84), “of course gives the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species. Thus there seems to be little doubt that the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin. . . . On the other hand, in some cases, as with the elephant and rhinoceros, none are destroyed by beasts of prey.”
We are inclined to think that neither the food limit nor the beasts of prey are a very important check on the multiplication of organisms. The lion, for example, was never so numerous as to reach the limit of its food supply. Before the white man obtained a foothold in Africa vast herds of herbivores were to be seen in those districts where lions were most plentiful. This is a most important fact, for, if the numbers of a species are not determined by those of the animals that prey upon it, the particular colour of an organism is probably not of any direct importance to it. This cuts away the foundation of some of the generally accepted theories of animal colouration.
“Climate,” writes Darwin (p. 84), “plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought seem to be the most effective of all checks. I estimated (chiefly from the greatly reduced numbers of nests in the spring) that the winter of 1854-55 destroyed four-fifths of the birds in my own grounds, and this is a tremendous destruction when we remember that 10 per cent. is an extraordinarily severe mortality from epidemics with man.”
In our opinion, Darwin did not lay nearly enough stress upon the importance of climate as a check on the increase of species. We have seen that he stated his belief that it is the most effective of all checks. But even this is not a sufficiently strong statement of the case. It seems to us that before this check all other checks pale into insignificance.
Darwin failed to notice the potent effects of damp. Damp is more injurious to most species than even cold or drought, as every one who has tried to keep birds in England knows. All entomologists are aware how harmful damp is to insects. Caterpillars seem to take cover under leaves to avoid damp rather than to hide themselves from birds, since these make a point, when searching for insects, of invariably looking carefully under leaves.
It is a well-known fact that a wet winter in England causes much mortality among rabbits. The increase of the rabbit in Australia is usually attributed to the fact that the little rodent has not so many predatory creatures to contend with there as it has in Europe. This is not so. In Australia the rabbit has to fight against eagles, other large birds of prey, carnivorous marsupials, feral cats, monitor lizards and large snakes, to say nothing of the well-organised and persistent attacks of man.
Were predacious creatures the most important foes of the rabbit it would never have obtained a firm foothold in Australia. Damp appears to be its chief enemy. In Australia this does not exist. Hence the remarkable increase of the species. Stronger evidence it would not be possible to advance of the potency of damp as a check on the increase of a species and of the comparative powerlessness of the attacks of raptorial creatures.
The failure of the sandgrouse to establish a footing in England is, we believe, due to the fact that it is constitutionally unfitted to withstand our damp climate.
The camel is an animal that revels in dry habitats, hence the difficulty of keeping camels in damp Bengal, although they seem to thrive well enough in the drier parts of India.
“When a species,” writes Darwin (p. 86), “owing to highly favourable circumstances, increases inordinately in numbers in a small tract, epidemics—at least, this seems generally to occur with our game animals—often ensue; and here we have a limiting check independent of the struggle for life. But even some of these so-called epidemics appear to be due to parasitic worms, which have from some cause, possibly in part through facility of diffusion amongst the crowded animals, been disproportionately favoured: and here comes in a sort of struggle between the parasite and its prey.”
Thus inadequately does Darwin deal with that bar to the increase of organisms, which is only second in importance to the effect of climate. The check occasioned by disease and parasites is one to which naturalists have as yet paid but little attention. The result is a very general misunderstanding of the true nature of the struggle for existence, in other words, of the modus operandi of natural selection.
The tsetse-fly in Africa is a far more important check on the increase of some animals than the lions and other beasts of prey. There are in that continent large tracts of country, known as tsetse-fly belts, in which neither horse, nor ox, nor dog can exist. If races of these animals were to arise which could withstand the bite of the tsetse-fly, these species might increase more rapidly than the rabbit in Australia has done, nor would it matter if the creatures in question were bright crimson, or any other conspicuous colour.
Take the case of the lion in Africa. The chief bar to the increase in numbers of this species appears to be the teething troubles to which the whelps are liable. Now suppose that a mutation were to occur in the lion. Suppose that several members of a litter were all bright blue, and that these suffered from no teething troubles. They would probably all grow up, and although at some disadvantage as hunters on account of their conspicuous colouring, they would nevertheless probably increase at the expense of the normally coloured lions, because of the immunity of their offspring from death from teething troubles. Zoologists would then be at a loss to explain their bright colouring. We should have all manner of ingenious suggestions raised, namely, that in the moonlight these creatures were really not at all conspicuous, indeed that they were obliteratively coloured. In other words, a totally wrong explanation of their colouring would be given and accepted. It is our belief that many of the explanations put forward and accepted of the colouration of existing species are wide of the mark.
As all bee-keepers are aware, the disease known as foul-brood works more havoc among their bees than all the insectivorous creatures put together.
Similarly throat disease among wood-pigeons does more towards keeping their numbers down than all the efforts of predacious birds.
A check on multiplication not mentioned by Darwin is that which is sometimes imposed by the individuals of the species on one another. Thus, in some animals, as, for example, the hyæna, the male occasionally devours his own young ones.
A check of a similar nature results from the habit which the Indian House Crow (Corvus splendens) has of interrupting the pairing operations of its neighbours.
Attributes of Successful Species
We are now in a position to sum up briefly the more important requisites for success in the struggle for existence.
These are not so much specialised structure as courage, a good constitution, mental capacity and prolificacy.
Few animals possess all these characteristics in a pre-eminent degree, for, to use the words of Mr Thompson Seton, “Every animal has some strong point or it could not live, and some weak point or the other animals could not live.” Courage may be of two kinds—active courage, like that of the Englishman, or passive courage, like that of the Jew.
As D. Dewar has said: In the struggle for existence, “An ounce of good solid pugnacity is worth many pounds of protective colouration.”
It is of course possible for an animal to possess too much courage. An excessive amount of courage will often cause a creature to fight unnecessary battles, which may lead to its premature death. This is perhaps the reason why the pugnacious black form of the leopard is not more numerous.
Under a good constitution we must include the power of resisting the rigours of climate, more especially damp, the ability to resist disease, and the enjoyment of a good digestion. When from any cause the normal food of a species becomes scarce, the members of that species will have to starve or supplement the normal diet with food of an unusual nature; and those that are endowed with a good digestion will be able to digest the new food and thus survive, while those which cannot assimilate food to which they are unaccustomed will become emaciated and perish. We see this in every hard winter in England, when the redwing, which, unlike other thrushes, cannot thrive on berries, is the first to die. Most of the more successful birds—the crows and gulls, for example—are omnivorous—that is to say, they are able to digest all manner of food.
Under mental capacity, we would include cunning and sufficient intelligence to adapt oneself to changed conditions. It is largely through man’s superior mental capacity that he has become the dominant species. It is true that he displays also courage and a good constitution, being able to adapt himself to life under the most diverse conditions; but this is, of course, in part due to his mental capacity, which enables him to some extent to adapt his environment to himself.
The advantages of prolificacy are so apparent that it is unnecessary to dilate upon them. Nearly as important as excessive fertility is the ability on the part of the parents to look after their young ones.
Every successful species possesses in a special degree at least one of the above attributes. It is interesting to take in turn the various species which are most widely distributed and consider to what extent they possess these several qualities.
Let us now consider a factor in evolution which is nearly as important as natural selection itself—we allude to the phenomenon of correlation.
Correlation
We may define correlation as the interdependence of two or more characters. This phenomenon is far more common than the majority of naturalists seem to think. It very frequently happens that one particular character never appears in an organism without being accompanied by some other character which we should not expect to be in any way related to it.
Darwin called attention to this phenomenon. “In monstrosities,” he writes, on page 13 of the Origin of Species (new edition), “the correlations between quite different parts are very curious, and many interesting instances are given in Isidore Geoffroy St Hilaire’s great work on this subject. Breeders believe that long limbs are almost always accompanied by an elongated head. Some instances of correlation are quite whimsical: thus cats which are entirely white and have blue eyes are generally deaf; but it has been lately stated by Mr Tait that this is confined to the males.
“Colour and constitutional peculiarities go together, of which many remarkable cases could be given among animals and plants. From the facts collected by Heusinger, it appears that white sheep and pigs are injured by certain plants, whilst dark-coloured individuals escape. Professor Wyman has recently communicated to me a good illustration of this fact: on asking some farmers in Virginia how it was that all their pigs were black, they informed him that the pigs ate the paint-root (Lachnanthes), which coloured their bones pink, and which caused the hoofs of all but the black varieties to drop off; and one of the ‘crackers’ (i.e. Virginia squatters) added, “we select the black members of a litter for raising, as they alone have a good chance of living.’
“Hairless dogs have imperfect teeth; long-haired and coarse-haired animals are apt to have, as is asserted, long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks large feet.
“Hence, if man goes on selecting, and thus augmenting, any peculiarity, he will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of the correlation of growth.”
The great importance of the principle of the correlation of organs is, that natural selection may indirectly cause the survival of unfavourable variations, or of variations which are of no utility to the organism, because they happen to be correlated with organs or structures that are useful.
Physiologists insist more and more upon the close interdependence of the various parts of the organism. All recent researches tend to show that each of the organs has, besides its primary function, a number of subordinate duties to perform, and that the removal of one organ reacts on all the others.
In face of these facts we should have expected those zoologists who have followed Darwin to have paid very close attention to the subject of correlation. As a matter of fact, the phenomenon seems to have been almost completely neglected. This is an example of the manner in which the superficial theories which to-day command wide acceptance have tended to bar the way to research.
There seems to be, in the case of some organisms, at any rate, a distinct correlation between their colouring and their constitution or mental characters. For example, the black forms of the cobra, the leopard, and the jaguar are notoriously bad-tempered.
“There is,” writes Col. Cunningham, on p. 344 of Some Indian Friends and Acquaintances, “much variation in the temper of different varieties of cobras, and, as is often so noticeable among other sorts of animals, there would seem to be a distinct correlation between darkness of colour and badness of temper. It is probably in part owing to a recognition of this that the cobras ordinarily seen in the hands of the so-called snake charmers are of a very light colour, although the choice may also be to some extent of æsthetic origin, seeing that the paler varieties are specially ornamental, due to the brilliancy of their markings and the great development of their hoods.” It would thus appear that there is also a correlation between the colour of the cobra and the size of its hood.
Hesketh Pritchard informs us, in Through the Heart of Patagonia, that the Gauchos assert that a “picaso” colt—that is to say, a black one with white points—is the reverse of docile. Similarly, black mice are said to be very hard to tame.
We have already called attention to the importance of courage and the power of resisting the rigours of climate in the struggle for existence. It is apparently because black is so frequently correlated with courage that it is seen comparatively often in nature, in spite of the fact that it is a very bad colour as regards protection from enemies. Those birds and beasts which are black are usually thriving species. The dominance of the crow tribe is a case in point. Crows, it is true, are not really courageous, but they are dangerous owing to their gregarious habits, and are dreaded by other creatures on account of their power of combination. In Birds of the Plains, D. Dewar records an instance of a number of crows killing in revenge so powerful a bird as the kite.
Since very many species seem to throw off melanistic variations, it may perhaps be asked, How is it that more black species do not exist?
The reply is twofold. In the first place, it is quite likely that in some organisms black variations are not correlated with courage or extreme pugnacity, and when such is the case the melanistic varieties will be more likely to be exterminated by foes, on account of their conspicuousness. It must be remembered that, other things being equal, the inconspicuously coloured organism has a better chance of survival than the showily coloured one. This is, of course, a very different attitude from that which insists on the all-importance to animals of protective colouration. Secondly, it is not difficult to see how too much courage may be fatal to an animal in leading it to take risks which a more timid creature would refrain from doing. This, as we have already suggested, is probably the reason why the black panther is so scarce. The black colour is readily inherited, so there must be some cause which tends to kill off the black varieties of the panther.
Lest it be thought the idea that excessive courage and pugnacity are harmful is mere fancy, let us quote from the account of the nesting habits of the White-rumped Swallow (Tachycineta leucorrhoa) given by Mr W. H. Hudson on p. 32 of Argentine Ornithology. He says that no matter how many nesting sites are available, there is always much fighting amongst these birds for the best places. “Most vindictively,” he writes, “do the little things clutch each other, and fall to the earth twenty times an hour, where they often remain struggling for a long time, heedless of the screams of alarm their fellows set up above them; for often, while they thus lie on the ground punishing each other, they fall an easy prey to some wily pussy who has made herself acquainted with their habits.”
We have already emphasised the importance to many species of possessing the power of resisting the effects of damp. In the case of some organisms favourable variations in this direction may possess a greater survival value than those in the shape of greater speed or physical strength.
Now, if there be any correlation between the power of resisting damp and the colour an animal bears, it is quite probable that animals of this colour, whether or no it be conspicuous, are likely to survive in preference to those who are more protectively coloured. There is some evidence that in certain cases, at any rate, resistance to climate is correlated with colour peculiarities. For example, some fanciers assert that yellow-legged poultry resist cold and damp better than those whose legs are not yellow. Fowls which have yellow legs have also yellow skins. In this connection the almost universal assumption of orange feet by domestic guinea-fowls is significant. Normally the feet of these birds are black, and their natural African habitat is a dry one.
A grey or white colour appears to be correlated with resistance to cold. In birds this may perhaps be explained by the fact that the feathers in some light-coloured varieties are longer than in those of normally-coloured ones. Thus mealy-coloured canaries have longer feathers than brightly-coloured ones.
The Arctic Skua, having no enemies to fear, stands in no need of protective colouration. It would therefore seem that the white-breasted form of this bird becomes more numerous as it nears the north pole, not because of the closer assimilation of its plumage to the colour of the snowy surroundings, but because the bird has to resist the greater degree of cold the farther north it finds itself. Similarly, in the region of the south pole the albino form of the Giant Petrel (Ossifraga gigantea) becomes common. Both these birds are themselves predatory and not liable to be preyed upon.
The curious china-white legs of some desert birds—as, for example, coursers and larks—would seem to indicate a power of resisting the hot rays radiating from the sand on which these creatures dwell.
White quills do not wear well either in domestic birds or in wild albinos. This may explain why it is that when a white wild species of bird has any black in its plumage the black is almost invariably on the tips of the wings.
White quill-feathers are one of the commonest variations observed in domesticated birds, nevertheless they are as rare as complete whiteness among birds in their natural state.
A chestnut or bay colour in mammals appears to be correlated with a high rate of speed, as in the thoroughbred horse. This perhaps explains why so many of the swiftest species of antelope, such as the hartebeests and sassaby (Damaliscus lunatus), are chestnut bay in colour. It is further a remarkable fact that in the Black-buck (Antilope cervicapra) and the Nilgai (Boselaphus tragocamelus) the females, which are faster than the males, are not black or grey like their respective males, but reddish.
Wild turkeys are bronze; tame ones are black more often than any other colour. This may be due to the fact that in them nigritude is correlated with the power to resist damp. Among human beings those races which live in very swampy districts are often intensely black.
It is a significant fact that those domestic animals which are bred for speed or for fighting purposes do not assume all the varied hues that characterise those that are allowed to breed indiscriminately. Racehorses, greyhounds, and homing pigeons furnish examples of this. Even more remarkable is the case of the Indian Aseel or game-cock. This is bred purely for fighting purposes, and is required to display extraordinary powers of endurance, since the spurs are cut off in order to prolong the fight. Thus it is that this Indian race of game-cocks shows little variation when compared with the English breed, which fights in a more natural manner. The hens of the Indian form seem never to show the colouration of the wild jungle fowl, although the cocks may do so. It would appear that hens having the colouration of their wild ancestors cannot breed cocks possessed of the requisite courage. The Aseel is said to be of the highest courage only when the legs, beak and iris are white.
There is, we believe, not the least doubt that many other connections between colour and various characteristics have yet to be discovered. It is high time that competent naturalists paid attention to this subject. A study of the question will almost certainly throw much light upon many phenomena of animal colouration which hitherto have not been satisfactorily explained. It is quite likely that the sandy hue displayed by birds and beasts which frequent desert regions may be due to a correlation with the power of withstanding intense dry heat rather than to its rendering them inconspicuous to their foes.
As other examples of correlation we may cite the correlation which seems to obtain between short canine teeth and the absence of a hairy covering to the body. This phenomenon is observed both in men and pigs. Hairless dogs almost invariably have their teeth but poorly developed.
Darwin called attention to the connection between a short beak and small feet in pigeons; we see the same phenomenon in the dwarf breed of ducks known as call-ducks.
A curious correlation exists between fowls’ eggs with brown shells and the incubating habit. Fanciers have long tried in vain to produce a hen that lays brown eggs without becoming “broody” at certain seasons.
Among fowls, long legs are invariably correlated with a short tail, as is well seen in the Malay breed. This correlation may explain the short tails of wading birds. Short-legged fowls, like Japanese bantams, have long tails, and it is significant that the short-legged Weka Rails (Ocydromus) of New Zealand have unusually long tails for the family. In this connection we may say that the tail-like plumes of the cranes are not tail-feathers, but the tertiary feathers of the wings. As egrets also have long trains of plumes growing from the back, it cannot be said that the short tail of the vast majority of the waders is due to the fact that these birds would be at a disadvantage were their caudal feathers long.
Isolation
Isolation is a most important factor in the making of species. It is a factor to which Darwin failed to attach sufficient importance, and one which has been to a large extent neglected by Wallaceians.
Divergence of Character
We have seen how a species can be improved or changed by natural selection. All those individuals which have varied in a favourable direction have been preserved, and allowed to leave behind them offspring that inherit their peculiarities, while those which have not so varied have perished without leaving behind any descendants. Thus the nature of the species has changed. The old type has given place to a new one. Instead of species A, species B exists. This is what Romanes has called monotypic evolution—the transformation of one species into another species. But any theory of the origin of species must be able to answer the question, Why have species multiplied? How is it that species A has given rise to species B, C, and D, or, while itself continuing to exist, has thrown off sister species B and C? How is it that in the course of evolution, species have not been transmuted in linear series instead of ramifying into branches? This ramification of a species into branches has been termed by Romanes polytypic evolution. It is easy to see how natural selection can bring about monotypic evolution, but how can it have effected polytypic evolution? To use Darwin’s phraseology, how is it that divergence of character has come about? Darwin’s reply to this question is (Origin of Species, p. 136), “from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.
“We can clearly discern this in the case of animals with simple habits. Take the case of a carnivorous quadruped, of which the number that can be supported in any country has long ago arrived at its full average. If its natural power of increase be allowed to act, it can succeed in increasing (the country not undergoing any change in its conditions) only by its varying descendants seizing on places at present occupied by other animals: some of them, for instance, being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations, climbing trees, frequenting water, and some perhaps becoming less carnivorous. The more diversified in habits and structure the descendants of our carnivorous animal become, the more places they will be enabled to occupy. What applies to one animal will apply throughout all time to all animals—that is, if they vary—for otherwise natural selection can effect nothing.” Darwin was, therefore, of opinion that natural selection is able to bring about polytypic evolution. Darwin tacitly assumes, in the illustration he gives, that the various races of the carnivorous animal are in some way prevented from intercrossing; for if they interbreed indiscriminately, these races will tend to be obliterated.
Isolation
“That perfectly free intercrossing,” writes Professor Lloyd Morgan (on p. 98 of Animal Life and Intelligence), “between any or all of the individuals of a given group of animals is, so long as the characters of the parents are blended in the offspring, fatal to divergence of character, is undeniable. Through the elimination of less favourable variations, the swiftness, strength, and cunning of a race may be gradually improved. But no form of elimination can possibly differentiate the group into swift, strong, and cunning varieties, distinct from each other, so long as all three varieties freely interbreed, and the characters of the parents blend with the offspring. Elimination may and does give rise to progress in any given group, as a group; it does not and cannot give rise to differentiation and divergence, so long as interbreeding with consequent interblending of characters be freely permitted. Whence it inevitably follows, as a matter of simple logic, that where divergence has occurred, intercrossing and interbreeding must in some way have been lessened or prevented.
“Thus a new factor is introduced, that of isolation or segregation. And there is no questioning the fact that it is of great importance. Its importance, indeed, can only be denied by denying the swamping effects of intercrossing, and such denial implies the tacit assumption that interbreeding and interblending are held in check by some form of segregation. The isolation explicitly denied is implicitly assumed.”
This is very sound criticism, and is not very materially affected by the fact that the intercrossing of varieties does not necessarily imply a blending of their characters in the offspring; for, as we have seen, some characters do not blend. No matter what form inheritance takes, in order that natural selection may cause polytypic evolution it must be assisted by isolation in some form or other.
Thus isolation is an important factor in evolution, though probably not so important as its more extreme advocates would have us believe. Wagner, Romanes, and Gulick have, in insisting upon the importance of the principle of isolation, rendered valuable service to biological science, but, in common with most men having a new theory, they have pushed their conclusions to absurd lengths.
As Romanes has pointed out, isolation may be discriminate or indiscriminate. “If,” he writes, on p. 5 of vol. iii. of Darwin and after Darwin, “a shepherd divides a flock of sheep without regard to their characters, he is isolating one section from the other indiscriminately; but if he places all the white sheep in one field, and all the black sheep in another field, he is isolating one section from the other discriminately. Or, if geological subsidence divides a species into two parts, the isolation will be indiscriminate; but if the separation be due to one of the sections developing, for example, a change of instinct determining migration to another area, or occupation of a different habitat on the same area, then the isolation will be discriminate, so far as the resemblance of instinct is concerned.”
Discriminate Isolation
Other names for indiscriminate isolation are separate breeding and apogamy. Discriminate isolation is also called segregate breeding and homogamy. The human breeder resorts to discriminate isolation in that he separates all those creatures from which he seeks to breed, from those from which he does not wish to breed. Natural selection itself is, therefore, a kind of discriminate isolator, since it isolates the fit by destroying all the unfit, and, inasmuch as it kills off all those creatures which it fails to isolate, it differs from other forms of isolation in preventing the inter-breeding of the unisolated forms and their giving rise to a different race. Thus it is clear that natural selection, unless aided by some other form of isolation, can give effect to only monotypic evolution. This is a point on which Romanes rightly insists strongly.
There are several other forms of discriminate isolation. Sexual selection would be one of these. Suppose, for example, that in any species there are large and small varieties formed, and like tends to breed with like, then the small individuals will breed with other small individuals, while large ones will mate with large ones; thus two races—a large one and a small one—will be evolved side by side, provided, of course, natural selection does not step in and destroy one of them.
Another kind of discriminate isolation may be due to the fact that one variety is ready to pair before the other; thus two races are likely to arise which breed at different seasons. It is unnecessary for us to discourse further on the subject of discriminate isolation; those interested in the subject should read vol. iii. of Darwin and after Darwin, by Romanes.
Indiscriminate Isolation
It is impossible to deny the importance of discriminate isolation as a factor in evolution. On this there can be no room for disagreement among biologists. It is when we come to the subject of indiscriminate isolation that we enter a region of zoological strife.
Is indiscriminate isolation per se a factor of evolution? Romanes, Gulick, and Wagner assert that it is, Wallace and his adherents assert that it is not.
As the burden of proof is on the former, they are entitled to the first hearing.
“We may well be disposed, at first sight,” writes Romanes (Darwin and after Darwin, p. 10), “to conclude that this kind of isolation can count for nothing in the process of evolution. For if the fundamental importance of isolation in the production of organic forms be due to its segregation of like with like, does it not follow that any form of isolation which is indiscriminate must fail to supply the very condition on which all the forms of discriminate isolation depend for their efficacy in the causing of organic evolution? Or, to return to one’s concrete example, is it not self-evident that the farmer who separated his flock into two or more parts indiscriminately, would not effect any more change in his stock than if he had left them all to breed together? Well, although at first sight this seems self-evident, it is, in fact, untrue. For, unless the individuals which are indiscriminately isolated happen to be a very large number, sooner or later their progeny will come to differ from that of the parent type, or unisolated portion of the parent stock. And, of course, as soon as this change of type begins, the isolation ceases to be indiscriminate; the previous apogamy has been converted into homogamy, with the usual result of causing a divergence of type. The reason why progeny of an indiscriminately isolated section of an originally uniform stock—e.g. of a species—will eventually deviate from the original type is, to quote Mr Gulick, as follows:—‘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 as long as the individuals of the two groups are kept from intergenerating.’”
The words of Mr Gulick require close scrutiny. We may admit that “no two portions of a species possess exactly the same average character,” but why should the two, if prevented from interbreeding yet subjected to similar climatic and other conditions, present the phenomenon of “increasing divergence?” The reason assigned by Romanes is the “Law” of Delbœuf, which runs:—“A constant cause of variation, however insignificant it may be, changes the uniformity of type little by little, and diversifies it ad infinitum.” From this “Law” it follows, says Romanes, on p. 13 of vol. iii. Darwin and after Darwin, that “no matter how infinitesimally small the difference may be between the average qualities of an isolated section of a species compared with the average qualities of the rest of that species, if the isolation continues sufficiently long, differentiation of specific type is necessarily bound to ensue.”
This deduction involves two important assumptions. The first is, that in each of the separated portions of the given species there is a constant cause of variation operating in one direction in the case of one portion and in another direction in the case of the other. This assumption is, unfortunately, not founded on fact. If we were to take one hundred race-horses and shut them up in one park and one hundred cart-horses and shut them up in another park, and prevent the interbreeding of the two stocks, we should, if Romanes’s tacit assumption be true, see the two types diverge more and more from one another. We know that as a matter of fact they will tend, generation after generation, to become more like one another. Galton’s Law of Regression, of which we have already spoken, and which is supported by ample evidence, clearly negatives this tacit assumption made by Romanes and Gulick. The second assumption upon which their reasoning is based is that there is no limit to the amount of change which can be effected by the accumulation of fluctuating variations; but, as we have already seen (on p. 70), there is a very definite limit and this limit is quickly reached.
Thus the arguments of Romanes and Gulick are fundamentally unsound.
Mollusca of Sandwich Isles
But the fact remains, and has to be accounted for, that, as a general rule, when two portions of a species are separated, so that they are prevented from interbreeding, they begin to diverge in character, and the longer they remain thus separated the greater becomes that divergence. This is an observed fact which cannot be gainsaid.
It was the observance of this fact which led Gulick to insist with such emphasis on the importance of geographical isolation as a factor in evolution. He discovered that the land mollusca of the Sandwich Islands fall into a great number of varieties.