These islands are very hilly, and Gulick found that each of the varieties is confined not merely to one island, but to one valley. “Moreover,” writes Romanes, on p. 16 ofDarwin and after Darwin, “on tracing this fauna from valley to valley, it is apparent that a slight variation in the occupants of valley 2, as compared with those of the adjacent valley 1, becomes more pronounced in the next, valley 3,still more so in 4, etc., etc. Thus it was possible, as Mr Gulick says, roughly to estimate the amount of divergence between the occupants of any two given valleys by measuring the number of miles between them. . . . The variations which affect scores of species, and themselves eventually run into fully specific distinctions, are all more or less finely graduated as they pass from one isolated region to the next; and they have reference to changes of form or colour, which in no one case presents any appearance of utility.”
Hitherto three different attempts have been made to explain this and allied phenomena:—
1. That it is the result of isolation.
2. That it is the result of natural selection.
3. That it is the result of the action of the environment on the organism.
Let us consider these in inverse order.
In the case of some organisms, more especially plants, invertebrates, and fish, the environment does exert a direct influence on their colouration. But, as we have seen, the changes in colour, etc., thus induced appear never to be transmitted to the offspring of the organisms so affected. They disappear when the offspring are removed to other surroundings.
On the other hand, local races or species—as, for example, the white-cheeked variety of sparrow found in India—usually retain theirexternal appearance when the environment is changed. In the one case the peculiarity is not inherited; in the other it is inherited.
The Wallaceian explanation is, of course, that the phenomenon is the result of natural selection. There must, say Wallace and his followers, be some differences in the environment, differences which we poor human beings cannot perceive, that have caused the divergence between the various isolated sections of the species. In the case of some local species this explanation is probably the correct one, but we have no hesitation in saying that natural selection is unable to offer a satisfactory explanation in a considerable number of instances. Take, for example, the case of the land mollusca of the Sandwich Islands. Mr Gulick worked for fifteen years at them, and states that so far as he is able to ascertain the environment in the fifteen valleys is essentially the same. “To argue,” writes Romanes, on p. 17 of vol. iii. ofDarwin and after Darwin, “that every one of some twenty contiguous valleys in the area of the same small island must necessarily present such differences of environment that all the shells in each are differently modified thereby, while in no one out of the hundreds of cases of modification in minute respects of form and colour can any human being suggest an adaptive reason therefore—to argue thus is merely to affirm an intrinsicallyimprobable dogma in the presence of a great and consistent array of opposing facts.”
Men of science not infrequently charge the clergy with adhering to dogma in face of opposing facts; it seems to us that many of the apostles of science are in this respect worse offenders than the most orthodox of Churchmen.
The example of the mollusca of the Sandwich Islands is by no means a solitary one. D. Dewar cited some interesting cases in a paper recently read before the Royal Society of Arts (p. 103 of vol. lvii. of the Society’s Journal):
“The Indian robins present even greater difficulties to those who profess to pin their faith to the all-sufficiency of natural selection. Robins are found in nearly all parts of India, and fall into two species, the brown-backed (Thamnobia cambaiensis) and the black-backed Indian Robin (Thamnobia fulicata). The former occurs only in Northern India, and the latter is confined to the southern portion of the peninsula. The hen of each species is a sandy brown bird with a patch of brick-red feathers under the tail, so that we cannot tell by merely looking at a hen to which of the two species she belongs. The cock of the South Indian form is, in winter, a glossy black bird, with a white bar in the wing, and the characteristic red patch under the tail. The cock of the northern species, as his name implies, has a sandy-brown back, which contrastsstrongly with the glossy black of his head, neck, and under parts. In summer the cocks of the two species grow more like one another owing to the wearing away of the outer edges of their feathers; but it is always possible to distinguish between them at a glance. The two species meet at about the latitude of Bombay. Oates states that in a certain zone, from Ahmednagar to the mouth of the Godaveri valley, both species occur, and they do not appear to interbreed.
“It seems impossible to maintain that natural selection, acting on minute variations, has brought about the divergence between these two species. Even if it be asserted that the difference in the colour of the feathers of the back of the two cocks is in some way correlated with adaptability to their particular environment, how are we to explain the fact that in a certain zone both species flourish?
“A similar phenomenon is furnished by the red-vented bulbul. This genus falls into several species, each corresponding to a definite locality and differing only in details from the allied species, as, for example, the distance down the neck to which the black of the head extends. There is a Punjab Red-vented Bulbul (Molpastes intermedius), a Bengal (Molpastes bengalensis), a Burmese (Molpastes burmanicus) and a Madras (Molpastes hæmorrhous) species.
“It does not seem possible to maintain the contention that these various species are the products of natural selection, for that would mean if the black of the head of the Punjab species extended further into the neck the bird could not live in that country.”
Thus, natural selection clearly is unable to explain some cases of divergence of character due to geographical isolation.
There remains the third explanation, that the divergence is the result of the simple fact of isolation.
We have already shown how insuperable are the objections to the view held by Romanes and Gulick.
It seems to us that explanation must lie in the fact that mutations occur every now and again in some species. If two portions of a species are separated and a mutation occurs in one portion and not in the other, and if the mutating form succeeds in supplanting the parent form in that isolated portion of the species in which it has appeared, we should have the phenomenon of two races or species differing in appearance although subjected to what appear to be identical environment.
This, of course, is pure conjecture. All that can be said of it at present is that it is not opposed to observed facts. That mutations do occur must be admitted. At present we aretotally in the dark as to what causes them. They arise at the most unexpected times.
In favour of the explanation based on “mutation” there is the interesting fact that geographical isolation does not by any means always cause divergence of character. This Romanes, with great fairness, freely admits. “There are,” he writes, on p. 133 of vol. iii. ofDarwin and after Darwin, “four species of butterflies, belonging to three genera (Lycæna donzelii,L. pheretes,Argynnis pales,Erebia manto), which are identical in the polar regions and the Alps, notwithstanding that the sparse Alpine populations have been presumably separated from their parent stocks since the glacial period.” Again, there are “certain species of fresh-water crustaceans (Apus), the representatives of which are compelled habitually to form small isolated colonies in widely separated ponds, and nevertheless exhibit no divergence of character, although apogamy has probably lasted for centuries.”
To these examples we may add that of the cormorants. These birds have an almost worldwide range. One species—our Cormorant (Phalacrocorax carbo)—occurs in every imaginable kind of environment. Isolation has not effected any changes in the appearance of this species. Yet in New Zealand there exist no fewer than fourteen other species of cormorant. New Zealand is a country where climatic conditionsare comparatively uniform, nevertheless it boasts of no fewer than fifteen out of the thirty-seven known species of cormorant. A possible explanation of this phenomenon may be found in the comparatively easy conditions under which cormorants live in New Zealand.[10]Under such circumstances mutants may be permitted by natural selection to survive, whereas in other parts of the world such mutants have not been able to hold their own.
Prof. Bateson has likened natural selection to a competitive examination to which every organism must submit. The penalty for failure is immediate death. The standard of the examination may vary with the locality.
Isolation, then, is a very important factor in the making of species, for without it, in some form, the multiplication of species is impossible.
Let us, in conclusion, briefly summarise what we now know of the method in which new species are made. We have studied the various factors of evolution—variation and correlation, heredity, natural selection, sexual selection, and the other kinds of isolation. How do these combine to bring new species into being, and to establish the same?
Let us first consider the factor known as natural selection, since this is the one on whichDarwin laid such great stress. Natural selection, although a most important factor in evolution, is not an indispensable one. Evolution is possible without natural selection.
Let us suppose that there is no such thing as natural selection; that the numbers of existing species are kept constant by the elimination of all individuals born in excess of the number required to maintain the species at the existing figure, and that the elimination of the surplus is effected, not by natural selection, but by chance, by the drawing of lots. Under such circumstances there may be evolution, existing species may undergo change, but the evolution will be determined solely by the lines along which variations occur.
If mutations take place along certain fixed lines, and tend to accumulate in the given directions, evolution will proceed along these lines quite independently of the utility to the organism of the mutations that occur. An unfavourable mutation will have precisely the same chance of survival as a favourable one.
If, on the other hand, mutations occur indiscriminately on all sides of the mean, then those mutations which happen to occur most frequently will have the best chance of survival, and they will mark the lines of evolution. But suppose that no mutation occurs more frequently than the others. Under such circumstances therewill be no evolution, unless, by some cause or other, portions of the species are isolated, because in the long run the mutations will neutralise one another.
Let us now suppose that natural selection comes into play. The old method of determining by lot which forms shall persist is replaced by selection on the fixed principle that the fittest shall survive. The mutations appear as before, and as before, of the large number that occur, only a few are permitted to survive. But now the survivors, instead of being a motley crowd, are a selected band, composed of individuals having many characteristics in common—a homogeneous company. Thus one result of natural selection is to accelerate evolution, by weeding out certain classes of individuals and preventing them breeding with those it has selected. On the other hand, natural selection will tend to diminish the number of species which have arisen through mutation, inasmuch as it weeds out many mutants which would have perished had their survival been determined by lot.
From this the kind of work performed by natural selection should be obvious. Natural selection does not make new species. These make themselves, or, rather, originate in accordance with the laws of variation.
“You can,” runs an old proverb, “bring ahorse to the drinking fountain, but you cannot make him drink.” You may be able to bring a child into the world, but you cannot secure its survival. Variation brings into being mutants, which are incipient species, but variation cannot determine their survival. It is at this stage that natural selection steps in.
But because natural selection allows certain mutations to persist, it is not correct to say that natural selection has caused these mutations or made or originated the species to which they give rise.
The Civil Service Commissioners do not make Indian civil servants: they merely determine which of a number of ready-made men shall become civil servants. Similarly, natural selection does not make new species, it simply decides which of a number of ready-made organisms shall survive and establish themselves as new species. Nor does natural selection always do as much as this; for it is not the only determinant of survival. Its position is sometimes comparable to that of the Medical Board which inspects and rejects the physically unfit of the candidates which have already been selected by some other authority.
The examination conducted by natural selection may be compared to a competitive one. A separate, independent examination is held for each particular locality; consequently the severity of the competition will vary with the locality.
In each competition some candidates pass with ease: they gain an unnecessarily high total of marks. So in nature do certain organisms, as, for example, the Leaf-butterflies (Kallimas), appear to be over-adapted to their environment. Other candidates manage to pass only by a very narrow margin: these are paralleled in nature by those species which are barely able to maintain themselves, which become extinct the moment the competition increases in severity.
The great bulk of the candidates fail to obtain sufficient marks to gain a place among the chosen few; these unsuccessful candidates correspond to the mutating forms which perish in the struggle for existence, to those individuals which happen to have mutated in unfavourable directions.
Even as many candidates have acquired knowledge of subjects in which they are not examined, so do many organisms possess characteristics which are of no utility to them in the struggle for existence.
Wallaceians expend much time and energy in misguided attempts to explain the existence of such characters in terms of natural selection.
Nature’s examination, like that held for entrance to the Indian Civil Service, is a liberal one, so that the qualifications of the successful candidates vary considerably. Provided a candidate is able to gain more marks than the other candidates for a vacancy, it matters not in whatsubjects the marks are gained. So is it in nature. Natural selection takes an organism as a whole. One species may have established itself because of its fleetness, a second because of its courage, a third because it has a strong constitution, a fourth because it is protectively coloured, a fifth because it has good digestive powers, and so on.
We thus perceive the part played by natural selection and other forms of isolation in the making of species. It is obvious that these do not make species any more than the Civil Service Commissioners manufacture Indian civil servants.
The real makers of species are the inherent properties of protoplasm and the laws of variation and heredity. These determine the nature of the organism; natural selection and the like factors merely decide for each particular organism whether it shall survive and give rise to a species.
The way in which natural selection does its work is comparatively easy to understand. But this is only the fringe of the territory which we call evolution.
We seem to be tolerably near a solution of the problem of the causes of thesurvivalof any particular mutation. This, however, is merely a side issue. The real problem is the cause of variations and mutations, or, in other words, how speciesoriginate. At present our knowledge of the causes of variation and mutationis practicallynil. We do not even know along what particular lines mutations occur.
We have yet to discover whether one mutation invariably leads to another along the same lines—in other words, whether mutating organisms behave as though they had behind them a force acting in a definite direction. The solution of these problems seems afar off. The hope of solving them lies, not in the speculations in which biologists of to-day are so fond of indulging, but in observation and experiment, especially the last.
The future of biology is largely in the hands of the practical breeder.
[1]The white, pied, and “Japan” individuals are not more different from the type than some variations occurring in wild birds.[2]This short-legged type of dog is sometimes seen among the ownerless and unselected pariah dogs of Indian towns; and a short-legged variety of the fowl may occur sporadically in Zanzibar, where the long-legged Malay is the prevalent breed.[3]“Effected” appears in the earlier editions, but in the later editions has given place to “affected,” probably a printer’s error.[4]Some egrets, such as the rock-egrets (Demiegretta) of eastern tropical coasts, are normally grey, but may be white, and this whiteness may be confined in individuals to the young or adult states.[5]After years of observation of these Indian geese, Finn is convinced they are now, at all events, pure Chinese; it is possible that they really were hybrids in Blyth’s time, but that fresh importations of geese from China, such as still occur, may have ultimately swamped the blood of the common goose. The fertility of the hybrid geese was, however, known to such early writers as Pallas and Linnæus. Darwin himself, at a later date, bred five young from a pair of such hybrids (Nature, Jan. 1, 1880, p. 207).[6]In this chapter we use the word Neo-Darwinism in its usually-accepted sense,i.e.as a name for that which should be called Wallaceism, for the doctrine of the all-sufficiency of natural selection.[7]Animal Colouration, p. 125. A book full of valuable facts and ideas on this most interesting subject.[8]Even these eggs, closely though they resemble in colouring the shingle, etc., on which they are laid, are discovered and eaten by gulls, as Mr A. J. R. Roberts points out inThe Bird Book.[9]Journal of the Bombay Natural History Society, Vol xv. (1903-4), p. 454.[10]Hutton and Drummond record other examples of this in the valuable work entitledThe Animals of New Zealand.
[1]The white, pied, and “Japan” individuals are not more different from the type than some variations occurring in wild birds.
[2]This short-legged type of dog is sometimes seen among the ownerless and unselected pariah dogs of Indian towns; and a short-legged variety of the fowl may occur sporadically in Zanzibar, where the long-legged Malay is the prevalent breed.
[3]“Effected” appears in the earlier editions, but in the later editions has given place to “affected,” probably a printer’s error.
[4]Some egrets, such as the rock-egrets (Demiegretta) of eastern tropical coasts, are normally grey, but may be white, and this whiteness may be confined in individuals to the young or adult states.
[5]After years of observation of these Indian geese, Finn is convinced they are now, at all events, pure Chinese; it is possible that they really were hybrids in Blyth’s time, but that fresh importations of geese from China, such as still occur, may have ultimately swamped the blood of the common goose. The fertility of the hybrid geese was, however, known to such early writers as Pallas and Linnæus. Darwin himself, at a later date, bred five young from a pair of such hybrids (Nature, Jan. 1, 1880, p. 207).
[6]In this chapter we use the word Neo-Darwinism in its usually-accepted sense,i.e.as a name for that which should be called Wallaceism, for the doctrine of the all-sufficiency of natural selection.
[7]Animal Colouration, p. 125. A book full of valuable facts and ideas on this most interesting subject.
[8]Even these eggs, closely though they resemble in colouring the shingle, etc., on which they are laid, are discovered and eaten by gulls, as Mr A. J. R. Roberts points out inThe Bird Book.
[9]Journal of the Bombay Natural History Society, Vol xv. (1903-4), p. 454.
[10]Hutton and Drummond record other examples of this in the valuable work entitledThe Animals of New Zealand.