Volume Ii Part 12 (1/2)
For example, Mr. Allen has studied in detail changes of size and colour among birds and mammals on the American continent; and he finds a wonderfully close sliding scale of both, corresponding stage by stage with gradual changes of climate. Very reasonably he attributes this to the direct influence of climatic conditions, without reference to natural selection--as does also Mr. Gould with reference to similar facts which he has observed among the birds of Australia. Against this view Mr. Wallace urges, ”that the effects are due to the greater or less need of protection.” But it is difficult to believe that such can be the case where so innumerable a mult.i.tude of widely different species are concerned--presenting so many diverse habits, as well as so many distinct habitats. Moreover, the explanation seems incompatible with the _graduated_ nature of the change, and also with the fact that not only colouration but size, is implicated.
We meet with a.n.a.logous facts in b.u.t.terflies. Thus _Lycaena agestis_ not only presents seasonal variations, (A) and (B); but while (A) and (B) are respectively the winter and summer forms in Germany, (B) and (C) are the corresponding forms in Italy. Therefore, (B) is in Germany the summer form, and in Italy the winter form--the German winter form (A) being absent in Italy, while the Italian summer form (C) is absent in Germany. Probably these facts are due to differences of temperature in the two countries, for experiments have shown that when pupae of sundry species of moths and b.u.t.terflies are exposed to different degrees of temperature, the most wonderful changes of colour may result in the insects which emerge. The remarkable experiments of Dorfmeister and Weismann in relation to this subject are well known. More recently Mr.
Merrifield has added to their facts, and concludes that the action of cold upon the pupae--and also, apparently, upon the larvae--has a tendency to produce dark hues in the perfect insect[110].
[110] _Trans. Entom. Soc._ 1889, part i. p. 79 _et seq._
But, pa.s.sing now from such facts of climatic variations over wide areas to similar facts within small areas, in an important _Memoir on the Cave Fauna of North America_, published a few years ago by the American Academy of Sciences, it is stated:--
”As regards change of colour, we do not recall an exception to the general rule that all cave animals are either colourless or nearly white, or, as in the case of Arachnida and Insects, much paler than their out-of-door relatives.”
Now, when we remember that these cave faunas comprise representatives of nearly all cla.s.ses of the animal kingdom, it becomes difficult, if not impossible, to imagine that so universal a discharge of colouring can be due to natural selection. It must be admitted that the only way in which natural selection could act in this case would be indirectly through the principle of correlation. There being no light in the caves, it can be of no advantage to the animals concerned that they should lose their colour for the sake of protection, or for any other reason of a similarly direct kind. Therefore, if the loss of colour is to be ascribed to natural selection, this can only be done by supposing that natural selection has here acted indirectly through the principle of correlation. There is evidence to show that elsewhere modification or loss of colour is in some cases brought about by natural selection, on account of the original colour being correlated with certain physiological characters (such as liability to particular diseases, &c.); so that when natural selection operates directly upon these physiological characters, it thereby also operates indirectly upon the correlated colours. But to suppose that this can be the explanation of the uniform diminution of colour in all inhabitants of dark caves would be manifestly absurd. If there were only one cla.s.s of animals in these caves, such as Insects, it might be possible to surmise that their change of colour is due to natural selection acting directly upon their physiological const.i.tutions, and so indirectly upon their colours. But it would be absurd to suppose that such can be the explanation of the facts, when these extend in so similar a manner over so many scores of species belonging to such different types of animal life.
With more plausibility it might be held that the universal discharge of colour in these cave-faunas is due, not to the presence, but to the absence of selection--i. e. to the cessation of selection, or panmixia.
But against this--at all events as a full or general explanation--lie the following facts. First, in the case of Proteus--which has often been kept for the purposes of exhibition &c., in tanks--the skin becomes dark when the animal is removed from the cave and kept in the light.
Secondly, deep-sea faunas, though as much exposed as the cave-faunas, to the condition of darkness, are not by any means invariably colourless.
On the contrary, they frequently present brilliant colouration. Thus it is evident that if panmixia be suggested in explanation of the discharge of colouring in cave-faunas, the continuance of colour in deep-sea faunas appears to show the explanation insufficient. Thirdly, according to my view of the action of panmixia as previously explained, no _total_ discharge of colouration is likely to be caused by such action alone. At most the bleaching as a result of the mere withdrawal of selection would proceed only to some comparatively small extent.
Fourthly, Mr. Packard in the elaborate _Memoir on Cave Fauna_, already alluded to, states that in some of the cases the phenomena of bleaching appear to have been induced within very recent times--if not, indeed, within the limits of a single generation. Should the evidence in support of this opinion prove trustworthy, of course in itself it disposes of any suggestion either of the presence or the absence of natural selection as concerned in the process.
Nevertheless, I myself think it inevitable that to some extent the cessation of selection must have helped in discharging the colour of cave faunas; although for the reasons now given it appears to me that the main causes of change must have been of that direct order which we understand by the term climatic.
As regards dogs, the Rev. E. Everest found it impossible to breed Scotch setters in India true to their type. Even in the second generation no single young dog resembled its parents either in form or shape. ”Their nostrils were more contracted, their noses more pointed, their size inferior, and their limbs more slender[111].” Similarly on the coast of New Guinea, Bosman says that imported breeds of dogs ”alter strangely; their ears grow long and stiff like those of foxes, to which colour they also incline ... and in three or four broods their barking turns into a howl[112].”
[111] _Variation_, &c. vol. i. p. 40.
[112] _Variation_, &c. vol. i. p. 40.
Darwin gives numerous facts showing the effects of climate on horses, cattle, and sheep, in altering, more or less considerably, the characters of their ancestral stocks. He also gives the following remarkable case with regard to the rabbit. Early in the fifteenth century a common rabbit and her young ones were turned out on the island of Porto Santo, near Madeira. The feral progeny now differ in many respects from their parent stock. They are only about one-third of the weight, present many differences in the relative sizes of different parts, and have greatly changed in colour. In particular, the black on the upper surface of the tail and tips of the ears, which is so constant in all other wild rabbits of the world as to be given in most works as a specific character, has entirely disappeared. Again, ”the throat and certain parts of the under surface, instead of being pure white, are generally grey or leaden colour,” while the upper surface of the whole body is redder than in the common rabbit. Now, what answer have our opponents to make to such a case as this? Presumably they will answer that the case simply proves the action of natural selection during the best part of 400 years on an isolated section of a species. Although we cannot say of what use all these changes have been to the rabbits presenting them, nevertheless we _must_ believe that they have been produced by natural selection, and therefore _must_ present some hidden use to the isolated colony of rabbits thus peculiarly situated. Four centuries is long enough to admit of natural selection effecting all these changes in the case of so rapidly breeding an animal as the rabbit, and therefore it is needless to look further for any explanation of the facts. Such, I say, is presumably the answer that would be given by the upholders of natural selection as the only possible cause of specific change. But now, in this particular case it so happens that the answer admits of being conclusively negatived, by showing that the great a.s.sumption on which it reposes is demonstrably false. For Darwin examined two living specimens of these rabbits which had recently been sent from Porto Santo to the Zoological Gardens, and found them coloured as just described. Four years afterwards the dead body of one of them was sent to him, and then he found that the following changes had taken place. ”The ears were plainly edged, and the upper surface of the tail was covered with blackish-grey fur, and the whole body was much less red; so that under the English climate this individual rabbit has recovered the proper colour of its fur in rather less than four years!”
Mr. Darwin adds:--
”If the history of these Porto Santo rabbits had not been known, most naturalists, on observing their much reduced size, their colour, reddish above and grey beneath, their tails and ears not tipped with black, would have ranked them as a distinct species.
They would have been strongly confirmed in this view by seeing them alive in the Zoological Gardens, and hearing that they refused to couple with other rabbits. Yet this rabbit, which there can be little doubt would thus have been ranked as a distinct species, as certainly originated since the year 1420[113].”
[113] _Variation_, &c. vol. i. p. 120.
Moreover, it certainly originated as a direct result of climatic influences, independent of natural selection; seeing that, as soon as individual members of this apparently new species were restored to their original climate, they recovered their original colouration.
As previously remarked, it is, from the nature of the case, an exceedingly difficult thing to prove in any given instance that natural selection has not been the cause of specific change, and so finally to disprove the a.s.sumption that it must have been. Here, however, on account of historical information, we have a crucial test of the validity of this a.s.sumption, just as we had in the case of the niata cattle; and, just as in their case, the result is definitely and conclusively to overturn the a.s.sumption. If these changes in the Porto Santo rabbits had been due to the gradual influence of natural selection guided by inscrutable utility, it is simply impossible that the same individual animals, in the course of their own individual life-times, should revert to the specific characters of their ancestral stock on being returned to the conditions of their ancestral climate. Therefore, unless any naturalist is prepared to contradict Darwin's statement that the changes in question amount to changes of specific magnitude, he can find no escape from the conclusion that distinctions of specific importance may be brought about by changes of habitat alone, without reference to utility, and therefore independently of natural selection.
II. _Food._
Although, as yet, little is definitely known on the subject, there can be no doubt that in the case of many animals differences of food induce differences of colour within the life-time of individuals, and therefore independently of natural selection.
Thus, sundry definite varieties of the b.u.t.terfly _Euprepia caja_ can be reared according to the different nourishment which is supplied to the caterpillar; and other b.u.t.terflies are also known on whose colouring and markings the food of the caterpillar has great influence[114].
[114] See especially, Koch, _Die Raupen und Schmetterling der Wetterau_, and _Die Schmetterling des Sudwestlichen Deutschlands_, whose very remarkable results of numerous and varied experiments are epitomized by Eimer, _Organic Evolution_, Eng. Trans. pp. 147-153; also Poulton, _Trans.