Darwin, and After Darwin Volume Ii Part 12

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.

Entom. Soc._ 1893.

Again, I may mention the remarkable case communicated to Darwin by Moritz Wagner, of a species of _Saturnia_, some pupae of which were transported from Texas to Switzerland in 1870. The moths which emerged in the following year were like the normal type in Texas. Their young were supplied with leaves of _Juglans regia_, instead of their natural food, _J. nigra_; and the moths into which these caterpillars changed were so different from their parents, both in form and colour, "that they were reckoned by entomologists as a distinct species[115]."

[115] Mivart, _On Truth_, p. 378.

With regard to mollusks, M. Costa tells us that English oysters, when turned down in the Mediterranean, "_rapidly_ became like the true Mediterranean oyster, altered their manner of growth, and formed prominent diverging rays." This is most probably due to some change of food. So likewise may be the even more remarkable case of _Helix nemoralis_, which was introduced from Europe to Virginia a few years ago. Under the new conditions it varied to such an extent that up to last year no less than 125 varieties had been discovered. Of these 67, or more than half, are new--that is, unknown in the native continent of the species[116].

[116] c.o.c.kerell, _Nature_, vol. xli. p. 393.

In the case of Birds, the Brazilian parrot _Chrysotis festiva_ changes the green in its feathers to red or yellow, if fed on the fat of certain fishes; and the Indian Lori has its splendid colouring preserved by a peculiar kind of food (Wallace). The Bullfinch is well known to turn black when fed on hemp seeds, and the Canary to become red when fed on cayenne pepper (Darwin). Starting from these facts, Dr. Sauermann has recently investigated the subject experimentally; and finds that not only finches, but likewise other birds, such as fowls, and pigeons, are subject to similar variations of colour when fed on cayenne pepper; but in all cases the effect is produced only if the pepper is given to the young birds before their first moult. Moreover, he finds that a moist atmosphere facilitates the change of colour, and that the ruddy hue is discharged under the influence either of sunlight or of cold. Lastly, he has observed that sundry other materials such as glycerine and aniline dyes, produce the same results; so there can be no doubt that organic compounds probably occur in nature which are capable of directly affecting the colours of plumage when eaten by birds. Therefore the presence of such materials in the food-stuffs of birds occupying different areas may very well in many cases determine differences of colouration, which are constant or stable so long as the conditions of their production are maintained.

III. _s.e.xual Selection._

Pa.s.sing on now to causes of specific change which are internal, or comprised within the organisms themselves, we may first consider the case of s.e.xual Selection.

Mr. Wallace rejects the theory of s.e.xual selection _in toto_, and therefore nothing that can be said under this head would be held by him to be relevant. Many naturalists, however, believe that Darwin was right in the large generalization which he published under this t.i.tle; and in so far as any one holds that s.e.xual selection is a true cause of specific modification, he is obliged to believe that innumerable specific characters--especially in birds and mammals--have been produced without reference to utility (other, of course, than utility for s.e.xual purposes), and therefore without reference to natural selection. This is so obvious that I need not pause to dilate upon it. One remark, however, may be useful. Mr. Wallace is able to make a much more effective use of his argument from "necessary instability" when he brings it against the Darwinian doctrine of s.e.xual selection, than he does when he brings it against the equally Darwinian doctrine of specific characters in general not being all necessarily due to natural selection. In the latter case, it will be remembered, he is easily met by showing that the causes of specific change other than natural selection, such as food, climate, &c., may be quite as general, persistent, and uniform, as natural selection itself; and therefore in this connexion Mr. Wallace's argument falls to the ground. But the argument is much more formidable as he brings it to bear against the theory of s.e.xual selection. Here he asks, What is there to guarantee the uniformity and the constancy of feminine taste with regard to small matters of embellishment through thousands of generations, and among animals living on extensive areas? And, as we have seen in Part 1, it is not easy to supply an answer. Therefore this argument from the "necessary instability of character" is of immeasurably greater force as thus applied against Darwin's doctrine of s.e.xual selection, than it is when brought against his doctrine that all specific characters need not necessarily be due to natural selection.

Therefore, also, if any one feels disposed to attach the smallest degree of value to this argument in the latter case, consistency will require him to allow that in the former case it is simply overwhelming, or in itself destructive of the whole theory of s.e.xual selection. And, conversely, if his belief in the theory of s.e.xual selection can survive collision with this objection from instability, he ought not to feel any tremor of contact when the objection is brought to bear against his scepticism regarding the alleged utility of all specific characters. For a.s.suredly no specific character which is apparent to our eyes can be supposed to be so refined and complex (and therefore so presumably inconstant and unstable), as are those minute changes of cerebral structure on which a psychological preference for all the refined shadings and many pigments of a complicated pattern must be held ultimately to depend. For this reason, then, as well as for those previously adduced, if any one agrees with Darwin in holding to the theory of s.e.xual selection notwithstanding this objection from the necessary instability of unuseful embellishments, _a fortiori_ he ought to disregard the objection altogether in its relation to useless specific characters of other kinds.

But quite apart from this consideration, which Mr. Wallace and his followers may very properly say does not apply to them, let us see what they themselves have made of the facts of secondary s.e.xual characters--which, of course, are for the most part specific characters--in relation to the doctrine of utility.

Mr. Wallace himself, in his last work, quotes approvingly a letter which he received in 1869 from the Rev. O Pickard-Cambridge, as follows:--

"I myself doubt that particular application of the Darwinian theory which attributes male peculiarities of form, structure, colour, and ornament to female appetency or predilection. There is, it seems to me, undoubtedly something in the male organization of a special and s.e.xual nature, which, of its own vital force, develops the remarkable male peculiarities so commonly seen, _and of no imaginable use to that s.e.x_. In as far as these peculiarities show a great vital power, they point out to us the finest and strongest individuals of the s.e.x, and show us which of them would most certainly appropriate to themselves the best and greatest number of females, and leave behind them the strongest and greatest number of progeny. And here would come in, as it appears to me, the proper application of Darwin's theory of Natural Selection; _for the possessors of greatest vital power being those most frequently produced and reproduced, the external signs of it would go on developing in an ever increasing exaggeration_, only to be checked where it became really detrimental in some respect or other to the individual[117]."

[117] _Darwinism_, pp.[typo: period missing in scan] 296-7: italics mine.

Here then the idea is, as more fully expressed by Mr. Wallace in the context, that all the innumerable, frequently considerable, and generally elaborate "peculiarities of form, structure, colour, and ornament," which Darwin attributed to s.e.xual selection, are really due to "the laws of growth." Diverse, definite, and constant though these specific peculiarities be, they are all but the accidental or advent.i.tious accompaniments of "vigour," or "vital power," due to natural selection. Now, without waiting to dispute this view, which has already been dealt with in the chapter on s.e.xual Selection in Part I, it necessarily follows that "a large proportional number of specific characters," which, while presenting "no imaginable use," are very much less remarkable, less considerable, less elaborate, &c., must likewise be due to this "correlation with vital power." But if the principle of correlation is to be extended in this vague and general manner, it appears to me that the difference between Mr. Wallace and myself, with respect to the principle of utility, is abolished. For of course no one will dispute that the prime condition to the occurrence of "specific characters," whether useful or useless, is the existence of some form which has been denominated a "species" to present them; and this is merely another way of saying that such characters cannot arise except in correlation with a general fitness due to natural selection. Or, to put the case in Mr. Wallace's own words--"This development [of useless specific characters] will necessarily proceed by the agency of natural selection [as a necessary condition] _and the general laws which determine the production of colour and of ornamental appendages_." The case, therefore, is just the same as if one were to say, for example, that all the ailments of animals and plants proceed from correlation with life (as a necessary condition), "and the general laws which determine the production" of ill-health, or of specific disease. In short, the word "correlation" is here used in a totally different sense from that in which it is used by Darwin, and in which it is elsewhere used by Wallace for the purpose of sustaining his doctrine of specific characters as necessarily useful. To say that a useless character A is correlated with a useful one B, is a very different thing from saying that A is "correlated with vital power," or with the general conditions to the existence of the species to which it belongs. So far as the present discussion is concerned, no exception need be taken to the latter statement. For it simply surrenders the doctrine against which I am contending.

IV. _Isolation._

It is the opinion of many naturalists who are well ent.i.tled to have an opinion upon the subject, that, in the words of Mr. Dixon, "Isolation can preserve a non-beneficial as effectually as natural selection can preserve a beneficial variation[118]." The ground on which this doctrine rests is thus clearly set forth by Mr. Gulick:--"The fundamental cause of this seems to lie in the fact that no two portions of a species possess exactly the same average characters; and, therefore, that the initial differences are for ever reacting on the environment and on each other in such a way as to ensure increasing divergence in each generation, as long as the individuals of the two groups are kept from intergenerating[119]." In other words, as soon as a portion of a species is separated from the rest of that species, so that breeding between the two portions is no longer possible, the general average of characters in the separated portion not being in all respects precisely the same as it is in the other portion, the result of in-breeding among all individuals of the separated portion will eventually be different from that which obtains in the other portion; so that, after a number of generations, the separated portion may become a distinct species from the effect of isolation alone. Even without the aid of isolation, any original difference of average characters may become, as it were, magnified in successive generations, provided that the divergence is not harmful to the individuals presenting it, and that it occurs in a sufficient proportional number of individuals not to be immediately swamped by intercrossing. For, as Mr. Murphy has pointed out, in accordance with Delbuf's law, "if, in any species, a number of individuals, bearing a ratio not infinitely small to the entire number of births, are in every generation born with a particular variation which is neither beneficial nor injurious, and if it be not counteracted by reversion, then the proportion of the new variety to the original form will increase till it approaches indefinitely near to equality[120]." Now even Mr. Wallace himself allows that this must be the case; and thinks that in these considerations we may find an explanation of the existence of certain definite varieties, such as the melanic form of the jaguar, the brindled or ring-eyed guillemot, &c. But, on the other hand, he thinks that such varieties must always be unstable, and continually produced in varying proportions from the parent forms. We need not, however, wait to dispute this arbitrary a.s.sumption, because we can see that it fails, even as an a.s.sumption, in all cases where the superadded influence of isolation is concerned. Here there is nothing to intercept the original tendency to divergent evolution, which arises directly out of the initially different average of qualities presented by the isolated section of the species, as compared with the rest of that species[121].

[118] _Nature_, vol. x.x.xiii. p. 100.

[119] _Divergent Evolution through c.u.mulative Segregation_, Linn.

Journ. Zoology, vol. xx. p. 215.

[120] _Habit and Intelligence_, p. 241.

[121] Allusion may here again be made to the case of the niata cattle. For here is a case where a very extreme variety is certainly not unstable, nor produced in varying proportions from the parent form. Moreover, as we have seen in the preceding chapter, this almost monstrous variety most probably originated as an individual sport--being afterwards maintained and multiplied for a time by artificial selection. Now, whether or not this was the case, we can very well see that it may have been. Hence it will serve to ill.u.s.trate another possibility touching the origin and maintenance of useless specific characters. For what is to prevent an individual congenital variation of any kind (provided it be not harmful) from perpetuating itself as a "varietal," and eventually, should offspring become sufficiently numerous, a "specific character"? There is nothing to prevent this, save panmixia, or the presence of free intercrossing. But, as we shall see in the next division of this treatise, there are in nature many forms of isolation. Hence, as often as a small number of individuals may have experienced isolation in any of its forms, opportunity for perpetuation will have been given to any congenital variations which may happen to arise. Should any of these be p.r.o.nounced variations, it would afterwards be ranked as a specific character. I do not myself think that this is the way in which indifferent specific characters _usually_ originate. On the contrary, I believe that their origin is most frequently due to the influence of isolation on the average characters of the whole population, as briefly stated in the text. But here it seems worth while to notice this possibility of their occasionally arising as merely individual variations, afterwards perpetuated by any of the numerous isolating conditions which occur in nature. For, if this can be the case with a varietal form so extreme as to border on the monstrous, much more can it be so with such minute differences as frequently go to const.i.tute specific distinctions. It is the business of species-makers to search out such distinctions, no matter how trivial, and to record them as "specific characters." Consequently, wherever in nature a congenital variation happens to arise, and to be perpetuated by the force of heredity alone under any of the numerous forms of isolation which occur in nature, there will be a case a.n.a.logous to that of the niata cattle.

As we shall have to consider the important principle of isolation more fully on a subsequent occasion, I need not deal with it in the present connexion, further than to remark that in this principle we have what appears to me a full and adequate condition to the rise and continuance of specific characters which need not necessarily be adaptive characters. And, when we come to consider the facts of isolation more closely, we shall find superabundant evidence of this having actually been the case.

V. _Laws of Growth._

Under this general term Darwin included the operation of all unknown causes internal to organisms leading to modifications of form or structure--such modifications, therefore, appearing to arise, as he says "spontaneously," or without reference to utility. That he attributed no small importance to the operation of these principles is evident from the last edition of the _Origin of Species_. But as these "laws of growth" refer to causes confessedly unknown, I will not occupy s.p.a.ce by discussing this division of our subject--further than to observe that, as we shall subsequently see, many of the facts which fall under it are so irreconcilably adverse to the Wallacean doctrine of specific characters as universally adaptive, that in the face of them Mr. Wallace himself appears at times to abandon his doctrine _in toto_.