Darwin, and After Darwin Volume I Part 14

[38] Were it not that some of Darwin's critics have overlooked the very point wherein the great value of protective colouring as evidence of natural selection consists, it would be needless to observe that it does so in the _minuteness_ of the protective resemblance which in so many cases is presented. Of course where the resemblance is only very general, the phenomena might be ascribed to mere coincidence, of which the instincts of the animal have taken advantage. But in the measure that the resemblance becomes minutely detailed, the supposition of mere coincidence is excluded, and the agency of some specially adaptive cause demonstrated. Again, it is almost needless to say, no real difficulty is presented (as has been alleged) by the cases above quoted of seasonal imitations, on the ground that natural selection could not act alternately on the same individual. Natural selection is not supposed to act alternately on the same individual. It is supposed to act always in the same manner, and if, as in the case of a regularly recurring change in the colours of the environment, correspondingly recurrent changes are required to appear in the colours of the animals, natural selection sets its premium upon those individuals the const.i.tutions of which best lend themselves to seasonal changes of the needful kind--probably under the influence of stimuli supplied by the changes of external conditions (temperature, moisture, &c.).

In the first place, we always find a complete correspondence between imitative colouring and instinctive endowment. If a caterpillar exactly resembles the colour of a twig, it also presents the instinct of habitually reposing in the att.i.tude which makes it most resemble a twig--standing out from the branch on which it rests at the same angle as is presented by the real twigs of the tree on which it lives.

Here, again, is a bird protectively coloured so as to resemble stones upon the rough ground where it habitually lives; and the drawing shows the att.i.tude in which the bird instinctively reposes, so as still further to increase its resemblance to a stone. (Fig. 109.)

[Ill.u.s.tration: FIG. 109.--_Oedicnemus crepitans_, showing the instinctive att.i.tude of concealment. Drawn from a stuffed specimen in the British Museum, 1/6 nat. size, with appropriate surroundings supplied.

To take only one other instance, hares and rabbits, like grouse and partridges--or like the plover just alluded to,--instinctively crouch upon those surfaces the colours of which they resemble; and I have often remarked that if, on account of any individual peculiarity of coloration, the animal is not able thus to secure concealment, it nevertheless exhibits the instinct of crouching which is of benefit to all its kind, although, from the accident of its own abnormal colouring, this instinct is then actually detrimental to the animal itself. For example, every sportsman must have noticed that the somewhat rare melanic variety of the common rabbit will crouch as steadily as the normal brownish-gray type, notwithstanding that, owing to its abnormal colour, a "n.i.g.g.e.r-rabbit" thus renders itself the most conspicuous object in the landscape. In all such cases, of course, there has been a deviation from the normal type in respect of colour, with the result that the inherited instinct is no longer in tune with the other endowments of the animal. Such a variation of colour, therefore, will tend to be suppressed by natural selection; while any variations which may bring the animal still more closely to resemble its habitual surroundings will be preserved. Thus we can understand the truly wonderful extent to which this principle of protective colouring has been carried in many cases where the need of it has been most urgent.

Not only colour, but structure, may be profoundly modified for the purposes of protective concealment. Thus, caterpillars which resemble twigs do so not only in respect of colour, but also of shape; and this even down to the most minute details in cases where the adaptation is most complete: certain b.u.t.terflies and leaf-insects so precisely resemble the leaves upon which, or among which, they live, that it is almost impossible to detect them in the foliage--not only the colour, the shape, and the venation being all exactly imitated, but in some cases even the defects to which the leaves are liable, in the way of fungoid growths, &c. There are other insects which with similar exactness resemble moss, lichens, and so forth. A species of fish secures a complete resemblance to bunches of sea-weed by a frond-like modification of all its appendages, and so on through many other instances. Now, in all such cases where there is so precise an imitation, both in colour and structure, it seems impossible to suggest any other explanation of the facts than the one which is supplied by Mr. Darwin's theory--namely, that the more perfect the resemblance is caused to become through the continuous influence of natural selection always picking out the best imitations, the more highly discriminative becomes the perception of those enemies against the depredations of which this peculiar kind of protection is developed; so that, in virtue of this action and re-action, eventually we have a degree of imitation which renders it almost impossible for a naturalist to detect the animal when living in its natural environment.

[Ill.u.s.tration: FIG. 110.--Imitative forms and colours in insects.

Drawn from nature (_R. Coll. Surg. Mus._).]

_Warning Colours._

In strange and glaring contrast to all these cases of protective colouring, stand other cases of conspicuous colouring. Thus, for example, although there are numberless species of caterpillars which present in an astonis.h.i.+ng degree the phenomena of protective colouring, there are numberless other species which not only fail to present these phenomena in any degree, but actually go to the opposite extreme of presenting colours which appear to have been developed for the sake of their conspicuousness. At all events, these caterpillars are usually the most conspicuous objects in their surroundings, and therefore in the early days of Darwinism they were regarded by Darwin himself as presenting a formidable difficulty in the way of his theory. To Mr.

Wallace belongs the merit of having cleared up this difficulty in an extraordinarily successful manner. He virtually reasoned thus. If the _raison d'etre_ of protective colouring be that of concealing agreeably flavoured caterpillars from the eye-sight of birds, may not the _raison d'etre_ of conspicuous colouring be that of protecting disagreeably flavoured caterpillars from any possibility of being mistaken by birds?

Should this be the case, of course the more conspicuous the colouring the better would it be for the caterpillars presenting it. Now as soon as this suggestion was acted upon experimentally, it was found to be borne out by facts. Birds could not be induced to eat caterpillars of the kinds in question; and there is now no longer any doubt that their conspicuous colouring is correlated with their distastefulness to birds, in the same way as the inconspicuous or imitative colouring of other caterpillars is correlated with their tastefulness to birds. Here then is yet another instance, added to those already given, of the verification yielded to the theory of natural selection by its proved competency as a guide to facts in nature; for a.s.suredly this particular cla.s.s of facts would never have been suspected but for its suggestive agency.

As in the case of protective imitation, so in this case of warning conspicuousness, not only colour, but structure may be greatly modified for the purpose of securing immunity from attack. Here, of course, the object is to a.s.sume, as far as possible, a touch-me-not appearance; so that, although dest.i.tute of any real means of offence, the creatures in question present a fict.i.tiously dangerous aspect. As the Devil's-coach-horse turns up his stingless tail when threatened by an enemy, so in numberless ways do many harmless animals of all cla.s.ses pretend to be formidable. But the point now is that these instincts of self-defence are often helped out by structural modifications, expressly and exclusively adapted to this end. For example, what a remarkable series of protective adjustments occurs in the life-history of the Puss Moth--culminating with so comical an instance of the particular device now under consideration as the following. I quote the facts from Mr. E. B. Poulton's admirable book on _The Colours of Animals_ (pp. 269-271).

[Ill.u.s.tration: FIG. 111.--The larva of Puss Moth (_C. vinula_) when undisturbed; full-fed; natural size.]

The larva of the Puss Moth (Cerura vinula) is very common upon poplar and willow. The circular dome-like eggs are laid, either singly or in little groups of two or three, upon the upper side of the leaf, and being of a reddish colour strongly suggest the appearance of little galls, or the results of some other injury to the leaf. The youngest larvae are black, and also rest upon the upper surface of the leaf, resembling the dark patches which are commonly seen in this position. As the larva grows, the apparent black patch would cover too large a s.p.a.ce, and would lead to detection if it still occupied the whole surface of the body. The latter gains a green ground-colour which harmonises with the leaf, while the dark marking is chiefly confined to the back. As growth proceeds the relative amount of green increases, and the dark mark is thus prevented from attaining a size which would render it too conspicuous. In the last stage of growth the green larva becomes very large, and usually rests on the twigs of its food-plant (Fig.

111). The dark colour is still present on the back but is softened to a purplish tint, which tends to be replaced by a combination of white and green in many of the largest larvae. Such a larva is well concealed by General Protective Resemblance, and one may search a long time before finding it, although a.s.sured of its presence from the stripped branches of the food-plant and the faeces on the ground beneath.

[Ill.u.s.tration: FIG. 112.--The larva of Puss Moth in its terrifying att.i.tude after being disturbed; full-fed; natural size.]

As soon as a large larva is discovered and disturbed it withdraws its head into the first body-ring, inflating the margin, which is of a bright red colour. There are two intensely black spots on this margin in the appropriate position for eyes, and the whole appearance is that of a large flat face extending to the outer edge of the red margin (see Fig. 112). The effect is an intensely exaggerated caricature of a vertebrate face, which is probably alarming to the vertebrate enemies of the caterpillar. The terrifying effect is therefore mimetic. The movements entirely depend on tactile impressions: when touched ever so lightly a healthy larva immediately a.s.sumes the terrifying att.i.tude, and turns so as to present its full face towards the enemy; if touched on the other side or on the back it instantly turns its face in the appropriate direction. The effect is also greatly strengthened by two pink whips which are swiftly protruded from the p.r.o.ngs of the fork in which the body terminates. The p.r.o.ngs represent the last pair of larval legs which have been greatly modified from their ordinary shape and use. The end of the body is at the same time curved forward over the back (generally much further than in Fig.

112), so that the pink filaments are brandished above the head.

_Mimicry._

Lastly, these facts as to imitative and conspicuous colouring lead on to the yet more remarkable facts of what is called mimicry. By mimicry is meant the imitation in form and colour of one species by another, in order that the imitating species may be mistaken for the imitated, and thus partic.i.p.ate in some advantage which the latter enjoys. For instance, if, as in the case of the conspicuously-coloured caterpillars, it is of advantage to an ill-savoured species that it should hold out a warning to enemies, clearly it may be of no less advantage to a well-savoured species that it should borrow this flag, and thus be mistaken for its ill-savoured neighbour. Now, the extent to which this device of mimicry is carried is highly remarkable, not only in respect of the number of its cases, but also in respect of the astonis.h.i.+ng accuracy which in most of these cases is exhibited by the imitation.

There need be little or virtually no zoological affinity between the imitating and the imitated forms; that is to say, in some cases the zoological affinity is not closer than ordinal, and therefore cannot possibly be ascribed to kins.h.i.+p. Like all the other branches of the general subject of protective resemblance in form or colouring, this branch has already been so largely ill.u.s.trated by previous writers, that, as in the previous cases, I need only give one or two examples.

Those which I choose are chosen on account of the colours concerned not being highly varied or brilliant, and therefore lending themselves to less ineffectual treatment by wood-engraving than is the case where attempts are made to render by this means even more remarkable instances. (Figs. 113, 114, 115.)

[Ill.u.s.tration: FIG. 113.--Three cases of mimicry. Drawn from nature: first two pairs nat. size, last pair 2/3 (_R. Coll. Surg. Mus._).]

[Ill.u.s.tration: FIG. 114.--Two further cases of mimicry; flies resembling a wasp in the one and a bee in the other. Drawn from nature: nat. size (_R. Coll. Surg. Mus._).]

[Ill.u.s.tration: FIG. 115.--A case of mimicry where a non-venomous species of snake resembles a venomous one. Drawn from nature: 1/3 nat. size (_R. Coll. Surg. Mus._).]

It is surely apparent, without further comment, that it is impossible to imagine stronger evidence in favour of natural selection as a true cause in nature, than is furnished by this culminating fact in the matter of protective resemblance, whereby it is shown that a species of one genus, family, or even order, will accurately mimic the appearance of a species belonging to another genus, family, or order, so as to deceive its natural enemies into mistaking it for a creature of so totally different a kind. And it must be added that while this fact of mimicry is of extraordinarily frequent occurrence, there can be no possibility of our mistaking its purpose. For the fact is never observable except in the case of species which occupy the same area or district.

Such being what appears to me the only reasonable view of the matter, I will now conclude this chapter on the evidences of natural selection as at all events the main factor of organic evolution, by simply adding ill.u.s.trations of two further cases of mimicry, which are perhaps even more remarkable than any of the foregoing examples. The first of the two (Fig. 115) speaks for itself. The second will be rendered intelligible by the following few words of explanation.

There are certain ants of the Amazons which present the curious instinct of cutting off leaves from trees, and carrying them like banners over their heads to the hive, as represented in Fig. 116, B, where one ant is shown without a leaf, and the others each with a leaf. Their object in thus collecting leaves is probably that of growing a fungus upon the "soil" which is furnished by the leaves when decomposing. But, be this as it may[39], the only point we are now concerned with is the appearance which these ants present when engaged in their habitual operation of carrying leaves. For it has been recently observed by Mr.

W. L. Sclater, that in the localities where these hymenopterous insects occur, there occurs also a _h.o.m.opterous_ insect which mimics the ant, leaf and all, in a wonderfully deceptive manner. The leaf is imitated by the thin flattened body of the insect, "which in its dorsal aspect is so compressed laterally that it is no thicker than a leaf, and terminates in a sharp jagged edge." The colour is exactly the same as that of a leaf, and the brown legs show themselves beneath the green body in just the same way as those of the ant show themselves beneath the leaf. So that both the form and the colouring of the h.o.m.opterous insect has been brought to resemble, with singular exactness, those belonging to a different order of insect, when the latter is engaged in its peculiar avocation. A glance at the figure is enough to show the means employed and the result attained. In A, an ant and its mimic are represented as about 2-1/2 times their natural size, and both proceeding in the same direction. It ought to be mentioned, however, that in reality the margin of the leaf is seldom allowed to retain its natural serrations as here depicted: the ants usually gnaw the edge of the real leaf, so that the margin of the false one bears an even closer resemblance to it than the ill.u.s.tration represents. B is a drawing from life of a group of five ants carrying leaves, and their mimic walking beside them[40].

[39] For a full account of this instinct and its probable purpose, see _Animal Intelligence_, pp. 93-6.

[40] Both drawings are reproduced from Mr. Poulton's paper upon the subject (_Proc. Zool. Soc._, June 16, 1891).

[Ill.u.s.tration: FIG. 116 PROTECTIVE MIMICRY]

CHAPTER IX.

CRITICISMS OF THE THEORY OF NATURAL SELECTION.

I will now proceed to consider the various objections and difficulties which have hitherto been advanced against the theory of natural selection.

Very early in the day Owen hurled the weight of his authority against the new theory, and this with a strength of onslaught which was only equalled by its want of judgment. Indeed, it is painfully apparent that he failed to apprehend the fundamental principles of the Darwinian theory. For he says:--

Natural Selection is an explanation of the process [of trans.m.u.tation] of the same kind and value as that which has been proffered of the mystery of "secretion." For example, a particular ma.s.s of matter in a living animal takes certain elements out of the blood, and rejects them as "bile." Attributes were given to the liver which can only be predicated of the whole animal; the "appetency" of the liver, it was said, was for the elements of bile, and "biliosity," or the "hepatic sensation," guided the gland to their secretion. Such figurative language, I need not say, explains absolutely nothing of the nature of bilification[41].

[41] _Anatomy of Vertebrates_, vol. iii. p. 794.

a.s.suredly, it was needless for Owen to say that figurative language of this kind explains nothing; but it was little less than puerile in him to see no more in the theory of natural selection than such a mere figure of speech. To say that the liver selects the elements of bile, or that nature selects specific types, may both be equally unmeaning re-statements of facts; but when it is explained that the term natural selection, unlike that of "hepatic sensation," is used as a shorthand expression for a whole group of well-known natural causes--struggle, variation, survival, heredity,--then it becomes evidence of an almost childish want of thought to affirm that the expression is figurative and nothing more. The doctrine of natural selection may be a huge mistake; but, if so, this is not because it consists of any unmeaning metaphor: it can only be because the combination of natural causes which it suggests is not of the same adequacy in fact as it is taken to be in theory.

Owen further objected that the struggle for existence could only act as a cause of the extinction of species, not of their origination--a view of the case which again shows on his part a complete failure to grasp the conception of Darwinism. Acting alone, the struggle for existence could only cause extermination; but acting together with variation, survival, and heredity, it may very well--for anything that Owen, or others who followed in this line of criticism, show to the contrary--have produced every species of plant and animal that has ever appeared upon the face of the earth.

Another and closely allied objection is, that the theory of natural selection "personifies an abstraction." Or, as the Duke of Argyll states it, the theory is "essentially the image of mechanical necessity concealed under the clothes, and parading in the mask, of mental purpose. The word 'natural' suggests Matter, and the physical forces.

The word 'selection' suggests Mind, and the powers of choice." This, however, is a mere quarrelling about words. Darwin called the principle which he had discovered by the name natural selection in order to mark the a.n.a.logy between it and artificial selection. No doubt in this a.n.a.logy there is not necessarily supposed to be in nature any counterpart to the mind of the breeder, nor, therefore, to his powers of intelligent choice. But there is no need to limit the term _selection_ (_se_ and _lego_, Gr. [Greek: lego]) to powers of intelligent choice. As previously remarked, a bank of sea-weed on the sea-sh.o.r.e may be said to have been selected by the waves from all the surrounding sand and stones. Similarly, we may say that grain is selected from chaff by the wind in the process of winnowing corn. Or, if it be thought that there is any ambiguity involved in such a use of the term in the case of "Natural Selection," there is no objection to employing the phrase which has been coined by Mr. Spencer as its equivalent--namely, "Survival of the Fittest." The point of the theory is, that those organisms which are best suited to their surroundings are allowed to live and to propagate, while those which are less suited are eliminated; and whether we call this process a process of selection, or call it by any other name, is clearly immaterial.

A material question is raised only when it is asked whether the process is one that can be ascribed to causation strictly natural. It is often denied that such is the case, on the ground that natural selection does not originate the variations which it favours, but depends upon the variations being supplied by some other means. For, it is said, all that natural selection does is to preserve the suitable variations _after they have arisen_. Natural selection does not _cause_ these suitable variations; and therefore, it is argued, Darwin and his followers are profoundly mistaken in representing the principle as one which _produces_ adaptations. Now, although this objection has been put forward by some of the most intelligent minds in our generation, it appears to me to betoken some extraordinary failure to appreciate the very essence of Darwinian doctrine. No doubt it is perfectly true that natural selection does not produce variations of any kind, whether beneficial or otherwise. But if it be granted that variations of many kinds are occurring in every generation, and that natural selection is competent to preserve the more favourable among them, then it appears to me unquestionable that this principle of selection deserves to be regarded as, in the full sense of the word, a natural cause. The variations being expressly regarded by the theory as more or less promiscuous[42], survival of the fittest becomes the winnowing fan, whose function it is to eliminate all the less fit in each generation, in order to preserve the good grain, out of which to const.i.tute the next generation. And as this process is supposed to be continuous through successive generations, its action is supposed to be c.u.mulative, till from the eye of a worm there is gradually developed the eye of an eagle.

Therefore it follows from these suppositions (which are not disputed by the present objection), that if it had not been for the process of selection, such development would never have been begun; and that in the exact measure of its efficiency will the development proceed. But any agency without the operation of which a result cannot take place may properly be designated the cause of that result: it is the agency which, in co-operation with all the other agencies in the cosmos, produces that result.

[42] The degree in which variability is indefinite, or, on the contrary, determinate, is a question which is not yet ripe for decision--nor even, in my opinion, for discussion. But I may here state the following general principles with regard to it.

(1) It is evident that up to some point or another variations _must_ be pre-determined in definite lines. Men do not gather grapes from thorns, figs from thistles, nor even moss-roses from sweet-briars.

In other words, "the nature of the organism" in all cases necessitates the limiting of variations within certain bounds.

(2) But when the question is as to what these bounds may be, we can only answer in a general way that, according to the general theory of evolution, they must be such as are imposed by heredity, coupled with the degree to which external conditions of life (and possibly also use-inheritance) are capable, in given cases, of modifying congenital characters. These are the only causes which the theory of descent can consistently recognise as producing variations in determinate directions.

(3) Inasmuch as variation presupposes the existence of parts that vary, and inasmuch as the variation of parts can only be in the alternative directions of increase or decrease around an average, it follows that, in the first instance at all events, every variation, if determinate, must be so only in one or other of these two opposite directions.

(4) In as far as variations are summated in successive generations, so as eventually to give rise to new structures, organs, mechanisms, &c., natural selection is theoretically competent to explain the facts, without our having to postulate the operation of unknown causes producing variations in determinate lines,--or not further than is stated in paragraphs 1 and 2.

(5) Nevertheless, it does not follow that there are not such other unknown causes; and, if there are, of course the importance of natural selection as a cause of adaptive modification would be limited in proportion to their number and the extent of their operation. But it is for those who, like the late Professors Asa Gray and Nageli, maintain the existence of such causes, to substantiate their belief by indicating them.

Take any a.n.a.logous case. The selective agency of specific gravity which is utilised in gold-was.h.i.+ng does not create the original differences between gold-dust and dust of all other kinds. But these differences being presented by as many different bodies in nature, the gold-washer takes advantage of the selective agency in question, and, by using it as a cause of segregation, is enabled to separate the gold from all the earths with which it may happen to be mixed. So far as the objects of the gold-washer are concerned, it is immaterial with what other earths the gold-dust may happen to be mixed. For although gold-dust may occur in intimate a.s.sociation with earths of various kinds in various proportions, and although in each case the particular admixture which occurs must have been due to definite causes, these things, in relation to the selective process of the washer, are what is called accidental: that is to say, they have nothing to do with the causative action of the selective process. Now, in precisely the same sense Darwin calls the mult.i.tudinous variations of plants and animals accidental. By so calling them he expressly says he does not suppose them to be accidental in the sense of not all being due to definite causes. But they are accidental in relation to the sifting process of natural selection: all that they have to do is to furnish the promiscuous material on which this sifting process acts.