Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/362

Rh 344 MIMICRY apparently overgrown with a creeping moss or jungermannia ; and Mr Belt discovered a larval form in Nicaragua whose body was prolonged into thin green filaments, precisely like the moss in which V: lurked. In other instances the insect probably uses its disguise rather to deceive its prey than to escape its enemies. Sir Joseph Hooker believes that an Indian Mantis deludes the little creatures which foiiu its food by its singular likeness to a leaf; while Sir Charles Dilke found one which had its head and fangs moulded into the deceptive appearance of an orchid, so that small flies were actually attracted in search of honey into its very jaws. Outside the class of insects, similar phenomena sometimes occur. Thus, according to Mr Bates, many showy little tropical spiders double themselves up at the base of leaf stalks so as to resemble flower buds, and thus delude the flies on which they prey. Even among the vertebrates Mr Belt mentions a green Nicaraguan lizard looking like the herbage by which it is surrounded, and decked with leaf-like expansions, which hide its predaceous nature from passing beetles or butterflies. These last instances are divided from true mimicry by a very narrow line. But they differ in the fact that some vague object only in the general environment is simulated, not a particular pro tected species, as in genuine mimetic resemblance. If we allow, however, that natural selection can produce the white colour of Arctic animals, and the sandy hue of the sole and the flounder, it is easy enough to extend the same principle to the leaf-insect and the stick-insect, or even to real mimicry, as in the case of the Lcptnlis and the Hcliconidie. Certain Phasmidse may at first have varied in the direction of green coloration, and these would naturally escape the eyes of birds more readily than their fellows. After the lapse of many generations, all the Phasmidaz of that special group would have become green, and the birds which preyed upon them would have learned in many cases to penetrate the disguise ; for, as Mr Belt has observed, each fresh deceptive resemblance in the prey is sure to be followed by increased keenness of discrimination in the enemies of the species. At this stage the ordinary green PJmsmidas would often be killed, while only those which happened to approximate rudely in the venation of their wings to leaves would now escape the sharper and more experienced eyes of the birds. Thus step by step the disguise would become more and more perfect, only the best-protected of each generation escaping on the average, while all the worse-protected would be dis covered and devoured. Given the usual luxuriance of tropical life, it is not difficult to understand how favourable variations mightcontinu- ally occur, until at length we get such perfect deceptions as those of the leaf-insects, the stick-insects, and the moss-grown larva?. The phenomena of true mimicry may be explained by a parallel genesis. Suppose, to begin with, a group of large and brilliant butterflies like the South-American HeliconidsR, protected by a nauseous taste and odour, and therefore never eaten by birds. To such insects slow flight and conspicuous hues are a positive protection, because they enable birds readily to discriminate them, and therefore prevent attacks, just as the banded body of the wasp and the hum of the bee prevent us from catching and killing them upon a window pane. Suppose, again, that in the same district there lives a widely different species of edible butterfly presenting some very slight and remote resemblance to the protected species. At first, no doubt, the resemblance will be merely an accidental one of general hue ; it may even be so slight as to deceive nobody except upon the most distant and casual glance. Now, suppose these edible butterflies to be devoured in large quantities by birds, then a few of them may happen to gain safety by associating with the flocks of inedible butterflies which the birds refuse. After a time, even if the habit of consorting with the protected species becomes fixed in the race, the birds will begin to recognize the edible insects amongst the flocks, especially such as vary most in the opposite direction from the protected species. On the other hand, they will overlook such as vary most in the same direction as the inedible kind ; and thus the least mimetic individuals will be destroyed, while the most mimetic will be left to pair with one another and to produce young, most of whom will present the like peculiarities. From generation to generation the birds will go on picking out every bad copy, and sparing all the best ones, till at last the two species become absolutely indistinguishable upon the wing. But the mimicry will never of course affect any but the most external and noticeable parts of the organism ; it will be to the last a mere matter of colour, shape of wing, visible appearance of legs or antenna?, and so forth. The underlying structural differences will remain as great as ever, though externally masked by the deceptive resemblance in form and hue. In like manner we may explain the genesis of the mimetic resemblance borne by Volucclla to the humble bee. Suppose an undisguised fly to enter the bees nest, it would be at once attacked and killed. But if it presented some very slight resemblance to the bee it might manage to lay its eggs undisturbed, and its larva? would then be able to feed quietly upon the larva? of the bee. With each new generation the more flimsy disguises would be more and more readily detected, and only those flies which varied most in the direction of resembling the bees would survive or lay their eggs in peace. On the other hand, those which actually succeeded would possess great advantages over their neighbours, because their larvse would thus obtain a safe and certain supply of food, and be guaranteed the protection of the bees nest. In this way the flies would at last, by constant survival of the best-adapted, come exactly to imitate the bees amongst which they lived. The theory of the origin of mimetic forms thus briefly sketched out is due to Mr Bates and Mr AVallace, and it explains all the facts more fully than any other. It shows us, first, why the mimicking organism always imitates a specially protected species ; secondly, why the two always inhabit the same district ; thirdly, why the mimicking species is always much rarer than the species mimicked ; fourthly, why the phenomenon is confined to a few groups only ; and fifthly, why several different mimicking species often imitate the same protected form. It also accounts for the absence of mimicry amongst large or dominant animals, and its comparative commonness amongst small and defenceless kinds. And by affiliating the whole of the phenomena upon the general principles of protective colouring it reduces a seemingly strange and marvellous fact to a particular case of a well-known law. Whatever theory be adopted, however, the facts and most of their implications remain the same. For, whether we suppose these imitative resemblances to be due to direct creative design or to survival of favourable variations, it is at least clear that the disguise subserves a function that it is purposive and not accidental. Hence we may draw from the phenomena of mimicry certain important psychological implications. On the hypothesis of evolution, it is obvious that the mimicry can never go further than the senses of the creatures against whom the disguise is advantageous would naturally carry it ; and even on the hypothesis of special design it is not likely that the imitation would be made more accurate than would be necessary for practical purposes of deception. There is much evidence in favour of this view. Mr B. T. Lowne, for example, who has carefully measured the curvature of the facets in the compound eyes of insects, upon which depends the minimum size of apprehensible objects, finds that the mimicry in the case of the flies parasitic upon bees nests has proceeded just so far as the structure of the bee s eye would lead us to expect, and no further. I n other words, so far as measurements of angular distance subtended can guide us, such a fly seems to be absolutely indistinguishable by a bee from one of his own species, within the limits of ordinary vision. The pictures cast upon the sensorium by the fly and by a brother bee are simply identical. In many other cases it can be shown that the mimicry seems specially intended to deceive the eyes of a particular class of animals ; while there is no case of mimicry where the only enemies or prey consist of plants or eyeless animals. Naturally there can be no mimicry without a creature to deceive ; the very conception implies an external nervous system to be acted upon, and to be acted upon deceptively. Thus mimicry in plants must have reference to the eyes of animals, in animals themselves to the eyes of one another. We may conclude, accord ingly, that if a leaf-insect is green with faint violet-brown veins to the wings, exactly like a certain leaf, in order to deceive sundry tropical birds, then those birds are capable of perceiving the forms and colours imitated to that particular degree. So the presence of mimicry in any group may guide us to a rough idea of the perceptive powers of those creatures whom the mimicry serves to deceive. The exact imitation of sand and coloured pebbles in the flat-fish is a fairly safe indication that the predaceous fish by whose selection they have been developed (through the weeding out of ill-protected variations) can pretty accurately distinguish form and colour. The long green pipe fish which cling around green sea-weed have probably acquired their existing hues to deceive the eyes of small sharks ; the Pliyllopteryx eques, a hippocampus which looks pre cisely like a piece of tangled and waving fucus (see figure, vol. xi. p. 852), has doubtless in the same way taken on its delusive like ness to the algre among which it lives. So the cricket which resembles its foe the sand-wasp must have gained its present shape and hue by deceiving its enemy, and therefore it sug gests the probability of highly developed vision on the part of the wasps. There seems every reason to believe that in many instances insects, spiders, and even lizards have developed mimetic or other deceptive resemblances in order to delude the eyes of in sects ; while in other cases the disguise has been unconsciously adopted to deceive fish, amphibians, reptiles, birds, and mammals. Moreover, we have some grounds for believing that the sense of colour is exceptionally strong in birds and in one or two insect orders ; and the mimicry of colour seems to have proceeded to the greatest length amongst animnls which arc most exposed to the attacks of these classes, or which would find it advantageous to deceive them. It may be added that these same classes have been most effective in producing the bright hues of flowers and fruits, on Mr Darwin s hypothesis, or are at least in any case most intimately correlated with such vegetable structures as fertilizers of blossoms and dispersers of seed. Mimicry is thus to some extent a rough gauge of the perceptive faculties of the species deceived by it.