THE IMITATIVE SIDE OF NATURE.
AVERY remarkable article in the new number of the Westminster Review, on the protective mimicry of the animal world, gives us a great number of most instructive illus- trations of the operation of the Darwinian law of natural selection in perpetuating and multiplying certain protective resemblances between different species of animals, and also between certain animals and certain vegetable products. The instances quoted are so remarkable and complex that we can only give one or two, which we may take as representative of the rest. And we do so not in the view, of course, of criticizing the natural philosophy of an evidently profound and accomplished naturalist, but only to show how much less, as it seems to us, Mr. Darwin's principle really explains than this writer, and perhaps the majority of the increasing and successful school of Darwinians, believe that it explains, concerning the ultimate principles of crea- tion. That Mr. Darwin's theory shows how, when a varia- tion advantageous to the life and multiplication of a species has really come into existence, that variation is perpetuated, and even rendered more perfect, by hereditary transmission, no one who understands it can doubt at all. But the weak side of the theory,—whieh seems even weaker in connection with this doctrine of protective mimicry, than in connection with other developments of the principle of utility,—seems to be in its assumption that accidental variations of species must take place equally in all directions and in unlimited numbers, of which only those which are useful to the creature, and give it a fresh chance in the conflict for existence, will tend to perpetua- tion. The indifferent variations, on the other hand, which neither help nor hinder, will, according to this theory, simply take their chance, and the unfavourable ones will be inevitably killed off, in consequence of the disadvantage with which they weight the scale against the individuals in whom they are produced. To take an instance of the advantages of Nature's "protective resemblances," and how they act to multiply a species. There is the ptarmigan, whose summer plumage is exactly like the lichen-coloured stones amidst which it sits, and whose winter plumage is white, so that it is undistinguishable from the snow which then covers the ground. Now, of course, a ptarmigan which did not cast off its white plum- age in summer would be so conspicuous as to get killed off ; and so, also, one which did not change its summer plumage for white in winter would for the same reason be soon killed off. It is clear, therefore, that these changes are protective, and the Darwinian theory explains why, once in existence, the families which possess these striking characteristics have an immense advantage over all other families of the same species of bird, and are sure to beat them in the competition. What the Darwinian theory does not explain, or pretend to explain, is, how these particularly useful changes come to be produced at all. Indeed, it is generally supposed that a thousand useless variations occur for every one that is successful ;—that ptarmigans becoming lichen-coloured in winter and white in summer may have come into existence, and found it a very bad speculation indeed ; that others have been white all the year round, and been cut off in such quan- tities in the summer that it did not answer to them at all ; that others, again, tried lichen colour all the year round, and fell in such quantities in the winter that they, too, were a short-lived race. The difficulty of this supposition of assuming unnumbered useless or positively disadvantageous varieties for every great stroke of Nature's, is that, when such a delicate operation as a double change of colour synchronizing with a double change of season is needed for success, one wonders almost that the stock on which experiment was made was not exhausted before the success was achieved. And this is more remarkable in other far more recondite cases of protective resemblance, or even mimicry. Thus, the accomplished writer in the Westminster describes a race of Indian butterflies, Kallima inachis, and the Malayan Kallima paralekta, which have a showy upper surface, but whose under surface is coloured like a withered leaf, with radiating lines like the veins of a leaf. Not only so, but it has irregular blotches, like the mildew or fungus on a leaf in decay. And not only so, but its habits are exactly those which enable it to act the part of a withered leaf successfully. The Westminster reviewer, who has himself observed them, thus describes them :— " These butterflies frequent dry forests and fly very swiftly. They were never seen to settle on a flower or a green leaf, but were many times suddenly lost sight of in a bush or tree of dead leaves. On such occasions they were generally searched for in vain, for while gazing intently at the very spot where one had disappeared, it would often sud- denly dart out, and again vanish twenty or fifty yards further on. On one or two occasions the insect was detected reposing, and it oould then be seen how completely it assimilates itself to the surrounding leaves. It site on a nearly upright twig, the wings fitting closely back to back, concealing the antenna) and head, which are drawn up between their bases. The little tails of the hind wing toach the branch, and form a perfect stalk to the leaf, which is supported in its place by the claws of the middle pair of feet, which are slender and inconspicuous. The irregular outline of the wings, gives exactly the perspective effect of a shrivelled leaf."
So, toe, there is an insect, the Phylliam, or "walking leaf," "in which not only are the wings perfect imitations of leaves in every detail, but the thorax and legs are flat, dilated, and leaf-like ; so that when the living insect is resting among the foliage
on which it feeds, the closest observation is often unable to distinguish between the animal and the vegetable." And then, again, there are harmless creatures which are protected by closely resembling very venomous creatures of the same genus, just as the disguise of a soldier's or policeman's outward dress will often secure immunity for the mildest and most lamb-like individuals from the attacks to which they would be subjected in their own. The most curious case of this kind is that of harm- less snakes which borrow a terror from poisonous snakes, such as Patroclus borrowed from the armour of Achilles :—
" There are in tropical America a number of venomous snakes of the genus Elaps, which are ornamented with brilliant colours disposed in a peculiar manner. The ground colour is generally bright red, on which are black bands of various widths and sometimes divided into two or three by yellow rings. Now, in the same country are found several genera of harmless snakes, having no affinity whatever with the above, but coloured exactly the same. For example, the poisonous Elaps fulvins often occurs in Guatemala with simple black bands on a coral- red ground ; and in the same country is found the harmless snake Plioceras equalis, coloured and banded in identically the same manner.. A variety of Elaps corallinus has the black bands narrowly bordered with yellow on the same red ground colour, and a harmless snake, Homalocraniam semicinctum, has exactly the same markings, and both are found in Mexico What adds much to the extraordinary character of these resemblances is the fact, that nowhere in the world but in America are there any snakes at all which have this style of colouring. Dr. Gunther, of the British Museum, who has kindly fur- nished the details here referred to, assures us that this is the case ; and that red, black, and yellow rings occur together on no other snakes in the world but on Elaps and the species which so closely resemble it."
A great number of instances of the same hind are accumulated by the Westminster reviewer, instances in which one kind of creature gains a distinct advantage by seeming to belong to a kind to which it does not belong,—in short, by imitating either alarming and un- pleasant animals which it is not agreeable for enemies to attack, or by imitating dull and uninteresting vegetables which it is no object for carnivore to attack.
Now, the great question that arises out of these curious and manifold instances of protective resemblance is one to which the Westininster reviewer devotes exceedingly little attention,— namely, whether it is more reasonable to ignore entirely any intellectual principle in these imitative strokes of Nature, and to explain them only as the prizes amongst a far greater num- ber of blanks, or whether, on the contrary, it is more reason- able to admit that there is a creative reason for the production of the special variation which succeeds, as well as an obvious, cause for its success when once produced. Is "the crawling leaf" only an accidental hit of Nature's, which multiplies and survives because it is a hit? Or is it one of the most typical instances of that imitative principle in the life of the lower universe, which is taken up again in the law of conscious imitation directly we reach the higher life of men? The man-orchis and bee-orchis, &c., are cases in which the vegetable world imitates the animal, and here, too, we believe, without any opening for the Darwinian principle of an advantage weighing on its side in the contest for existence. Again, directly we enter the regions of the lowest animal mind, we have the parrot imitating all the sounds and notes it hears without apparently much chance of greater gain than loss, by virtue of such an instinct. If by imitating ferocious animals it gains some protection, by imitating helpless and defenceless animals it must lose as much as in the other case it gains. Pure imitation seems to have a real root even in the life of what we call unconscious nature ; and directly we reach the earliest stages of conscious and rational life, in the children of our own race, it has a prolific vigour, the vitality of which no one- doubts or denies. Is the imitative side of unconscious nature a mere accident, which has got stereotyped and struck our observa- tion only when it has tended to promote the safety of particular species ?—or is it an anticipation of the imitative side of mental phenomena,—an anticipation due, in fact, to the intellectual unity of the divine cause ?
The argument, as the Westminster reviewer puts it, is as follows : —As regards mere colour, we see that among domestic animals colours that would be unfavourable to their safety in a wild state, are as common as those which would favour their safety. Thus, though you never find white mice wild, or white cats wild, simply because the white specimens are so much more certain to be seen and destroyed by their enemies than specimens of a less conspi- cuous colour, you no sooner keep them under protection than speci- mens of white colour turn up again in numbers. Hence he argues from analogy that in Arctic climates only the white specimens have remained—the others being picked off—and in more complex cases you must suppose that every approximation towards a useful or protective resemblance is selected out of a multitude of other varia- tions, till the complete protective resemblance is reached. Thus a ptarmigan which happened to have lighter-coloured plumage for its winter coat than for its summer would have a better chance for life, and of its offspring those whose plumage returned towards the old identity would be picked off soonest, whilst those in which the new change was slightly exaggerated would have a still more protected life. Or, to take the case of the Phyllium, the "walking-leaf." The Westminster reviewer would suppose that such extraordinary and minute resemblance would be the work of many centuries ; that resemblance in colour to the leaves fed upon would be, perhaps, the first step in the direction of protection ; that then any iusect with a greater general resemblance of shape to the leaf than another, would have a safer life and a larger progeny; and so little by little the change would goon, till all the minute shaping and graining of the leaf, in the first in- stance accidentally introduced on the insect's surface, had been strengthened and preserved by a like process.
We confess this explanation seems to us totally inadequate. It explains, no doubt, the protection and multiplication of specimens with these curious advantages, but it does not explain at all how they oriyinate, especially how such an extraordinary concurrence of distinct elements of resemblance are combined in any one speci- men. It must be remembered that many of the elements in the final resemblance would be rather disadvantageous than advantageous without the others. The harmless snake Plioce- rus equalis, if it began by being coral-red without black bands, would take nothing by this degree of approximation to the appear- ance of a deadly snake. Or if it began by being all black, it would take nothing by that amount of resemblance. It is the alternating red and black which constitutes the protection. So the butterflies which act the part of withered leaves would lose by hanging like leaves to the trees, if they hung their brilliant side outwards ; and they would take nothing by resembling withered leaves, if they did not bang like leaves to the trees. It is the simultaneous combination of the two quali- ties — or rather of the habit with the quality — which is their salvation.
Besides, as we have said, there are such a multitude of cases of imitation or resemblance in nature which are not protective in any sense. The laws of light, which cause shadows to mimic sub- stances, and which compel the water to reflect the hills around, and make
"The swan on sweet St. Mary's Lake Float double, swan and shadow,"
—laws which involve, indeed, the whole process of photography,— are surely inorganic evidences that the universe has been con- ceived in a mind which resolves to mark it by startling like- nesses as well as startling differences. It may be, as the West- minster Review thinks, that the mere design to protect from extinction is but a poor solution of such extraordinary protective resemblances and protective mimicries as he points out, between insects (probably of no great moral use to the world) and the leaves or trees in which they are harboured. To us the curious vegetable imitations of animal nature (as in the orchises), and the still more curious animal imitations of vegetable nature, suggest less what in higher moral regions we call Providence, than art,— but art of the highest order they assuredly do suggest. That the human device which deceived Macbeth, of Birnam Wood coming to Dunsinane, should be anticipated in the lowest orders of animal life, where the leaves seem to crawl off the trees and to take a locomotion of their own, does strike us as a very curious demonstration of mind in Nature, though it may seem absurd to interpret it as due solely to the purpose of protecting the life of an insect not, as far as we know at present, particu- larly important to the divine ends of the Universe. But that the resemblance, as resemblance, has an intellectual purpose above and beyond what we may call the practical results of the resemblance, seems to us, if it be a superstition, a superstition of the kind which no man with both thought and imagination could doubt. No doubt the "argument from design" should be reserved chiefly for cases in which a moral purpose worthy of a divine Father can be discerned. But the argument from beauty and har- mony applies as mach to the lowest foundations of the physical Universe, as to those moral parts of creation which we know to belong to the eternal purposes of God.
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