LECTURE IV

THE COLORATION OF ANIMALS AND ITS RELATION TO THE PROCESSES OF SELECTION

Biological significance of colours—Protective colours of eggs—Animals of the snow-region—Animals of the desert—Transparent animals—Green animals—Nocturnal animals—Double colour-adaptation—Protective marking of caterpillars—Warning markings—Dimorphism of colouring in caterpillars—Shunting back of colouring in ontogeny—'Sympathetic' colouring in diurnal Lepidoptera—In nocturnal Lepidoptera—Theoretical considerations—The influence of illumination in the production of protective colouring,Tropidoderus—Harmony of protective colouring in minute details—Notodonta—Objections—Imitation of Strange objects,Xylina—Leaf-butterflies,Kallima—Hebomoja—Nocturnal Lepidoptera with leaf-markings—Orthoptera resembling leaves—Caterpillars of the Geometridæ.

Wehave seen what Darwin meant by natural selection, and we understand that this process really implies a transformation of organisms by slow degrees, in the direction of adaptive fitness—a transformation which must ensue as necessarily as when a human selector, prompted by conscious intention, tries to improve an animal in a particular direction, by always selecting the 'fittest' animals for breeding. In nature, too, there is selection, because in every generation the majority succumb in the struggle for life, while on an average those which survive, attain to reproductive maturity, and transmit their characters to their descendants, are those which are best adapted to the conditions of their life—that is, which possess those variations of most advantage in overcoming the dangers of life. Since individuals are always variable in some degree, since their variations can be inherited by their progeny, and since the continually repeated elimination of the majority of those descendants is a fact, the inference from these premisses must be correct; there must be a 'natural selection' in the direction of a gradually increasing fitness and effectiveness of the forms of life.

We cannot, however, directly observe this process of natural selection; it goes on too slowly, and our powers of observation are neither comprehensive nor fine enough. How could we set about investigating the millions of individuals which constitute the numerical strength of a species on a given area, to find out whether they possess some variable character in a definite percentage, and whether this percentage increases in the course of decades or centuries? Andthere is, furthermore, the difficulty of estimating the biological importance of any variation that may occur. Even in cases where we know its significance quite well in a general way, we cannot estimate its relative value in reference to the variation of some other character, though that other may also be quite intelligible. Later on, we shall speak of protective colouring, and in so doing we shall discuss the caterpillars of one of the Sphingidæ, which occur in two protective colours, some being brown, others green. From the greater frequency of the brown form we may conclude that brown is here a better adaptation than green, but how could we infer this from the character itself, or from our merely approximate knowledge of the mode of life of the species, its habits, and the dangers which threaten it? A direct estimation of the relative protective value of the two colours is altogether out of the question. The survival of the fittest cannot be proved in nature, simply because we are not in a position to decide,a priori, what the fittest is. For this reason I was forced to try to make the process of natural selection clear by means of imagined examples, rather than observed ones.

But though we cannot directly follow the uninterrupted process of natural selection which is going on under natural conditions, there is another kind of proof for this hypothesis, besides that which consists in logically deducing a process from correct premisses; I should like to call this the practical proof. If a hypothesis can be made to explain a great number of otherwise unintelligible facts, it thereby gains a high degree of probability, and this is increased when there are no facts to be found which are in contradiction to it.

Both of these criteria are fulfilled by the selection-hypothesis, and indeed the phenomena which may be explained by it, and are intelligible in no other way, present themselves to us in such enormous numbers, that there can be no doubt whatever as to the correctness of the principle; all that can be still disputed is, how far it reaches.

Let us now turn our attention to this practical way of proving the theory by the facts which it serves to interpret, beginning with a consideration of the external appearance of organisms, their colour and form.

The Colour and Form of Organisms.

Erasmus Darwin had in many cases already rightly recognized the biological significance of the colouring of an animal species, and we may be sure that many of the numerous good observers of earlier times had similar ideas. I can even state definitely that Rösel von Rosenhof, the famous miniature-painter and naturalist of Nürnbergin the middle of the eighteenth century, recognized clearly, and gave beautiful descriptions of what we now call colour-adaptation. It is true that he gave them only as isolated instances, and was far from recognizing the phenomenon of colour-adaptation in general, or even from inquiring into its causes. From the time of Linné, the endeavour to establish new species overshadowed all the finer observation of life-habits and inter-relations, and, later on, after Blumenbach, Kielmeyer, Cuvier, and others, the eager investigation of the internal structure of animals also tended to divert attention from these œcological relations. In systematic zoology, colour ranked only as a diagnostic character of subordinate value, because it is often not very stable, and indeed is sometimes very variable; it was therefore found preferable to keep to such relatively stable differences as are to be found in the form, size, and number of parts.

Charles Darwin was the first to redirect attention to the fact that the colouring of animals is anything but an unimportant matter; that, on the contrary, in many cases it is of use to the animal, e.g. in making it inconspicuous; a green insect is not readily seen on green leaves, nor a grey-brown one on the bark of a tree.

It is plain that the origin of such a so-called 'sympathetic' coloration, harmonizing with the usual environment of the animal, can be easily interpreted in terms of the principle of selection; and it is equally evident that it cannot be explained by the Lamarckian principle of transformation. Through the accumulation of slight useful variations in colour, it is quite possible for a green or a brown insect to arise from a previous colour, but a grey or a brown insect could not possibly have become a green one simply by getting into the habit of sitting on a green leaf; and still less can the will of the animal or any kind of activity have brought the change about. Even if the animal had any idea that it would be very useful to it to be coloured green, now that it had got into the habit of sitting on a leaf, it could not have done anything towards attaining the desirable green colour. Quite recently the possibility of a kind of colour-photography on the skin of the animal has been suggested, but there are many species whose colouring is in contrast to their environment, so that the skin in these cases does not act as a photographic plate, and it would, therefore, have to be explained how it comes to pass that it functions as such in the sympathetically coloured animals. I do not ask for proof of the chemical composition of the stuff which is supposed to be sensitive to light. Whether this be iodide of silver or something quite different, the question remains the same: how comes it that it has only appeared in animals to which a sympatheticcolouring is advantageous in the struggle for life? And the answer, from our point of view, must read: it has arisen through natural selection in those species to which a sympathetic colouring is useful. Thus even if the supposition that sympathetic colouring is due to automatic photography on the part of the skin were correct, we should still have to regard it as an outcome of natural selection; but it is not correct—at least in general—as the above objection shows, and as will be further apparent from many of the phenomena of colour-adaptation which I shall now adduce.

To explain sympathetic coloration, then, we must assume, with Darwin and Wallace, a process of selection due to the fact that, as changes took place in the course of time in the colouring of the surroundings, those individuals on an average most easily escaped the persecution of their enemies which diverged least in colour from their surroundings, and so, in the course of generations, an ever greater harmony with this colouring was established. Variations in colouring crop up everywhere, and as soon as these reached such a degree as to afford their possessors a more effective protection than the colouring of their fellows, then natural selection of necessity stepped in, and would only cease to act when the harmony with the environment had become complete, or, at least, so nearly so that any increase of it could not heighten the deception.

Of course, it is presupposed in the working out this selective process that the species has enemies which see. This is the case, however, with most animals living on the earth or in the water, unless they are of microscopic minuteness. Many animals, too, are subject to persecution not only in their adult state, but at almost every period of their life, and so, in general, we should expect that many of them would have attained at each stage that coloration of body that would render them least liable to discovery by their enemies.

And this is in reality the case: numerous animals are protected in some measure by so-called sympathetic colouring, from the egg to the adult state.

Let us begin with the egg, and of course there is no need to speak of any eggs except those which are laid. Of these many are simply white in colour, e.g. the eggs of many birds, snakes, and lizards, and this seems to contradict our prediction; but these eggs are either hidden in earth, compost, or sand, as in the case of the reptiles, or they are laid in dome-shaped nests, or concealed in holes in trees, as in many birds; thus they require no protective colouring.

In other cases, however, numerous eggs, especially of insects andbirds, possess a colouring which makes it very difficult to distinguish them from their usual surroundings. Our large green grasshopper (Locusta viridissima) lays its eggs in the earth, and they are brown, exactly like the earth which surrounds them. They are enough in themselves to refute the hypothesis that sympathetic colouring has arisen through self-photography, for these eggs lie in total darkness in the ground. Insect-eggs which are laid on the bark of trees are often grey-brown or whitish like it, and the eggs of the humming-bird hawk-moth (Macroglossa stellatarum), which are attached singly to the leaves of the bedstraw, have the same beautiful light-green colour as these leaves, and, in point of fact, green is a predominant colour of the eggs in a very large number of insects.

But the eggs of many birds, too, exhibit 'sympathetic' colouring; thus the curlew (Numenius arquata) has green eggs, which are laid in the grass; but the red grouse (Lagopus scoticus) lays blackish-brown eggs, exactly of the colour of the surrounding moor-soil; and it has been observed that they remain uncovered for twelve days, for the hen lays only one egg daily, and does not begin to brood until the whole number of twelve is complete. Herein lies the reason of the colour-adaptation, which the eggs would not have required, if they had always been covered by the brooding bird.

The eggs of birds are frequently not of one colour only; those of the Alpine ptarmigan (Lagopus alpinus), for instance, are ochre-yellow with brown and red-brown dots, resembling the nest, which is carelessly constructed of dry parts of plants. Sometimes this mingling of colours reaches an astonishing degree of resemblance to surroundings, as in the golden plover (Charadrius pluvialis), whose eggs, like those of the peewit (Vanellus cristatus), are laid among stones and grasses, not in a true nest, but in a flat depression in the sand, and, protected by a motley speckling with streaking of white, yellow, grey and brown, are excellently concealed. Perhaps the eggs of the sandpipers and gulls are even better protected, for their colouring is a mingling of yellow, brown, and grey, which imitates the sand in which they are laid so perfectly, that one may easily tread on them before becoming aware of them.

But let us now turn from eggs to adult animals. Darwin first pointed out that the fauna of great regions may exhibit one and the same ground-colouring, as is the case in the Arctic zone and in the deserts. The most diverse inhabitants of these regions show quite similar coloration, namely, that which harmonizes with the dominant colour of the region itself. It is not only the persecuted animals, which need protection, that are sympathetically coloured in thesecases, the persecutors themselves are likewise adapted, and this need not surprise us, when we remember that the very existence of a beast of prey depends on its being able to gain possession of its victims, and that therefore it must be of the greatest use to it to contrast as little as possible with its surroundings, and thus be able to steal on its quarry unperceived. Those that are best adapted in colour will secure the most abundant food, and will reproduce most prolifically; and they will thus have a better prospect of transmitting their usual colouring to their offspring. The Polar bear would starve if he were brown or grey, like his relatives; among the ice and snow of the Polar regions his victims, the seals, would see him coming from afar.

In the Arctic zone the adaptation of the colouring of the animals to the white of the surroundings is particularly striking. Most of the mammals there are pure white, or approximately white, at least during the long winter; and it is easily understood that they must be so if they are to survive in the midst of the snow and ice,—both beasts of prey and their victims. For the latter the sympathetic colouring is of 'protective' value; for the former, of 'aggressive' value (Poulton). Thus we find not only the Polar hare and the snow-bunting white, but also the Arctic fox, the Polar bear, and the great snowy owl; and though the brown sable is an exception, that is intelligible enough, for he lives on trees, and is best concealed when he cowers close to the dark trunk and branches. For him there would be no advantage in being white, and therefore he has not become so.

Desert animals are also almost all sympathetically coloured, that is, they are of a peculiarly sandy yellow, or yellowish-brown, or clayey-yellow, or a mixture of all these colours; and here again the beasts of prey and their victims are similarly coloured. The lion must be almost invisible from a short distance, when he steals along towards his prey, crouching close to the ground; but the camel too, the various species of antelope, the giraffe, all the smaller mammals, and also the horned viper (Vipera cerastes), the Egyptian spectacled snake (Naja haje), many lizards, geckos, and the great Varanus, numerous birds, not a few insects, especially locusts, show the colours of the desert. It is true that the birds often have very conspicuous colours, such as white on breast and under parts, but the upper surface is coloured like the desert, and conceals them from pursuers whenever they cower close to the ground. It has even been observed that a locust of the genusTryxalisis of a light sand-colour in the sandy part of the Libyan desert, but dark brown in its rocky parts, thus illustrating a double adaptation in the same species.

Another group, which agrees in colour with the general surroundings, is that of the 'glass-animals,' as they have been called, though perhaps 'crystal animals' is a better term. A great number of simple free-swimming marine forms, and a few fresh-water ones, are quite colourless, and perfectly transparent, or have at most a bluish or greenish tinge, and on this account they are quite invisible as long as they remain in the water. In our lakes there lives a little crustacean about a centimetre in length, of the order of water-fleas (Leptodora hyalina), a mighty hunter among the smallest animals, which swims forward jerkily with its long swimming-appendages, and widely spreads its six pairs of claws, armed with thorny bristles, like a weir basket, to seize its prey. We may have dozens of these in a glass of water without being able to see a single one, even when we hold the glass against the light, for the creatures are crystal-clear and transparent, and have exactly the same refractive power as the water. It requires a very sharp scrutiny and a knowledge of the animals to be able to detect in the water little yellowish stripes, which are the stomachs of the animals filled with food in process of digestion, for which, as we can readily understand, invisibility cannot very well be arranged. If the water be then strained through a fine cloth, a little gelatine-like mass of the bodies of theLeptodorawill remain on the sieve.

A great many of the lower marine animals are equally transparent, and as clear as water; most of the lower Medusæ, the ctenophores, various molluscs, the barrel-shaped Salpæ, worms, many crustaceans of quite different orders, and above all an enormous number of larvæ of the most diverse animal groups. I can remember seeing the sea at the shore at Mentone so full of Salpæ, that in every glass of sea-water drawn at random there were many of them, and sometimes a glass held a positive animal soup. But one did not see them in the glass of water, and only those who knew what to look for recognized them by the bluish intestinal sac that lies posteriorly in the invisible body. But when the water was poured off through a fine net, there remained on the filter a large mass of a crystalline gelatinous substance.

It is obvious that this must serve as a protective arrangement, for the animals are not seen by their pursuers; but it is not anabsoluteprotection, for they have many pursuers who do not wait till they see their prey, but are almost constantly snapping the mouth open and shut, leaving it to chance to bring them their prey.No protective arrangement, however, affords absolute security; it protects against some enemies, perhaps against many, but never against all.

But now let us turn to a group of a different colouring, the greenanimals. We are familiar with our big grass-green grasshopper, and we know how easily it is overlooked when it sits quietly on a high grass-stem, surrounded by grasses and herbage; the light grass-green of its whole body protects it most effectively from discovery: for myself, at least, I must confess that in a flowery meadow I have stood right in front of one, and have looked close to it for a long time without detecting it. In the same way countless insects of the most diverse groups—bugs, dipterous flies, sawflies, butterflies—and especially the larvæ (caterpillars) of the last, are of the same green as the plants on which they live, and this again applies to the predaceous species, as well as the species preyed upon. Thus the rapacious praying-mantis (Mantis religiosa) is as green as the grass in which it lurks motionless for its victim—a dragonfly, a fly, or a butterfly.

There are also green spiders, green amphibians like the edible frog, and especially the tree-frog, green reptiles like lizards and the tree-snakes of tropical forests. It is always animals which live among green that are green in colour.

We may wonder, for a moment, why there are so few green birds, since they spend so much of their time among the green leaves. But this paucity of green birds is only true of temperate climates. In Germany we have only the green woodpecker, the siskin, and a few other little birds, and even these are not of a bright green, but are rather greyish-green. The explanation lies in the long winter, when the trees are leafless. In the evergreen forests of the tropics there are numerous green birds belonging to very diverse families.

Yet another group with a common colour-adaptation deserves mention—the beasts of the night. They are all more or less grey, brown, yellowish, or a mixture of these colours, and it is obvious that, in the duskiness of night, they must blend better with their environment on this account. White mice and white rats cannot exist under natural conditions, since they are conspicuous in the night, and the same would be true of white bats, nightjars, and owls; but all of these have a coloration suited to nocturnal habits.

A very remarkable fact is that in many animals the colour-adaptation is a double one. Thus the Arctic fox is white only in winter, while in summer he is greyish-brown; the ermine changes in the same way, and the great white snowy owl of the Arctic regions has in summer a grey-brown variegated plumage. Many animals which are subject to persecution also change colour with the seasons, like the mountain hare (Lepus variabilis), which is brown in summer and pure white in winter, the Lapland lemming, and the ptarmigan (Lagopus alpinus), which do the same. It has beendoubted whether natural selection can explain this double coloration, but I do not know where the difficulty lies, and there is certainly no other principle whose aid we can evoke. The mountain hare must have had some sort of colour before it attained to seasonal dimorphism. Let us assume that it was brown, that the climate became colder and the winter longer, then those hares would have most chance of surviving which became lighter in winter, and so a white race was formed. Poulton has shown that the whiteness is due to the fact that the dark hairs of the summer coat grow white as they lengthen at the beginning of winter, and the abundance of new hairs which complete the winter coat are from the first white throughout. If the white hairs were to persist throughout the summer it would be very disadvantageous to their wearer; so a double selection must take place, in summer the individuals which remain white, in winter those which remain brown, being most frequently eliminated, so that only those would be left which were brown in summer and white in winter. This double selection would be favoured by the fact that there would be, in any case, a change of fur at the beginning of summer; the winter hairs fall out and the fur becomes thinner. The process does not differ essentially from that which takes place in any species when two or more parts or characters, which are not directly connected, have to be changed, such as, for instance, colour and fertility. The struggle for existence will in this case be favourable, on the one hand, to the advantageously coloured, and on the other to the most fertile, and though the two characters may at first only occur separately, they will soon be united by free crossing, until ultimately only those individuals will occur which are at once the most favourably coloured and the most fertile. So in this case there remain only those which are brown in summer and white in winter.

We must ascribe to the influence of the processes of selection the exact regulation of the duration of the winter and summer dress, which has been carefully studied in the case of the variable hare. In the high Alps it remains white for six or seven months, in the south of Norway for eight months, in Northern Norway for nine months, and in Northern Greenland it never loses its white coat at all, as there the snow, even in summer, melts only in some places and for a short time. But apart from concealment there is certainly another adaptation involved here—namely, the growth of the hair as a protection against the cold. From an old experiment made in 1835 by Captain J. Ross, and recently brought to light again by Poulton, we learn that a captive lemming kept in a room in winter did not change colour until it was exposed to the cold. The constitution ofanimals which become white in winter is thus so organized that the setting in of cold weather acts as a stimulus which incites the skin to the production of white hairs. This predisposition also we must refer to the influence of natural selection, since it must have been very useful to the species that the winter coat should grow just when it was necessary as a protection against cold. This explains at the same time why the predisposition to respond to the stimulus of cold by a growth of winter fur finds expression earlier in those colonies of Arctic animals, such as the hare, which live in Lapland, than in those which live in the south of Norway.

But that it is not thedirectinfluence of cold which colours the hair of a furred animal white we can see from our common hare (Lepus timidus), which, in spite of the winter's cold, does not become white, but retains its brown coat, and not less so from the mountain hare (Lepus variabilis), which in the south of Sweden also remains brown, although the winter there may be exceedingly cold. But as the covering of the ground with snow is not so uninterrupted there as in the higher North, a white coat would be not a better protection than a brown one, but a worse. The white colouring of Arctic animals is therefore not directly due to the influence of the climate, as has often been maintained, but is due to it indirectly, that is, through the operation of natural selection. I have tried to make this clear by means of this example, so that we may not have to repeat it in considering those which are to follow.

But all attempts at any other explanation are even more decidedly excluded when we turn our attention to more complicated cases of colour-adaptation, which are not confined to the simple, general coloration, but are helped by markings and colour-patterns, that is, by schemes of colour.

Thus numerous caterpillars exhibit definite lines and spots on their ground-colouring, which, in one way or another, aid in protecting them from their enemies.

Fig. 2.Longitudinally stripedcaterpillar of a Satyrid.After Rösel.

The green grass-eating caterpillar of many of ourSatyridæhas two or more darker or lighter lines running down the sides of its body, which make it much less conspicuous among the grasses on which it feeds than if it were a uniform green mass (Fig. 2). Not infrequently the colour and form present a remarkably close resemblance to the inflorescences or fruit-ears of the grasses. Caterpillars marked thus are never found on the leaves of trees, where they would immediately catch the eye. It is true that longitudinal striping often occurs on caterpillars which live on other plants besides grass, but as these other plants grow among the grasses theprotective efficacy is just the same. This is the case with the Pieridæ (Garden Whites).

All the caterpillars of our Sphingidæ, on the other hand, which live on bushes and trees, have on the sides of the segments light oblique stripes, seven in number, which are disposed to the longitudinal axis of the body at the same angle as the lateral veins of a leaf of their food-plant have to the mid-rib. It cannot of course be said that the caterpillar thereby gains the appearance of a leaf, indeed, if one sees it apart from its food-plant it does not look in the least like a leaf, but among the leaves of a bush or tree this marking secures it in a high degree from discovery. Thus the caterpillar of the eyed hawk-moth (Smerinthus ocellatus), when it is sitting among the crowded foliage of a willow, is often very difficult to find, because its large green body does not appear as a single green spot, but is divided by the oblique lateral stripes into sections like the half of a willow leaf, so that even a searching glance is led astray, there being nothing to focus attention on the animal as distinguished from its surroundings (Fig. 3). As a boy I often had the interesting experience of overlooking a caterpillar which was sitting just before me, until after a time I chanced to hit upon the exact spot in the field of vision.

Fig. 3.Full-grown caterpillar of the EyedHawk-moth,Smerinthus ocellatus.sb, the subdorsalstripe.

In the majority of these caterpillars with oblique stripes, the likeness to the half of a leaf is heightened by the fact that the light oblique row is accompanied by a broader coloured band, suggesting the shade of the leaf's mid-rib. The caterpillar ofSphinx ligustrihas a lilac band, and that ofSphinx atroposa blue one. In both cases it is difficult to believe that such striking colours can secure the animals from discovery, yet among the blending shadows of the leaf-complex of their food-plant they greatly increase their resemblance to a leaf-surface. Of the death's-head caterpillar (Sphinxatropos) this sounds almost incredible, for this form is chiefly a bright golden yellow, and the narrow white oblique stripes have sky-blue borders becoming darker towards the under side; but it must not be forgotten that the potato is not the true food-plant of the species, for it lives, in its true home in Africa, and also in the south of Spain, on wild solanaceous plants, which, we are informed by Noll, have precisely these colours—golden-yellow and blue in the blossom, the fruit, and in part also in the leaves and stem. There the caterpillars sit the whole day long on the plants, while with us they have formed the habit of feeding only in the twilight and at night, and concealing themselves in the earth by day, a habit that is found in other caterpillars also, and which we must again ascribe to a process of natural selection.

Fig. 4.Full-grown caterpillar of the Elephant Hawk-moth (Chærocampa elpenor) in its"terrifying attitude."

Some caterpillars exhibit other, more complex markings, which do not protect them by rendering them difficult to detect, but by terrifying the enemy who has discovered them, and warning him away. Such terrifying or aggressive colours are to be found, for instance, in the caterpillars of the Sphingid genusChærocampain the form of large eye-like spots, which occur in pairs close together on the fourth and fifth segments of the animal. Children and those unfamiliar with animals take these for true eyes; and as the caterpillar, when it is threatened by an enemy, draws in the head and anterior segments, so that the fourth one is greatly distended, the eye-spots seem to stand on a thick head (Fig. 4), and it cannot be wondered at that the smaller birds, lizards, and other enemies are so terrified that they refrain from attacking. Even hens hesitate to seize such a caterpillar in its defiant attitude, and I once looked on for a long time in a hen-coop while one hen after another rushed to pick up a caterpillar I had placed there, but, when close to it, hastily drew back the head already prepared to strike. Even a gallant cock was a long time in making up his mind to attack the terrible beast, and drew back repeatedly before he at length ventured to strike a deadly blow with his bill. After the first stroke the caterpillar, of course, was lost. Thus even this disguise is only arelativeprotection, effective only against smaller enemies. But that these are really frightened away, I had once an opportunity of observing, when I puta caterpillar of the common elephant hawk-moth (Chærocampa elpenor) in the feeding-trough of a hencoop, and a sparrow flew down to feed from the trough. It descended at first with its back to the caterpillar and fed cheerily. But when by chance it turned round, and spied the caterpillar, it scurried hastily away.

Fig. 5.The Eyed Hawk-moth in its 'terrifying attitude.'

Among Lepidoptera, too, eye-spots often occur on the wings, and to some extent, at least, they have in this case also the significance of warning marks. Take, for instance, the large blue and black eye-spots on the posterior wings of the eyed hawk-moth (Smerinthus ocellatus). When the insect is sitting quietly the two spots are not visible, as they are covered by the anterior wings, but as soon as the creature is alarmed it spreads all four wings, and now both eyes stand boldly out on the red posterior wings and alarm the assailant, as they give the impression of the head of a much larger animal (see Fig. 5). There are also eye-like spots which have not this significance and effect, as, for instance, the 'eye-spots' on the train-feathers of the peacock and the Argus pheasant, or the little eye-like spots on the under surface of many diurnal butterflies. In the first case, it is a matter of decoration; in the second, perhaps of the mimicry of dewdrops, which increases still further the resemblance to a withered leaf; but there are undoubtedly many cases in which the eye-spots serve as means of frightening off enemies, and these cases are especially common among butterflies.

Such warning marks are in no way contradictory to the sympathetic colouring of the rest of the body, and indeed we usually find them in combination with it. In some cases the eye-spot, though very conspicuous, is covered, as in the eyed hawk-moth, when at rest,by the sympathetically coloured parts—in this instance the anterior wings. In other cases eye-spots of considerable size lie clearly exposed, but exhibit the same sympathetic colours as the whole of the rest of the wing-surface. In this case they do not interfere with the protective influence of general colouring, because they are only visible from a very short distance. This is the case in the largeCaligospecies of South America, which only fly for a short time in the early morning and in the evening, remaining concealed throughout the day in dark shadowy places, where the mingled colouring of brown, grey, yellow, and black on the under surfaces of the wings prevents their being recognized from a distance as butterflies at all. But even the best sympathetic colouring is not an absolute protection, and when the insect is discovered by an enemy near at hand, the terrifying mark, a large deep-black spot on the posterior wing, comes into effect, and scares the assailant away.

Fig. 6.Under surface of the wings ofCaligo.

In such cases the sympathetic colouring was probably the first to arise, and the eye-spot was developed later by a new process of selection, brought about by the necessity of protecting the species more effectively than by mere inconspicuousness alone. In many cases it can be proved that the power of scaring off an enemy did not begin with the formation of the eye-spot, but with the development of a new instinct. When the caterpillar ofChærocampa elpenoris attacked it immediately assumes the defiant attitude described above, but the same striking attitude is assumed by the caterpillars of the allied American genusDarapsa, as I learn from an old illustration by Abbot and Smith, although this form possesses no eye-spots (Fig. 7). Thus, then, metaphorically speaking, the caterpillar at first attempted to scare off its enemy by a terrifying attitude alone, and it was only subsequently, in the course of the phyletic evolution, that the eye-spots were added, in the elephant hawk-moths and other species, to heighten the terrifying effect. But that the eye-spot did not make its appearance suddenly is proved by severalAmerican species ofSmerinthus, in which they are much less perfectly developed than in the European species. In these Sphingidæ, too, the defiant attitude was evolved earlier than the eye-spots, as we may see from our poplar hawk-moth (Smerinthus populi), which, when alarmed, spreads out all four wings in the same peculiar manner which in the eyed hawk-moth (Smerinthus ocellatus) displays the eye-spots; it strikes about with its wings as if to scare off the enemy, an effect which will certainly be more surely achieved if, at the same time, a pair of eyes becomes suddenly visible.

Fig. 7.Caterpillar of a North AmericanDarapsain its "terrifying attitude" (afterAbbot and Smith).

Sympathetically coloured caterpillars are, however, by no means the only ones; there are some with such striking, glaring colours that, far from rendering their possessors inconspicuous, they make them visible from a long way off; but this apparent contradiction of the theory of the colour-adaptation of animals that require protection has been explained by the acuteness of Alfred Russel Wallace. We know that among insects, and also among caterpillars, there are many which have a repulsive taste. In any case, certain caterpillars are rejected by many birds and lizards. Such species are, therefore, relatively safe from being devoured. If they were protectively coloured, or if, moreover, they resembled caterpillars with an agreeable taste, they would gain little advantage from their unpalatability; for the birds would at first take them for eatable, and would only discover their repulsiveness on attempting to eat them. But a caterpillar which has received a single stroke from a bird's bill is doomed to death. It must therefore be of the greatest advantage for unpalatable caterpillars, and unpalatable animals generally, to be in their colouring as conspicuously distinguishable as possible from the edible species. Hence, then, the glaring colours, which we can now refer without any further difficulty to the process of natural selection, for every individual of an ill-tasting species that is more conspicuously coloured than its fellows must have an advantage over them, and must have a better chance of surviving, because it will be less easily mistaken for a member of an edible species.

I should like to discuss one other phenomenon, which is well calculated to give us a deeper insight into the transformation processes of organisms—I refer to the remarkable dimorphism of colour which occurs in many of the species of caterpillar just described.

The caterpillar of the convolvulus hawk-moth (Sphinx convolvuli)is in its full-grown stage green, like the wild convolvulus on which it lives, or brown like the ground on which its food-plant grows. It thus shows a double adaptation, each of which is capable of protecting it to a certain extent, and we might think to thesameextent. But that is not so, the brown colouring is a more effective protection than the green, as we may learn from two facts. In the first place, the four young stages of the caterpillar are green, and it only becomes brown in the last stage, though sometimes even then it remains green. This shows that the brown is a relatively modern adaptation, and it could not have arisen had it not been better than the original green. In the second place, the green-coloured caterpillars of the convolvulus hawk-moth are nowadays much less numerous than the brown ones, and this implies that the latter survive oftener in the struggle for existence. We have here an interesting case of an easily recognizable process of selection still going on between the old green and the newer brown variety.

It is hardly necessary to ask why the brown colour should in this case be a better protection than the green, for it is obvious that such a large green body as that of the full-grown convolvulus-caterpillar would be but badly concealed among the little leaves of the convolvulus plant in spite of its green colour; while the brown caterpillar, on the brown soil, with its pebbles, hollows, and irregular shadows, is excellently protected, especially if it passes the day concealed in the ground, as is actually the case.

Our view is materially strengthened by the fact that the same phenomenon of double colouring occurs in several allied species of Sphingidæ, but in a manner which shows us that we have to do with a similar process of transformation, only at a more advanced stage. The caterpillar ofChærocampa elpenor(Fig. 4) shows the same state of things as that of the convolvulus hawk-moth; it is brown or green, and the green form is the less common. But in the two other European species ofChærocampathe full-grown caterpillar is always brown, and indeed it becomes brown in the fourth stage, instead of, likeChærocampa elpenor, only in the fifth and last. Another indigenous sphingid species,Deilephila vespertilio, only remains green during the first two stages, and assumes in the third stage the grey-brown colour which it afterwards retains. The dark colour has obviously prevailed among the full-grown caterpillars for a considerable length of time, for it is in this, the largest and most conspicuous stage, that the change of colour must have been most necessary, and consequently the process of selection must have begun in it, and only after the more protective brown became general would it have extended to the nextstage below, if it were of use there too, and, later on, to still earlier stages in the life-history.

One might be inclined to ascribe this shunting back of a new character from the later to the earlier stages of development to purely internal forces, which brought it about of necessity, and quite independently of whether the extension of the character was useful or injurious. We shall come back to this later, and try to find out how far this is the case, but in the meantime we may regard at least so much as established, that this shunting back does not take place everywhere and without limits, but that natural selection calls a halt as soon as its effect would be injurious.

Fig. 8.Caterpillar of the Buckthorn Hawk-moth,Deilephila hippophaës.A, Stage III.B, Stage V.r, ring-spots.

There could be no continuance of insect-metamorphosis if every character of the final stage had to be shunted back to the one next below, for then, for instance, the characters of the butterfly must, in the course of the phyletic evolution, be carried back to the pupa and larva. But even in the larval stage alone it can be seen that this carrying back is kept within well-defined limits. Thus, for instance, in the dimorphic caterpillars of the Sphingidæ the brown of the full-grown stage never comes so far down as the earliest stages, for the little caterpillars are all green, like the leaves and stems on which they sit. On the other hand, there are species in which the green persists, as apparently the most advantageous colour. Thus in the buckthorn hawk-moth (Deilephila hippophaës) (Fig. 8), which lives in the warm valleys of the Alps, and especially in Valais, the caterpillars are grey-green in all stages, and are exactly of the shade of the lower surface of the buckthorn leaves; they possess no oblique lines, for these would not make them more like the leaves, as the full-grown caterpillars are much bigger than an individual leaf of buckthorn,on which, moreover, the lateral veins are not very conspicuous. Nevertheless the caterpillar enjoys very fair security, as it does not feed through the day, but only in twilight and at night; it passes the daytime concealed in the dry leaves and earth about the base of the bush. Its resemblance to the leaves is very great, and is increased by the fact that it bears on the last segment a comparatively large orange-coloured spot (r), exactly the colour of the buckthorn berry, which ripens just at the time that the caterpillar attains its full growth.

But butterflies are as much persecuted, and have as much need of protection, as caterpillars, and among them, too, we find many instances of protective colouring, which are the more interesting in that they occur, as a rule, only on such parts of the body as remain visible when the insect is at rest, which is exactly what we should expect if the coloration has been wrought out in the course of natural selection. But it is well known that the resting position of diurnal Lepidoptera is quite different from that of the nocturnal forms, and is not even the same among all families, and in accordance with this we find the sympathetic colouring occurs on quite different areas in the different families.

The reason why the butterflies only require to be protected by their colour in the sleeping or resting position is that no colour whatever could make a flying butterfly invisible to its enemies, because the background against which its body shows is continually changing during its flight, and, moreover, the movement alone is enough to betray it, even if it is of a dull colour.

Thus, in general, only those parts of a butterfly's wing that are invisible at rest could safely bear bright or conspicuous colour, while the visible portions had to acquire sympathetic coloration through natural selection.

As the diurnal butterflies, when at rest, turn their wings upward and bring them together, it is only the under side which is sympathetically coloured, and that only as far as it is visible, that is, the whole of the posterior wing, and as much of the anterior one as is not covered by it. Many diurnal butterflies, when at rest, fold the anterior wing so far back that only its tip remains visible, and in such cases only this tip is protectively coloured, while in other forms, which have not this habit, almost the whole surface of the wing is sympathetically coloured.

A very simple protective colouring is exhibited by our 'lemon butterfly' (Rhodocera rhamni), in which the under surface is a whitish yellow, which protects the insect well when it settles onthe dry leaves on the ground in the light woods which it is fond of frequenting.

Our gayest diurnal butterflies, the species ofVanessa, all have the under surface of a dusky colour, sometimes passing into a blackish brown, as in the peacock-butterfly,Vanessa(v. io), sometimes more into greyish brown, or brown-yellow, or reddish brown. They are never simple colours, but always consist of mixtures of different colour-tones—indeed, there is often a complex mingling of many colours, as grey, brown, black, white, green, blue, yellow, and red, made up of dots, strokes, spots, and rings, into a wonderful and very constant pattern, which, taken as a whole, has the effect of being uniform, and harmonizes with the soil, or with the highway, on which the species loves to settle, with much greater accuracy than a monochrome grey or brown would do. When the 'painted lady' (Vanessa cardui) settles on the ground it is hardly distinguishable from it, and this species in particular has a preference for settling on the ground. Other species ofVanessa, such as the peacock and the Camberwell beauty (Vanessa antiopa), are underneath of a dark blackish grey, or even black; when resting they press themselves into the darkest corners and crevices, and are thus most effectively secured from discovery.

Many diurnal Lepidoptera, on the other hand, especially the wood-butterflies of the family Satyridæ, have the habit of resting on the trunks of trees, asSatyrus proserpinadoes on the great beech-trunks of the forest clearings. These large butterflies, coloured conspicuously on the upper surface in deep velvety black and white, are marked on the under surface exactly to match the whitish bark of the great beech, covered over with white, grey, blackish-brown, and yellow spots, and the butterfly whose flight one has just been carefully following disappears as it suddenly alights on such a tree-trunk. As I have already stated, the protective colour only extends over as much of the insect as is seen when it is at rest. As the anterior wings are folded far back between the posterior ones, the protective colouring is limited to the whole surface of the posterior wing, and the tip of the anterior one, as far as that is visible in the resting attitude; the protectively coloured area is somewhat sharply bounded, and it is often of very different extent in quite nearly allied species, according to whether the species folds the anterior wing far back or not. Thus in our common small tortoiseshell-butterfly (Vanessa urticæ) the protective area is considerably wider than in the large tortoiseshell (Vanessa polychloros), much as the two resemble each other in other details.

This harmony between the wing tips and the posterior wings isnowhere wanting, where the under side is protectively coloured at all, but in many cases the protective colouring spreads over almost the whole of the anterior wings, and these are then not folded far back when at rest, as will be seen later in the so-called leaf-butterflies.

There is one genus of diurnal butterflies which seems to contradict the law that all the surface that is visible in the resting position exhibits the protective coloration—the South American wood-butterflies of the genusAgeronia. They have on the upper surface a very complicated bark-like pattern of mingled grey on grey, and this confirms the usual rule, for we know that these butterflies—a striking exception among all the other diurnal forms—settle with outspread wings on the trunk of a tree in exactly the same attitude as many of the nocturnal Lepidoptera of the family of the Loopers or Geometridæ, in which the upper surface is also deceptively like the bark of the tree on which they rest.

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