Evolution and Adaptation

by Thomas Hunt Morgan

The behavior of young birds, more especially those that take care of themselves from the moment they leave the egg, furnishes a number of cases of instincts that are protective. If, for example, a flock of young pheasants is suddenly disturbed, the birds at once squat down on the ground, and remain perfectly quiet until the danger is past. Their resemblance to the ground is so perfect that they are almost invisible so long as they remain quiet. If, instead of remaining still, they were to attempt to run away when disturbed, they would be much more easily seen.

Certain solitary wasps (Ammophila) have the habit of stinging caterpillars and spiders, and dragging them to their nests, where they are stored away for the future use of the young that hatch from the eggs laid by the wasp on the body of the prey. As a result of the sting which the wasp administers to the caterpillar, the latter is paralyzed, and cannot escape from the hole in which it is stored, where it serves as food for the young wasp that emerges from the egg. It was originally claimed by Forel that the wasp stings the caterpillar in such a way that the central nervous system is always pierced, and many subsequent naturalists have marvelled at the perfection of such a wonderful instinct. But the recent results of the Peckhams have made it clear that the act of the wasp is not carried out with the precision previously supposed, although it is true that the wasp pierces the caterpillar on the lower surface where the ventral chain of ganglia lies. The habit of this wasp is not very dissimilar from that shown by many other kinds of wasps that sting their captive in order to quiet it. We need not imagine in this case that the act carries with it the consciousness that the caterpillar, quieted in this way, will be unable to escape before the young wasps have hatched.

The resemblance in color of many animals to their natural backgrounds has in recent years excited the interest and imagination of many naturalists. The name of protective coloration has been given to this group of phenomena. The following cases which have less the appearance of purely imaginative writing may serve by way of illustration. A striking example is that of the ptarmigan which has a pure white coat in winter, and a brown coat in summer. The white winter plumage renders the animal less conspicuous against the background of snow, while in summer the plumage is said to closely resemble the lichen-covered ground on which the bird rests. The snowy owl is a northern bird, whose color is supposed to make it less conspicuous, and may serve either as a protection against enemies, or may allow the owl to approach its prey unseen. It should not pass unnoticed, however, that there are white birds in other parts of the world, where their white color cannot be of any use to them as a protection. The white cockatoos, for example, are tropical birds, living amongst green foliage, where their color must make them conspicuous, rather than the reverse.

The polar bear is the only member of the family that is white, and while this can scarcely be said to protect it from enemies, because it is improbable that it has anything to fear from the other animals of the ice-fields, yet it may be claimed that the color is an adaptation to allow the animal to approach unseen its prey.

In the desert many animals are sand-colored, as seen for instance in the tawny color of the lion, the giraffe, the antelopes, and of many birds that live on or near the ground.

It has been pointed out that in the tropics and temperate zones there are many greenish and yellowish birds whose colors harmonize with the green and yellow of the trees amongst which they live; but on the other hand we must not forget that in all climes there are numbers of birds brilliantly colored, and many of these do not appear to be protected in any special way. The tanagers, humming-birds, parrots, Chinese pheasants, birds of paradise, etc., are extremely conspicuous, and so far as we can see they must be much exposed on account of the color of their plumage. Whether, therefore, we are justified in picking out certain cases as examples of adaptation, because of an agreement in color between the organism and its surroundings, and in neglecting all others, is, as has been already said, a point to be further examined.

Not only among mammals and birds have many cases of protective coloration been described by writers dealing with this subject, but in nearly every group of the animal kingdom similar cases have been recognized. The green and brown color of lizards may protect them, the green color of many frogs is supposed to conceal them as they sit amongst the plants on the edge of a stream or pond. The gray-brown color of the toad has been described as a resemblance to the dry ground, while the brilliant green of several tree-frogs conceals them very effectively amongst the leaves. Many fishes are brilliantly colored, and it has even been suggested that those living amongst corals and sea-anemonies have acquired their colors as a protection, but Darwin states that they appeared to him very conspicuous even in their highly colored environment.

Amongst insects innumerable cases of adaptive coloration have been described. In fact this is the favorite group for illustrating the marvels of protective coloration. A few examples will here serve our purpose. The oft-cited case of the butterfly Kallima is, apparently, a striking instance of protective resemblance. When at rest the wings are held together over the back, as in nearly all butterflies, so that only the under surface is exposed. This surface has an unquestionably close resemblance to a brown leaf. It is said on no less authority than that of Wallace that when this butterfly alights on a bush it is almost impossible to distinguish between it and a dead leaf. The special point in the resemblance to which attention is most often called is the distinct line running obliquely across the wings which looks like the midrib of a leaf. Whether the need of such a close resemblance to a leaf is requisite for the life of this butterfly, we do not know, of course, and so long as we do not have this information there is danger that the case may prove too much, for, if it should turn out that this remarkable case is accidental the view in regard to the resemblance may be endangered.

Amongst caterpillars there are many cases of remarkable resemblances in color between the animal and its surroundings. The green color of many of those forms that remain on the leaves of the food-plant during the day will give, even to the most casual observer, the impression that the color is for the purpose of concealment; and that it does serve to conceal the animal there can be no doubt. But even from the point of view of those who maintain that this color has been acquired because of its protective value it must be admitted that the color is insufficient, because some of these same green caterpillars are marvellously armed with an array of spines which are also supposed to be a protection against enemies. Equally well protected are the brown and mottled geometrid caterpillars. These have, moreover, the striking and unusual habit of fixing themselves by the posterior pairs of false legs, and standing still and rigid in an oblique position on the twigs to which they are affixed. So close is their resemblance to a short twig, that even when their exact position is known it is very difficult to distinguish them.

Grasshoppers that alight on the ground are, in many cases, so similar to the surface of the ground that unless their exact location is known they easily escape attention, while the green color of the katydid, a member of the same group of orthoptera, protects it from view in the green foliage of the trees where it lives. The veinlike wings certainly suggest a resemblance to a leaf, but whether there is any necessity for so close an imitation may be questioned.