Evolution and Adaptation

by Thomas Hunt Morgan

Structural Adaptations

The most striking cases of adaptations are those in which a special, in the sense of an unusual, relation exists between the individual and its surroundings. For example, the foreleg of the mole is admirably suited for digging underground. A similar modification is found in an entirely different group of the animal kingdom, namely, in the mole-cricket, in which the first legs are also well suited for digging. By their use the mole-cricket makes a burrow near the surface of the ground, similar to, but of course much smaller than, that made by the mole. In both of these cases the adaptation is the more obvious, because, while the leg of the mole is formed on the same general plan as that of other vertebrates, and the leg of the mole-cricket has the same fundamental structure as that of other insects, yet in both cases the details of structure and the general proportions have been so altered, that the leg is fitted for entirely different purposes from that to which the legs of other vertebrates and of other insects are put. The wing of the bat is another excellent case of a special adaptation. It is a modified fore-limb having a strong membrane stretched between the fingers, which are greatly elongated. Here we find a structure, which in other mammals is used as an organ for supporting the body, and for progression on the ground, changed into one for flying in the air.

The tails of mammals show a number of different adaptations. The tail is prehensile in some of the monkeys; and not only can the monkey direct its tail toward a branch in order to grasp it, but the tail can be wrapped around the branch and hold on so firmly that the monkey can swing freely, hanging by its tail alone. The animal has thus a sort of fifth hand, one as it were in the middle line of the body, which can be used as a hold-fast, while the fingered hands are put to other uses. In the squirrels the bushy tail serves as a protection during the winter for those parts of the body not so thickly covered by hair. The tail of the horse is used to brush away the flies that settle on the hind parts of the body. In other mammals, the dog, the cat, and the rat, for example, the tail is of less obvious use, although the suggestion has been made that it may serve as a sort of rudder when the animal is running rapidly. In several other cases, as in the rabbit and in the higher apes, the tail is very short, and is of no apparent use; and in man it has completely disappeared.

A peculiar case of adaptation is the so-called basket on the third pair of legs of the worker honey-bee. A depression of the outer surface of the tibia is arched over by stiff hairs. The pollen collected from the stamens of flowers is stowed away in this receptacle by means of the other pairs of legs. The structure is unique, and is not found in any other insects except the bees. It is, moreover, present only in the worker bees, and is absent in the queen and the males.

The preceding cases, in which the adapted parts are used for the ordinary purposes of life of the individual, are not essentially different from the cases in which the organ is used to protect the animal from its enemies. The bad taste of certain insects is supposed to protect them from being eaten by birds. Cases like this of passive protection grade off in turn into those in which, by some reflex or voluntary act, the animal protects itself. The bad-smelling horns of the caterpillar of the black swallow-tailed butterfly (Papilio polyxenes) are thrust out when the animal is touched, and it is believed that they serve to protect the caterpillar from attack. The fœtid secretion of the glands of the skunk is believed to serve as a protection to the animal, although the presence of the nauseous odor may lead finally to the extermination of the skunk by man. The sting of bees and of wasps serves to protect the individual from attack. The sting was originally an ovipositor, and used in laying the eggs. It has, secondarily, been changed into an organ of offence.

The special instincts and reflex acts furnish a striking group of adaptations. The building of the spider’s web is one of the most remarkable cases of this kind. The construction of the web cannot be the result of imitation, since, in many instances, the young are born in the spring of the year following the death of the parents. Each species of spider has its own type of web, and each web has as characteristic a form as has the spider itself. It is also important to find that a certain type of web may be characteristic of an entire family of spiders. Since, in many cases, the web is the means of securing the insects used for food, it fulfils a purpose necessary for the welfare of the spider.

The making of the nests by birds appears to be also in large part an instinctive act; although some writers are inclined to think that memory of the nest in which the young birds lived plays a part in their actions, and imitation of the old birds at the time of nest-building may, perhaps, also enter into the result. It has been stated that the first nest built by young birds is less perfect than that built by older birds, but this may be due to the bird’s learning something themselves in building their nests, i.e. to the perfecting of the instinct in the individual that makes use of it. In any case much remains that must be purely instinctive. The construction of the comb by bees appears to be largely, perhaps entirely, an instinctive act. That this is the case was shown by isolating young workers as soon as they emerged from the cell, and before they could have had any experience in seeing comb built. When given some wax they set to work to make a comb, and made the characteristic six-sided structures like those made by the bees in a hive. The formation of so remarkable a structure as the comb is worthy of admiration, for, with the greatest economy of material, a most perfect storeroom for the preservation of the honey is secured. This adaptation appears almost in the nature of foresight, for the store of honey is used not only to feed the young, but may be drawn on by the bees themselves in time of need. It is true that a comparison with other kinds of bees makes it probable that the comb was first made for the eggs and larvæ, and only later became used as a storehouse, but so far as its form is concerned there is the same economy of constructive materials in either case.

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.

There can be little doubt in some of these cases that the color of the animal may be a protection to it, but as has been hinted already, it is another question whether it acquired these colors because of their usefulness. Nevertheless, if the color is useful to its possessor, it is an adaptation in our sense of the word, without regard to the way in which it has been acquired. Even, for instance, if the resemblance were purely the outcome of chance in the sense that the color appeared without relation to the surroundings, it would still be an adaptation if it were of use to the animal under the ordinary conditions of life.

In the lower groups numerous cases in which animals resemble their surroundings could be given. Such cases are known in crustacea, worms, mollusks, hydroids, etc., and the possible value of these resemblances may be admitted in many instances.

It is rather curious that so few cases of adaptive coloration have been described for plants. No one supposes that the slate color of the lichen is connected with the color of the rocks on which it grows, in the sense that the resemblance is of any use to the lichen. Nor does the color of the marine red algæ serve in any way to protect the plants so far as is known. The green color of nearly all the higher plants is obviously connected with the substance, chlorophyl, that is essential for the processes of assimilation, and has no relation to external objects. But when we come to the colors of flowers we meet with curious cases of adaptation, at least according to the generally accepted point of view. For it is believed by many naturalists that the color of the corolla of flowering plants is connected with the visits of insects to the flowers, and these visits are in many cases essential for the cross-fertilization of the flowers. This adaptation is one useful to the species, rather than the individual, and belongs to another category.

The leaf of the Venus’s fly-trap, which suddenly closes together from the sides when a fly or other light body comes to rest on it, is certainly a remarkable adaptation. A copious secretion of a digestive fluid is poured out on the surface of the leaf, and the products of digestion are absorbed. There can be no question that this contrivance is of some use to the plant. In other insectivorous plants, the pitcher plants, the leaves are transformed into pitchers. In Nepenthes a digestive fluid is secreted from the walls. A line of glands secreting a sweet fluid serves to attract insects to the top of the pitcher, whence they may wander or fall into the fluid inside, and there being drowned, they are digested. A lidlike cover projecting over the opening of the pitcher is supposed to be of use to keep out the rain.

In Utricularia, a submerged water-plant, the tips of the leaves are changed into small bladders, each having a small entrance closed by an elastic valve opening inwards. Small snails and crustaceans can pass into this opening, to which they are guided by small outgrowths; but once in the cup they cannot get out again, and, in fact, small animals are generally found in the bladders where they die and their substance is absorbed by forked hairs projecting into the interior of the bladder.

The cactus is a plant that is well suited to a dry climate. Its leaves have completely disappeared, and the stem has become swollen into a water-reservoir. “It has been estimated that the amount of water evaporated by a melon cactus is reduced to one six-hundredth of that given off by any equally heavy climbing-plant.”

Fig. 1.—The fertilization of Aristolochia Clematitis.
A, portion of stem with flowers in axil of leaf in different stages.
B and C, longitudinal sections of two flowers, before and after fertilization. (After Sachs.)

Sachs gives the following account of the fertilization process in Aristolochia Clematitis, which he refers to as a conspicuous and peculiar adaptation. In Figure [1 A] a group of flowers is shown, and in Figure [1 B and C] a single flower is split open to show the interior. In B a small fly has entered, and has brought in upon its back some pollen that has stuck to it in another flower. The fly has entered through the long neck which is beset with hairs which are turned inwards so that the fly can enter but cannot get out. In roaming about, the pollen that is sticking to its back will be rubbed against the stigmatic surface. “As soon as this has taken place the anthers, which have been closed hitherto, dehisc and become freely accessible,” as a result in the change in the stigma and of the collapse of the hairs at the base of the enlargement which has widened. The fly can now crawl under the anthers, and, if it does so, new pollen may stick to its back. At this time the hairs in the throat dry up, and the fly can leave its prison house, Figure [1 C]. If the fly now enters another flower this is fertilized by repeating the process. The unfertilized flowers stand erect with widely open mouths. As soon as they have been fertilized they bend down, as seen in Figure [1 A], and at the same time the terminal flap bends over the open mouth of the throat, “stopping the entrance to the flies, which have now nothing more to do here.”