Moreover, there are cases in which we can prove that specific differences are of an adaptive nature. When, of two nearly related species of frog, the spermatozoon of one possesses a thick head and that of the other a thin head, and when at the same time the micropyle through which alone the spermatozoon can make its way into the ovum is wide in the first species and narrow in the second, we have before us a specific character which is obviously adaptive.
In order to gain clearness as to the significance of natural selection in the restricted sense, that is of personal selection, it seems to me much more important to study the different groups of animals and plants with special reference to what they undoubtedly exhibit in the way of adaptation. For that reason I discussed different groups of adaptations in detail in some of the preceding lectures, although, or rather because, they all teach us that every part of every species, whether animal or plant, even every secretion, and indeed every habit, every inherited instinct, is subject to adaptation to the conditions of life. It seems difficult to refuse to admit that this is the natural impression which this study conveys, and it is strengthened as our knowledge increases; that every essential part of a species is not merely regulated by natural selection, but is originally produced by it, if not in the species under consideration at the time, then in some ancestral species; and, further, that every part can adjust itself in a high degree to the need for adaptation. It was not without a purpose that I discussed the phenomenon of mimicry so fully, for it, above all others, teaches us how great a power of adaptation the organism possesses, and what insignificant and small parts may be transformed, in a remarkable degree, in accordance with some actual need. We saw that a butterfly might assume a colouring which diverged entirely from that of its nearest relatives, but which caused it to resemble an immune species of a different family, and thereby protected it more effectively from persecution. Such a case can no more be due to a dominating phyletic force than to a chance and sudden displacement of the state of equilibrium of the determinant system; it can depend only on natural selection, that is, on a sifting out of the diverse variations offered by germinal selection, and the unhampered expression and augmentation of those favoured.
But it is not only these minute variations, insignificant in relation to the whole structure of the animal, which can be determined by natural selection. The same applies to the phyletic evolution as a whole; even that is not directed by the assumed internal principle of development.
Adaptations, from their very nature, can only depend upon selection, and not upon an internal principle of evolution, since that could take no account whatever of external circumstances, but would cause variations in the organism altogether independently of these. Thus, in considering the origin of any of the larger groups of animals, we may exclude a phyletic power as the guide of its evolution as soon as we can prove that all its essential structural relations, as far as they diverge from those of nearly related groups, are adaptations. We may not be able to do this for nearly all of the animal groups, and it will hardly be possible in regard to a single group of plants, because our insight into the biological significance of characters, which means more than the functional significance of the individual parts, and their correlation as parts of a whole, is seldom sufficiently intimate or thorough. But among animals we can do this in regard to some groups; one of these is the order of whales or Cetaceans.
Fig. 130. Skeleton of a Greenland
Whale with the contour of the body.
Ok, upper jaw. Uk, lower jaw. Sch,
shoulder-blade. OA, upper arm.
UA, bones of fore-arm. H, hand.
Br, vestige of the pelvis. Fr, vestige
of the femur. Tr, vestige of the lower
part of the leg. After Claus.
Cetaceans, as is well known, belong to the Mammalia, that is to say, to a class whose structure was built for life on the land. The ancestors of Cetaceans were similar to the other mammals, and possessed a coat of hair and four legs, and a body the mass of which was so distributed that it could be borne by those four legs. But all the modern Cetaceans live in the sea, and they have therefore entirely changed their bodily form; they have become spindle-shaped like fishes, well adapted for cleaving the water, but incapable of moving upon land. At the same time, their hind-legs have completely disappeared, and can now be demonstrated only as rudiments within the mass of muscle (Fig. 130, Br, Tr, Fr), while the fore-legs have been transformed into flippers, in which, however, the whole inherited, but greatly shortened, skeleton of the mammalian arm is concealed (OA, UA, H). The skin has lost its covering of hair so completely that in some cases no traces of it are demonstrable except in the embryo. All these changes are adaptations to an aquatic life, and could not have been produced independently of the influence of external conditions. But there is much more than this. A thick layer of blubber under the skin gives this warm-blooded animal an effective protection against being cooled down by the surrounding water, and at the same time gives it the appropriate specific gravity for life in the sea; an enormous tail-fin similar to that of fishes, but placed horizontally, forms the chief organ of locomotion, and for this reason the hind-legs became superfluous and degenerated. Similarly, the muscles of the ear have also disappeared, for the hearing organ of this aquatic type is no longer suited for receiving the sound-waves through an air-containing trumpet, but receives them by a shorter route from the surrounding water, directly through the bones of the skull. Remarkable changes in the respiratory and circulatory organs make prolonged submersion possible, and the displacement of the external nares from the snout to the forehead enables the animal to draw breath when it comes up from the depths to a frequently stormy surface. It would take a long time to enumerate all that can be recognized as adaptive in these remarkable aquatic mammals to a life in what to their ancestors would have been a strange and hostile element. Let us study particularly the case of the whalebone whales, for instance the Greenland whale, and we are at once struck by the enormous size of the head, which makes up about a third of the whole body (Fig. 130). Can this, which has such an important effect in determining the whole type of animal, be an outcome of some internal power of development? By no means! It is rather an adaptation to the mode of nutrition peculiar to this swimming mammal, for it does not, like dolphins and toothed whales, feed on large fishes and Cephalopods, but on minute delicate molluscs—Pteropods and pelagic Gastropods, on Salpæ, and the like, which often cover the surface of the Arctic Ocean in endless shoals, sometimes extending for many miles. To enable the whale to sustain life on such minute morsels it was necessary that it should be able to swallow enormous quantities; teeth were therefore useless, and they have become rudimentary, and can only be demonstrated in the embryo as rudiments (dental germs) in the jaw; but in place of these there hang from the roof of the mouth-cavity great plates of 'whalebone,' a quite peculiar product of the mucous membrane of the mouth, the ends of which are frayed into fibres, and form a sieve-net for catching the little animals which are engulfed with the sea-water. The mouth-cavity itself has become enormous, so that great quantities of water at a time can be strained through the net of whalebone-plates.
When I mention that peculiar changes have occurred also in the internal organs, that the lungs have elongated longitudinally and thus enable the animal more readily to lie in the water in a horizontal position, that peculiar arrangements exist within the nostrils and the larynx which enable the animal to breathe and swallow simultaneously, and that the diaphragm lies almost horizontally because of the length of the lung, I think I have said enough to indicate that not only does almost everything about the whale diverge from the usual mammalian type, but that all these deviations are adaptations to an aquatic life. If everything that is characteristic, that is, typical either of the order or of the family to which animals belong, depends upon adaptation, what room is left for the activity of an internal power of evolution? How much is left of the whale when the adaptations are subtracted? Nothing more than the general scheme of a mammal; but this was implicit in their ancestors before the whales originated at all. But if what makes whales what they are, that is, the whole 'scheme' of a whale, has originated through adaptation, then the hypothetical evolutionary power—wherever its seat may be—has had no share in the origin of this group of animals.
I said all this more than ten years ago, but the idea of an internal directive evolutionary force is firmly rooted in many minds, and new modifications of the idea are always cropping up, and of these the most dangerous seem to me to be those which are not clear in themselves, but suppose that the use of a shibboleth like 'organic growth' means anything. That organic growth is at the base of the phyletic evolution of organisms may be maintained from any scientific standpoint whatsoever, from ours as well as from Nägeli's, for no one is so extreme and one-sided as to regard the process of evolution as due solely to internal or solely to external factors. The process may thus always be compared to the growth of a plant, which likewise depends on both internal and external influences. But that is saying very little; we have still to show how much and how little is effected by these internal and external factors, what their nature precisely is, and what relation they bear to one another. There is thus a great difference between believing, with Nägeli, that 'the animal and plant kingdoms must have become very much what they actually are, even had there been on the earth no adaptation to new conditions and no competition in the struggle for existence,' and sharply emphasizing, in accordance with the facts just discussed, that, in any case, a whole order of mammals—the Cetaceans—could never have arisen at all if there had been no adaptation.
The same thing could be proved in regard to the class of Birds, for in them too we are able to recognize so many adaptive features, that we may say everything about them that makes them birds depends upon adaptation to aerial life, from the articulations of the backbone to the structure of the skull and the existence of a bill; from the transformation of the fore-limbs into wings, and of the hind-limbs to very original organs of locomotion on land or swimming organs in water, to the structure of the bones, the position, size, and number of the internal organs, down even to the microscopic structure of numerous tissues and parts. What could be more characteristic of a class of animals than feathers are of birds? They alone are enough to distinguish the class from all other living classes; an animal with feathers can now be nothing but a bird, and yet the feather is a skin-structure which has arisen through adaptation, a reptilian scale which has been so transformed that an organ of flight could develop from its anterior extremity. We find it thus even in the two impressions of the primitive bird Archcæopteryx, which have been preserved for us in the Solenhofen slate since the Jurassic period in the history of the earth. And into what detail does adaptation go in the case of the feathers! Is not the whole structure, with its quill, shaft, and vane, precisely adapted to its function, although that is purely passive? What I have just said of the whole class of Birds holds true for this individual structure, the feather; everything about it is adaptation, and indeed illustrates adaptation in two directions, for in the first place the feathers, by spreading a broad, light, and yet resistant surface with which to beat the air, act as organs of flight, while they are also the most effective warmth-retaining covering conceivable. In both these directions their achievements border on the marvellous. I need only recall the most recent discovery in this domain, the proof recently given by the Viennese physiologist, Sigmund Exner, that the feathers become positively electric in their superficial layer, and negatively electric in their deeper layer, whenever they rub against one another and strike the air. But they are rubbed whenever the bird flies or moves, and the consequence of the contrast in the electric charging of the two layers is that the covering feathers are closely apposed over the down-feathers, while, on the other hand, the similar charging of the down-feathers makes them mutually repel each other, with the result that a layer of air is retained between them, and thus there is between the skin and the covering feathers a loose thicket of feathers uniformly penetrated by air—the most effective warmth-preserver imaginable. The electric characters of the feathers—and the same is true of the hairs of animals—are thus not indifferent characters, but with an appreciable biological importance, and the same is true of the almost microscopical series of little hooklets which attach the barbs of the covering feathers to one another, and thus form a relatively firm but exceedingly light wing-surface which offers a strong resistance to the air. But as we must regard these hooklets as adaptations, so must we also regard the electrical characters of the feathers, and we must think of them as having arisen through natural selection, as Exner himself has insisted.