When we consider such cases as this we are led to the conclusion that the usual conception of adaptation is not adequate. We require something more than function or utility to express the difference between the two kinds of characters to be distinguished. For example, the absence of pigmentation from the lower sides of Flat-fishes may have no utility whatever, but we see that it differs from the specific markings of the upper side in the fact that it shows a relation to or correspondence with a difference of external conditions—namely, the incidence of light, while in such a case as the red spots of the Plaice we can discover no such correspondence.
We know that the American artist and naturalist Thayer has shown that the lighter colour of the ventral side of birds and other animals aids greatly in reducing their visibility in their natural surroundings, the diminution in coloration compensating for the diminution in the amount of light falling on the lower side, so that the upper and lower sides reflect approximately the same amount of light, and contrast, which would be otherwise conspicuous, is avoided. But the white lower sides of Flat-fishes are either not visible at all, or, if visible, are very conspicuous, so that the utility of the character is very doubtful.
We may distinguish then between characters which correspond to external conditions, functions, or habits, and those which do not. The word 'adaptation,' which we have hitherto used, does not express satisfactorily the peculiarities of all the characters in the former of these two divisions. If we consider three examples—enlarged hind-legs for jumping as in kangaroo or frog, absence of colour from the lower sides of Flat-fishes, and, thirdly, the finlets on the lower side of Zeugopterus—we see that they represent three different kinds of characters, all related to habits or external conditions. We may say that the third kind are correlated with some other character that has a relation to function or external conditions, as the extension of the fins on the under side of Zeugopterus is correlated with the enlargement of the fins, whose function is to cause the adhesion of the fish to a vertical surface.
With regard to the specific characters of the species of Zeugopterus nothing is known of peculiarities in mode of life which would give an importance in the struggle for existence to the concrescence of the pelvic fins with the ventral in punctatus, to the absence of this character and the elongation of the first dorsal ray in unimaculatus, or to the absence of both characters in norvegicus. No use is known for any of the other specific characters, which tend in each case to form a series. Thus in size norvegicus is the smallest, unimaculatus larger, and punctatus largest, the last reaching a of 8-1/2 inches. The subcaudal fin-flaps are developed in norvegicus, most in punctatus; each has four rays in norvegicus and unimaculatus, six in punctatus. The shortening and spinulation of the scales are greatest in punctatus, least in norvegicus. In punctatus there are teeth on the vomer, in unimaculatus none, in norvegicus they are very small.
If we consider fishes in general, we see that there is no evidence of any relation between many of the most important taxonomic characters and function or external conditions. In the seas Elasmobranchs and Teleosteans exist in swarming numbers side by side, but it is impossible to say that one type is more adapted to marine life than the other. There is good reason to believe that bony fishes were evolved from Elasmobranchs in fresh water which was shallow and foul, so that lungs were evolved for breathing air, and that marine bony fishes are descended from fishes with lungs; but no reason has been given for the evolution of bone in place of cartilage or for the various kinds of scales. Professor Houssaye, on the other hand, believes that the number and position of fins is adapted to the shape and velocity of movement of each kind of fish.
If we turn to other groups of animals we find everywhere similar evidence of the distinction between adaptive and non-adaptive characters. Birds are adapted in their whole organization for flight, the structure of the wing, of the sternum, breast muscles, legs, etc., are all co-ordinated for this end. But how do we know that feathers in their origin were connected with flight? It seems equally probable that feathers arose as a mutation in place of scales in a reptile, and the feathers were then adapted for flight. Nothing shows the distinction better than convergent adaptation. Owls resemble birds of prey in bill and claw and mode of life, yet they are related to insect-eating swifts and goat-suckers and not to eagles and hawks. Swifts and swallows are similar in adaptive characters, but not in those which show relationship. It may be said that the characters believed to show true affinities were originally adaptive, but we do not know this. Similarly, in reptiles the Chelonia are distinguished by the most extraordinary union of skin-bones and internal skeleton enclosing the body in rigid armour: it may be said that the function of this is protection, that it is adaptation, and can be explained by natural selection, but the adaptation in this case is so indefinite that it is difficult to be convinced of it.
Systematists have always distinguished between adaptive characters and those of taxonomic value—those which show the true affinities—and they are perfectly right: also they have always distrusted and held aloof from theories of evolution which profess to explain all characters by one universal formula. In my opinion, those who, like Weismann, consider all taxonomic characters adaptive, are equally mistaken with Bateson and his followers, who regard all characters as mutational. No system of evolution can be satisfactory unless it recognises that these two kinds of characters are distinct and quite different in their nature. But it may be asked, What objection is there to the theory of natural selection as an explanation of adaptations? The objection is that all the evidence goes to show that the necessary variations only arose under the given conditions, and, further, that the actions of the conditions and the corresponding actions of the organism give rise to stimuli which would produce somatic modifications in the same direction as the permanent modifications which have occurred. My view is, then, that specific characters are usually not adaptations, that other characters of taxonomic value are some adaptive and some unrelated to conditions of life, and that while non-adaptive characters are due to spontaneous blastogenic variations or mutations, adaptive characters are due to the direct influence of stimuli, causing somatic modifications which become hereditary, in other words, to the inheritance of acquired characters. It has become a familiar statement that every individual is the result of its heredity and its environment. The thesis that I desire to establish is that the heredity of each individual and each type is compounded of variations or changes of two distinct origins, one external and one internal; that is to say, of variations resulting from changes originating in the germ-cells or gametes, and of modifications produced originally in the soma by the action of external stimuli, and subsequently affecting the gametes.
When we study the characters of animals in relation to sex we find that in many cases there are conspicuous organs or characters present in one sex, usually the male, which are absent or rudimentary in the other. The conception of adaptation applies to these also, since we find that characters consist often of weapons such as horns, antlers, and spurs, used in sexual combat, of copulatory or clasping organs such as the pads on a frog's forefeet, of ornamental plumage like the peacock's tail serving to charm the female, or of special pouches as in species of pipe-fish and frog for holding the eggs or young. Darwin attempted to explain sexual adaptation by sexual selection. The selective process in this case was supposed to be, not the survival of individuals best adapted to secure food or shelter or to escape from enemies, but the success of those males which were victorious in combat, or which were most attractive to the females, and therefore left the greater number of offspring which inherited their variations. But, as Darwin himself admitted, this theory of selection does not in any way explain the differences between the sexes—in other words, the limitation of the characters or organs to one sex—since there is no reason in the process of selection itself why the peculiarity of a successful male should not be inherited by his female offspring as well as by his male offspring. The real problem, then, is the sex-limited heredity, and we shall consider later whether in this kind of heredity also there are characters of internal as well as external origin, blastogenic as well as somatogenic.
CHAPTER II
Mendelism And The Heredity Of Sex