The same is true of the eyes of deep-sea animals. It was believed at one time that all the inhabitants of dark regions had lost their eyes. This is the case with many cave animals and the inhabitants of the lightless depths of our lakes, but in the abyssal zone of the sea it is only some fishes and Crustaceans whose eyes have degenerated to the vanishing point. Moreover, the disappearance apparently occurs in species which are restricted to the ocean-floor in their search for food, which therefore can make more use of their tactile organs than of their eyes, for while the ocean-floor undoubtedly contains over wide areas an abundance of food for these mud-eaters, it is only partly illuminated, that is, only in places where there are luminous animals such as polyp-colonies, &c. The fact that so many of the animals of the great depths are luminous obviously conditions, not that most of the immigrants into the abyssal zone should lose their eyes as useless, but that they should adapt them to the light which is very weak in comparison with that of the superficial layers. The eyes of deep-sea fishes, for instance, are either enormously large, and therefore suited for perceiving the faint light of the depths, or they have varied in another and very characteristic manner: they have become elongated into a cylinder, which projects far beyond the level of the head. It looks almost as if the animals were looking through an opera-glass, and Chun has called these eyes 'telescope-eyes.' A. Brauer has recently shown what far-reaching variations of the original eye of fishes were necessary in order to transform it into an organ for seeing in the dark. These variations, however, have occurred in the eyes of the most diverse animals in the deep sea, and not only do different families of deep-sea fishes possess 'telescope-eyes,' but Crustaceans and Cephalopods as well. Even our owls possess quite a similar structure, although it does not project beyond the head in the same way. Here again we have to deal with the phenomenon which Oscar Schmidt in his time called convergence, that is, corresponding adaptations to similar conditions in animal forms not genealogically connected with one another. These telescope-eyes are not all descended from one species which chanced in one of the 'mutation-periods' suddenly to produce this combination of harmonious adaptations, but they have risen independently through variation progressing step by step in the direction of the required end, that is to say, through natural selection based upon germinal selection. Only thus can their origin be understood.
But what is time of eyes adapted to darkness is true in some measure of all eyes, for the eyes of animals are not mere decorative points which might be present or absent; they cannot have arisen in any animal whatever through sudden mutation—they have been laboriously acquired with difficulty, by the slow increase of gradually perfecting adaptations; they are parts which bear the most precise internal correlation with the whole organization of the animal, and which can only cease to exist when they become superfluous. Thus the origin of eyes seems to me only conceivable on the basis of germinal selection controlled towards what is purposeful by natural selection, that is to say, on a basis of fluctuating variation, and not through chance.
This is the case with all adaptations. Just as the eyes of animals are adaptations which utilize the light-waves in the interest of the organism and its survival, the same is true of all the sense-organs, tactile organs, smelling and tracking organs, organs of hearing, and so on. The animal cannot do without these; first the lower sense-organs arose and then the higher; the increasingly high organization of the animal conditioned this, and a multicellular animal without sensory structures is inconceivable. The same may be said of the nervous system as a whole, whose function it is to translate into action the stimuli received through the sense-organs, whether directly or by means of intervening nerve-cells, which form central organs of ever-increasing complexity of composition. As telescope-eyes have evolved in some groups of deep-sea animals, independently of one another, and certainly not through the fortuitous occurrence of a mutation, but under the compulsion of necessity in competition, so all the organs we have just named, the whole nervous system with all its sense-organs, must have arisen through the same factors of evolution in numerous independent genealogical lines. And it must not be supposed that this is all; what is true of the sense-organs—that they are necessities—is undoubtedly true also of all parts and organs of the animal body, both as a whole and in every detail. It cannot be demonstrated in all cases, but it is nevertheless certain that this applies also to all the organs of movement, digestion, and reproduction, to all animal groups and also to the differences between them, even although these may not always be obvious adaptations to the conditions of life. What part is left for mutation to play if almost everything is an adaptation? Possibly the specific differences; and these in point of fact cannot in many cases be interpreted with certainty as adaptations, though this can hardly be taken as a proof that they are not. Possibly also the geometrical skeletons of many unicellulars, in which again we cannot recognize any definite relation to the mode of life. It is easy enough to conceive of the wondrously regular and often very complex siliceous skeleton of the Radiolarians or Diatoms as due to saltatory mutations, and 'leaps' of considerable magnitude must certainly have been necessary to produce some of the manifold transformations here as everywhere else. But whether these are or are not without importance for the life of the organisms, we are in the meantime quite unable to decide. Here too it is well to be cautious in concluding that these organic 'crystallizations' are without importance, and therefore to infer that they have arisen suddenly from purely internal causes. One of the experts on Diatoms, F. Schütt, has shown us that differences in length in the skeletal process of the Peridineæ have a definite relation to their power of floating in the sea-water, that the long skeletal arms or horns which these microscopic vegetable organisms extend into the surrounding water form a float-apparatus, for their friction against the particles of the water prevents sinking and enables them to float for a considerable time at approximately the same level. These skeletal forms are thus adaptations, and Chun has recently been able to corroborate the conclusion that this adaptation is exactly regulated, for the length of these horns varies with the specific gravity of the different ocean-currents, species with 'monstrously long' horns occurring, for instance, in the Gulf of Guinea, which is distinguished by its low salinity and high temperature (Fig. 131, A), while in the equatorial currents with higher salinity and cooler water, and thus a higher specific gravity, there is a predominance of species of Peridineæ with 'very short' processes and relatively undeveloped float-apparatus (Fig. 131, B). It could be seen clearly in the course of the voyage that the long-armed Peridineæ became more abundant as the ship passed from the North Equatorial current into the Gulf of Guinea, and that by and by they held the field altogether, but later, when the 'Valdivia' entered into the South Equatorial current, they disappeared 'all at once.' Thus in this case, in which the veil over the relations between form and function in unicellular organisms has been lifted a little, we recognize that the smallest parts of the cell-body obey the laws of adaptation, and consistent thinking must lead us to the conviction that even in the most lowly organisms the whole structure in all its essential features depends upon adaptation.
Fig. 131. Peridineæ: species of
Ceratium. A, from the Gulf of
Guinea. B, from the South Equatorial
currents. After Chun.
If the horns of the Peridineæ grow to twelve times the usual length in adaptation to life in sea-water with a salinity increased to the extent of .002 per cent., then undoubtedly not only the protoplasmic particles of the body which form the horns, but all the rest as well, may be capable of adaptation; and if the Peridinium protoplasm has this power of adapting itself to the external conditions, then the capacity for adaptation must be a general character of all unicellular organisms, or rather of all living substance. As will be seen later on, we shall be brought to the same conclusion by different lines of evidence. But a recognition of this must greatly restrict the sphere of operation which we can attribute to saltatory mutations in the sense in which the term is used by De Vries, for adaptations from their very nature cannot arise suddenly, but must originate gradually and step by step, from 'variations' which combine with one another in a definite direction under the influence of the indirect, that is, selective influence of the conditions.
According to the theory of De Vries it seems as if 'variations,' augmented by selection, could never become constant, and that even the degree to which they can be augmented is very limited. As far as this last point is concerned, De Vries seems to me to overlook the fact that every increase in a character must have limits set by the harmony of the parts, which cannot be exceeded unless other parts are being varied at the same time. Artificial selection, in fact, in many cases reaches a limit which it cannot pass, because it has no control over the unknown other parts which ought to be varied, in order that the character desired may be increased still further. Natural selection would in many cases be able to accomplish this, provided that the variation is useful. But of what use is it to the beetroot when its sugar-content is doubled, or to the Anderbeck oats to be highly prized by man? And yet many individual characters have been very considerably increased in domesticated animals by selection: of these we need only call to mind the Japanese cock with tail-feathers twelve feet long.
But undoubtedly these artificial variations do not usually 'breed true' in the sense that De Vries's mutations of Œnothera lamarckiana did, that is to say, they only transmit their characters in purity with the continual co-operation of artificial selection. This at least appears to be the case, according to De Vries, in the ennobled cereal races, which, if cultivated in quantities, rapidly degenerate. In many animal breeds, however, this is not the case to the same degree; many, indeed the majority, of the most distinct races of pigeon breed true, and only degenerate when they are crossed with others.