But much more complicated cases than this might be cited, if we were in a position to estimate exactly the functional value of the individual parts of the wing-venation in the different insects, for it is well known that this venation serves the systematist as a basis for the definition of genera, especially in Lepidoptera and Hymenoptera. That is to say, it varies from genus to genus in a characteristic manner, obviously corresponding to the differences in the wing-form, and in the flight itself. But, unfortunately, we are still far from being able to make more than quite general hypotheses as to the meaning of the lengthening and strengthening, or conversely, the degeneration or elimination, of this or that vein. From extreme cases, however, as for instance the rich venation in good fliers with large wings, and the scanty venation in poor fliers with small wings, we learn at least so much, that the degree and even the manner of venation bears a definite relation to the function of the wing, and this we might have assumed. But these wing-veins, in as far as they serve as a support for the weak wing-membrane, are purely chitinous structures, skeletal parts which are not even renewed from time to time like the skeletal parts of the leg and many other parts of the insect. As they are laid down at first in the pupa as soft strings of cells, so they remain, and they only begin to be used when they are completely hardened. They can therefore never have been caused to vary through use in the course of the phyletic development of species and genera, and the Lamarckian principle can have no part in their transformations. But if they follow the most subtle changes, which we cannot precisely demonstrate, of the whole wing-surface and in the mode of flight, as a man is followed by his shadow, there must be some other principle which adapts the organ to its function, and which is able continually to adapt the large number of individual wing-veins in the manner most advantageous for the general function. Here, therefore, we have a state of matters exactly corresponding to that obtaining in the transformation of actively functioning parts which form a system with common co-operative action, as, for instance, in the case we first discussed, that of the stag's antlers.
Other even more complicated examples of harmonious adaptation of passively functioning parts are afforded by the markings of animals, such as those of the butterfly's wing. The colours have only a passive rôle, whether they be due to pigments alone, or to structure, or to both combined. When the coloration of a surface undergoes adaptive variation, this cannot be due to any action of the colour, but must depend on adaptation through selection. Yet it is well known that there are many butterfly-wings whose surfaces exhibit different colours and different shades of colour on their different parts, and that in such a way that together they form a picture, that of a leaf, a piece of bark, a stone overgrown with lichen, an eye, and so on. In such a case the individual colour-spots stand in a particular, indirect relation to each other; although they are independent of each other in their variation, they are not indifferent and due to chance, for together they produce a common picture; this is harmonious adaptation of many parts, where the Lamarckian principle is absolutely excluded.
It may, perhaps, be objected that this mimetic picture does not arise all at once, but very slowly in the course of long series of generations, and, indeed, of species. This must of course be so; the simple beginnings are complicated and perfected through the course of long ages. This is implied in the principle of selection as we understand it. But does any one suppose that the gigantic antlers of the giant-stag were developed in a few generations? In this case, too, must not numerous races have succeeded each other before the primitive antlers attained this enormous size? If this must be assumed there was abundance of time for the adaptation, through germinal variations, of the secondarily varying parts, the muscles, tendons, nerves, and bones, for all these parts function actively, and can without difficulty meet, in the individual life, the increased claims made upon them by a slight increase in the size of the antlers. For the certain and indubitable consequence of exercise, of increased use, is the strengthening of the functioning parts.
Thus the appropriate germinal variation of the secondarily varying parts may be delayed for a little without the individual being any the less effective, or being obliged to succumb in the struggle for existence. I do not, however, assert for a moment that the whole explanation of the phenomena of co-adaptation is included in this; on the contrary, I hope soon to be able to show that we may in such cases assume a preponderance of variational tendencies in a favourable direction, and that there is thus an indirect connexion between the utility of a variation and its actual occurrence. In the first place, however, I must refer to the other group of facts which I have indicated, which show, likewise, that the simultaneous co-adaptation of different parts may arise in certain circumstances, although the Lamarckian principle be excluded. These are the facts presented to us by the sterile forms of those insects, which, like bees, termites, and ants, live together in large societies.
Ants and bees are of special interest to us in this connexion, because they have long been carefully watched by a number of distinguished naturalists, and most of their vital functions have been precisely studied. Ever since the days of 'Old Peter Huber' in Geneva there have again and again been excellent observers who have devoted almost the whole of their life-work and talents to the more complete study of these wonderful animals. These insects are of interest to us here, because, in the course of the social life, a type of individual has arisen which diverges in structure in many parts of the body from both the male and the female, although it is sterile and does not reproduce, or does so in so few instances that the fact is of no moment in considering the origin of the present bodily structure. As is well known, these neuters, or better, workers, are, among ants and bees, females which differ from the true females not only in their smaller size and their infertility, but in many other points as well. Among ants, for instance, they are absolutely wingless, and at the same time they have a much smaller and differently formed thorax and a larger head. But the most striking point is the difference in their instincts, for while the females, concerned only with reproduction, pair and lay eggs, it is the workers who feed and clean the helpless emerging larvæ, and put them in places of safety, who carry the pupæ into the warm sunshine, and afterwards back again to the sheltered nest, who make this nest itself, and keep it in order, after having collected or prepared the material for it; it is they alone who defend the colony against the attacks of enemies, who undertake predatory expeditions, attacking the nests of other ants, and engaging in obstinate combats with them.
How can all these peculiarities have arisen, since the workers do not reproduce, or do so only exceptionally, and, in any case, are incapable of pairing, and therefore—among bees at least—only produce male offspring? Obviously it cannot have been through the transmission of the effects of use and disuse, since they leave no offspring to which anything could be transmitted.
Herbert Spencer has attempted to maintain the position that the characters of the workers of to-day already existed in the pre-social state, that is, before the ants began to form colonies, and that, therefore, they have not been newly evolved but only preserved. But, even if this be conceded in regard to the care of the brood and the building instinct, so much remains that could not have existed at that stage, that the problem of the origin of these new characters remains unsolved. The wings, for instance, among ants, can only have been lost when females appeared which did not reproduce, for the pairing of ants is associated with a nuptial flight high in the air. The wings are not merely absent in the workers, they do not even develop in the pupæ; they are, as Dewitz showed, present even now in the larva in the form of imaginal disks, but from the pupa-stage onwards they degenerate, and the segments of the thorax to which they are attached likewise appear small and modified. A variation of the germ-plasm must therefore have taken place, and to this is due the fact that the wing-primordia no longer develop, and that the thorax has a different development from what it had at the time when the animals were still fertile.
It has indeed been said that there is no need for assuming a variation of the germ-plasm, since the degeneration of the wing might be produced by inferior nourishment. This opinion is based on the fact that, among bees, the workers do actually arise from female larvæ which have received a meagre diet poor in nitrogenous elements, while the same female larvæ supplied with an abundant diet rich in nitrogen develop into queens.
But even though we may assume that there is a similar difference in the mode of feeding among most ants, because the workers are considerably smaller than the fertile females, it would be quite erroneous to conclude that the difference between the two types rests solely on the effect of differences in diet. The elimination of an individual organ has never yet been determined by bad and scanty nourishment; it is the whole animal with all its parts that degenerates and becomes small and weakly. Often as caterpillars of different species have been placed on starvation diet, whether for experimental purposes or to procure very small butterflies, it has never yet happened that a single organ, such as antenna, leg, or wing, has thereby been eliminated or caused to degenerate. I have myself instituted many such experiments with the maggots of the blue-bottle fly, by supplying them from their earliest youth with just as little food as possible without actually starving them to death, yet never have these larvæ given rise to flies in which the wings were absent or rudimentary.
Nor did these starved flies ever exhibit degenerate ovaries; they were always completely developed and equipped with the full number of ovarian-tubes. It was to decide this particular point that these experiments were instituted, for my opponents maintained that degeneration of the ovaries was a direct result of inferior nourishment. But that is not the case. Special investigations in regard to ants, undertaken at my request by Miss Elizabeth Bickford, showed that the anatomical results reached by earlier investigators, like Adlerz and Lespès, in regard to the degeneration of the ovaries in workers, were absolutely correct, and that the 'degeneration' consists not merely in the fact that the ovarian-tubes and ovum-primordia remain small, but also in a diminution of the number of ovarian-tubes (Fig. 105); the workers have always fewer ovarian-tubes than the females of the same species, and—what is of especial importance—the reduction in the number of ovarian-tubes has been effected to a different extent in different species of ants. In the red wood-ant (Formica rufa) the workers still possess from twelve to sixteen ovarian-tubes; in the meadow-ant (Formica pratensis) only eight, six, or four; in Lasius fuliginosus there are usually only two (one on either side); and in the little turf-ant (Tetramorium cæspitum) there are none at all. We have here, therefore, a phylogenetic process of degeneration, which has reached different degrees in the different species, and has only been completed in one (Tetramorium). The case stands as I previously stated it: 'The elimination of a typical organ is not an ontogenetic process, but a phylogenetic one,' it depends not upon 'the mere influences of nutrition which affect the development of the individual, but always on variations in the germ-plasm, which, to all appearance, can only come about in the course of a long series of generations'[14].