The Evolution Theory, Vol. 2 of 2

OBJECTIONS TO THE THESIS THAT FUNCTIONAL
MODIFICATIONS ARE NOT TRANSMITTED

It was Herbert Spencer, the English philosopher, who first brought the argument of co-adaptation into the field against my view of the non-inheritance of functionally acquired modifications. He pointed out that many, if not, indeed, most modifications of bodily parts, to be effective, implied further changes, often very numerous, in other parts, and these latter must therefore have changed simultaneously with the part which was being changed under the control of natural selection; this, however, is only conceivable as due to an inheritance of the changes caused by use, since a simultaneous alteration of so many parts through natural selection would be impossible. If, for instance, the antlers of our modern stag were to grow to the size of those of the Giant Stag of the Irish peat-bogs, which measured over ten feet across from tip to tip, this would mean—as has already been shown—a simultaneous thickening of the skull, and to bear the heavy burden, a strengthening of the ligamentum nuchæ, of the muscles of the neck and back, of the bones of the legs and their muscles, and, finally, of all the nerves supplying the muscles; and how could all this happen simultaneously with, and in exact proportion to the growth of the antlers, if it depended—as natural selection assumes—on chance variations of all these parts? What if the appropriately favourable variation in one of these organs did not occur? A harmonious variation of all the parts—bones, muscles, nerves, ligaments—which unite in a common activity, is an inadmissible assumption, because, in many cases, such co-operating groups of organs have in the course of evolution developed in opposite directions. In the giraffe, for instance, the fore-legs are longer than the hind-legs, which is the reverse of what obtains in the majority of ruminants; in the kangaroo the hind-legs, on the contrary, have developed to a disproportionate size, while the fore-legs have degenerated into relatively small grasping arms. Co-operating parts, like the fore and hind limbs, may thus follow opposite paths of evolution; their variations need not always be directed to the same end.

The difficulty presented by these so-called co-adaptations or harmonious correlations cannot be denied, and we must also admit that, if the results of exercise were inherited, the explanation of the phenomenon would, in many cases—but not, indeed, in all—be easy, because the adaptation of the secondarily varying parts in each individual life would correspond exactly to the altered function of the part, and would be transmitted to the descendants, and in them would again be subject to such a degree of variation, according to the principle of histonal selection, as might be conditioned by the further progress of the primary variation. The simplicity of the explanation is striking, if only it were at the same time correct! But there are whole series of facts, or rather of groups of facts, which prove that the causes of co-adaptation do not lie in the inheritance of functional modifications, and this must be recognized, even though we may not yet be in a position to state the causes of co-adaptation, and to say whether natural selection suffices to explain it or not.

I must first point out that co-adaptations occur not only in actively, but also in passively functioning parts. Very numerous instructive examples are to be found among the Arthropods, whose whole skeleton belongs to this category. It has been objected that this is not wholly passive, but that, like the bones of vertebrates, it is stimulated by the contraction of the muscles and incited to functional reaction, and that it thickens at places where strong muscles are inserted, and becomes or remains thin where it is not exposed to any strain from the muscles. But this is not the case, for the chitinous skeleton can only offer resistance to the muscular contractions when it is no longer soft, as it is immediately after it is secreted. As soon as it has become hard, it can no longer be altered, and can at most be worn away externally by long use. The proof of this lies in the necessity for moulting, which is indispensable to all Arthropods as long as they continue to grow, but does not occur later. Every one who has followed the growth of an insect or a crustacean knows well that the moultings or ecdyses are often accompanied by great changes, and hardly ever occur without some slight changes in the form of the body, especially of the limbs, with their teeth, bristles, spines, and so on. These new or transformed parts are formed before the throwing-off of the old chitinous shell, and under its protection, and they are brought about by an elaboration or transformation of the living soft matrix of the skeleton, the hypodermis, which consists of cells, and is the true skin. They must thus have arisen in the ancestors of our modern Arthropods in the same way, that is, not by a gradual modification during use, but by a slight sudden transformation before use. The steps in the transformation may have been very small, a bristle may have become a little longer in the second stage of life than it was in the first, or instead of five bristles a particular spot may bear six in the second or third stage of life; but the variations in the phyletic development must always be caused by germ-variations which effect from within the variation in the relevant stage of development. But the part which has varied can only function after it has become firm and immodifiable.

If these circumstances be kept clearly in mind, they furnish a quite overwhelming mass of proof against the views of the Lamarckians.

Furthermore, it is not even true that the thickest parts of the external skeleton are those at which the muscles are inserted. The wing-covers of beetles offer the best proof to the contrary, for there are no muscles at all in them, yet they are, in many species, the hardest and thickest part of the whole chitinous coat of mail. The reason is not far to seek; they protect the wings and the soft skin of the back, which lies concealed beneath them, and the muscles are inserted in this!—a relation which can be explained only by its suitability to the end, and not as due to any direct effect.

When we remember the origin—which we have just described—of the external skeleton from the soft layer of cells underneath it, the thickness of the chitinous skeleton, which is very different at different places in the same animal, but always adapted to its end, furnishes a case of co-adaptation in parts which have a purely passive function. The thickened part cannot be due to the insertion of a muscle, but it is always there in advance, from internal causes, so that the muscle finds sufficient resistance. Close to it there may lie, perhaps, the edge of a segment, and at this spot the chitinous skeleton becomes almost suddenly thinned to a joint membrane capable of being bent or folded, not because there was no pull from the muscles at this spot, but in order that the two segments may be connected movably. Thus, nowhere in the whole body of the Arthropod can the adaptation of the skeleton, in regard to thickness and power of resistance, be regulated by function itself, but only by processes of selection which imparted to each spot the thickness it required, in order to be effective in its function, whether that be offering resistance to the strain of the muscles, or giving suppleness to a joint, or affording the necessary hardness for biting the prey, or for boring into wood or earth, or merely for protecting the animal from external injuries.

There are, however, many individual functions of the Arthropods the exercise of which depends on the simultaneous change of several skeletal parts; as, for instance, many of the 'singing' or vocal apparatuses in insects. In quite recent times such vocal organs have been discovered in ants, in which they consist of a small striated region on the surface of the third abdominal segment, and a sharp ridge on the segment in front; the latter is rubbed against the former by the movements of the two segments. Quite a similar 'stridulating organ' has long been known in the bee-ant (Mutilla), and the whistling sound produced by it is easily heard by our ears; moreover August Forel has heard it in the large wood-ant (Camponotus ligniperdus), and has described it as an 'alarm-signal,' which the animals give each other on the approach of danger—an observation which has recently been confirmed by Wasmann and extended by Robert Wroughton in regard to Indian ants. All these arrangements for producing sound depend always on two organs, of which one resembles the bow, the other the strings of a violin; the one is of no value without the other, and they must therefore have developed simultaneously, yet they cannot have arisen through use, and the inheritance of the results of use, because they are both dead chitinous parts, which are never strengthened by rubbing against each other with the movements of the abdomen, but are rather worn away.

The same is true of the chirping organs of grasshoppers, beetles, and crickets; in all cases they consist of two different parts, which together produce a sound, and which therefore must have arisen simultaneously, and the origin of which cannot be referred to the inheritance of the results of exercise, but rather to selection. It is thus possible that co-adaptation of at least two parts may take place even when the hypothetical Lamarckian principle is altogether excluded.

When I say that we have here a case of two parts adapted to each other, that is, strictly speaking, understating the case, for, in the crickets and locusts, for instance, there is a whole series of peg-like chitinous papillæ (Fig. 86), the so-called 'bridge,' each of which must have arisen by itself through variation of the corresponding spot of skin. At least I can see no ground for the assumption that the chitinous surfaces on which the 'bridge' is now placed would necessarily, from internal reasons, have varied precisely in the line of the bridge as it has done.

Instructive examples of the co-adaptation of several parts to a common action in organs which are not subject to the Lamarckian principle are afforded by the diverse arrangements for cleaning the antennæ the bearers of the smelling-organ which are so important to the life of insects (Fig. 102). Here even the adaptation of an indented area on the tibia of the anterior leg to the cylindrical form of the antenna which passes through it, is sometimes so striking (Fig. 102, tak) that it might be thought that it must have arisen through a gradual wearing out; yet this is impossible, since we have to do with hard dead chitinous surfaces, and moreover not with a solid mass, like a hone, which is worn down by the knife, but with a hollow, thin-walled tube. In ants, bees, and ichneumon-flies this minute, semi-circular indentation contains small, pointed, triangular saw-teeth, closely set like those of a comb (tak), and the apparatus is made usable by the fact that a firm spine (tisp), fused to the end of the tibia, overhangs the notch and presses the antenna towards it. In many species this spine is double, or it is furnished with a thin comb or lappet (Fig. 102, L), or with rows of teeth, or with short bristles; in short, it may be equipped in the most different ways. Not infrequently, as in wasps of the genera Sphex, Scolia, Ammophila, the spine itself is also bent in a semicircle on the surface directed towards the notch, and this may be effected in very different ways, either by a bending of the whole thickness of the spine, or by the presence of a comb which is concave on its inner surface. I should never come to an end if I were to enumerate all the remarkable details which may be found in the two main parts of this apparatus, and which show very clearly how essential a co-operation of the two is in fulfilling the function of cleaning the antennæ. This fitting together of the two main parts cannot have been brought about in accordance with the Lamarckian principle; the adaptation must therefore have come about in some other way.

The same thing is shown by the legs and other appendages of insects and crustaceans, which are adapted for the most diverse functions, and the individual sections of which must be correlated if the function is to be possible. Let us consider only the claw structures in crustaceans and scorpions. Here, too, it seems as if the outgrowth of the last joint of the leg, which functions as the arm of the claw, must have arisen as a direct effect of use, through the pressure of an object held fast by the last joint, the movable half of the claw. Frequently, moreover, tooth-like protuberances occur on the fixed blade of the claw (Fig. 103). But how could these have arisen as a direct effect of pressure, since they are always preformed during the soft state of the appendage before use, and are only made use of after it is fully hardened. The soft crustaceans, the so-called 'butter-crabs' which have just cast their shells, creep carefully away and avoid using their limbs until they have become hard again. Here, too, we have the co-adaptation of two parts which vary independently, and which cannot be affected by the Lamarckian principle.

But the appendages furnish more complex examples of mutual adaptation. Thus the individual sections of the anterior leg of the mole-cricket (Gryllotalpa) have varied greatly, yet quite differently, and the whole together forms a most effective digging-tool. With it the animal digs out the earth before it to right and to left, and to do this it makes with both legs simultaneous outward movements, which are otherwise quite unusual among insects, and does so with such strength that Rösel von Rosenhof saw two bodies each weighing three pounds pushed away in this manner. In this case four chief parts of the leg (Fig. 104), the coxa (cox), the femur (fe), the tibia (tib), and the tarsi (tars) are so adapted to each other in form, joints, thickness of skeleton, and size, that they cannot have varied otherwise than in relation to each other, but each piece has done so in an individual manner. Most remarkable of all is the short broad tibia, equipped with four large, hard teeth, which has to perform the digging in the ground after the manner of a spade, while the disproportionately thin and weak tarsal joints, the last of which bears two perfectly straight spines instead of claws, are directed upwards, and do not touch the ground, being no longer used for walking. Rösel supposed, probably correctly, that they are used for cleaning the spade when it becomes clogged up with earth, since the animal cannot clean it with its mouth. These quite unusually formed parts of the limb cannot have become what they are as the direct results of use, because, for one thing, it would have been not their broad surfaces, but their narrow edges, which would most easily cut through the earth, that would have been directed outwards. The peculiar curving, first concave, then convex, of the outer surface of the digging foot is exactly what is best adapted for cutting into the earth and for the pushing aside which follows, but it is not what it would have become if the chitin-wall had yielded to the pressure of the earth and adapted itself to it. But, as we are again dealing with the chitinous skeleton, there can be no question of the direct effect of use, and, it seems to me, it must be admitted that here we have a case of co-adaptation of at least seven different parts, which have varied independently of each other, without any assistance from the Lamarckian principle.

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].

Against this proposition an observation by O. vom Rath has been cited. According to it, three drone larvæ which had been accidentally fed by the workers with royal food exhibited striking retrogressive peculiarities in their sexual organs. The testes contained only immature sperms (just before emergence from the pupa), and the copulatory organ was entirely wanting. That a certain degree of fatty degeneration of the testes should be caused by the 'unusual fattening' is not surprising, but it seems to me very questionable whether the absence of the copulatory organ can be referred to the abnormal diet; it ought to be definitely decided, by the investigation of numerous cases, whether some abnormal peculiarity in the constitution of the germ-plasm in these eggs was not the true cause. Hitherto, unfortunately, I have not been able to procure the fresh material necessary to decide this point[15]. From all this it must be evident that we are not justified in regarding either the absence of wings or the degeneration of the ovaries as a direct result of the inferior nourishment supplied to the workers in the larval state: but should any one still have doubt on this point I may mention that, among our indigenous ants, there are two species in which the workers are just as large as the fertile females, and that in tropical America a species (Myrmecocystus megalocola) occurs in which the workers are larger than the true females; this must mean that they have received more food than the females, though perhaps not the same mixture of food.

From all the facts we have discussed we can confidently conclude that the differences in structure, which distinguish the workers from the true females, do not depend upon the influence, in the individual lifetime, of a poorer diet, but upon variation in the primary constituents of the germ; we must conceive of the germ-plasm of ants as containing, in addition to male and female ids, special ids of workers, in which the determinants of wing and ovary are degenerate in some degree, while the determinants of other parts, such as the brain, are more highly developed. The manner of feeding, however, and perhaps the mingling with the food of a special secretion of the salivary glands, acts as a stimulus which determines whether one kind of id or another is to be liberated, that is, to become active and to enter on the path of development.

A proof of this view is to be found, it seems to me, in the existence of transition forms between workers and true females, which was first brought to general knowledge by Forel. Perhaps it would be better to call these 'mongrel forms' for their various parts do not maintain a medium between the two types, but many parts follow the type of the worker, and others that of the true female. Thus Forel twice found a nest of the red wood-ant which contained a large number of these mixed forms, all of which possessed the small head and large curved thorax of the queen, but otherwise resembled the workers in size and appearance, and also in the degeneration of the ovaries. Many of them were very small, only 5 mm. in length, and had probably received very little food, and, according to the theory of direct influence, these should have been pure workers. That they possessed the head and thorax of a queen is a proof that the characters of both forms of individual were present in the germ-plasm as primary constituents, or indeed entire ids. In normal circumstances only one kind of these ids would have become active, either the worker-id or the queen-id, but in abnormal circumstances they might both be liberated to activity simultaneously, and then they would stamp one part of the body with the character of a queen, another with that of a worker. Forel observed one of these nests in two successive years, and both times found the mixed forms in large numbers[16]. In the second year he found a great number of newly-emerged individuals of this type. I have already inferred from this observation that the mixed forms were probably in both years the offspring of the same mother, and this may well have been the case. My further conclusion, that the mixed forms must be due to some abnormality in the constitution of the germ-plasm of the maternal eggs, no longer appears to me so convincing as it did formerly, because, in the interval, we have learnt, through that indefatigable investigator of ants, Pater Wasmann, that there is another possible explanation of these mixed forms; it, too, is based upon a hypothesis, but it is so interesting that I must briefly outline it to you.

Like Forel and myself, Pater Wasmann had supposed that the reason of this kind of mixed form (the so-called pseudogynous worker) lay in an abnormality of the constitution of the germ-plasm, but he now regards it as the result of a change in the mode of rearing instituted by the workers with respect to the constitutionally female or queen larvæ, because there was a scarcity of workers. The hypothesis sounds very daring, but it is well founded, at least in so far that there really is a reason why a scarcity of females must occur at certain times in some colonies of ants, and this might certainly determine the workers in charge of the larvæ to feed females with worker food, so as to rear them to render the necessary assistance.

This reason lies in the occasional presence of a parasitic beetle, Lomechusa strumosa, whose larvæ, curiously enough, are cared for and fed by the ants as though they were their own, and in return they eat up the larvæ of the ants, often destroying them in large numbers. Wasmann informs us that the parasitic larvæ grow up just at the time at which the ants are rearing their workers, and it is these, therefore, which fall victims to the Lomechusa-larvæ, and the result is that a scarcity of young workers must soon make itself felt. The workers seek to make this good by rearing as workers all the larvæ previously destined for queens. But this only succeeds partially, because the development towards true females has already begun; thus mixed forms arise.

This explanation would be rather in the air if we did not know that, among bees, such changes in the manner of rearing are by no means uncommon. Indeed they occur regularly when the queen of a hive perishes and no more 'female' eggs are in store; young worker larvæ are then fed with royal food, and these develop into queens. There can thus be no doubt that these insects have it in their power to liberate to activity either the female ids or the worker ids by a specific mode of feeding, and there is nothing contrary to reason in admitting the possibility of an alternation of this influence in the course of development, for something analogous occurs in regard to secondary sexual characters, as, for instance, the appearance of male decorative colours in ducks that have become sterile.

But this change in the mode of rearing bee-larvæ gives rise to pure queens and not to mixed forms, and we must therefore regard it as undecided whether Wasmann's explanation is correct in this case, and whether an abnormality in the constitution of the germ-plasm may not be the true cause of this or other kinds of mixed forms among ants. In any case the 'Lomechusa hypothesis' rests upon the assumption of different kinds of ids in the germ-plasm, as Pater Wasmann expressly states, and the differences between the worker and queen-ants have their cause in this, and not directly in the kind of larval food.

If there were not different ids corresponding to the different kinds of individuals in the germ-plasm a kind of polymorphism might indeed have arisen in the colony through differences in nutrition, but it could not have been of the kind we now see—that is, a sharply defined differentiation of persons, in adaptation to their different functions. This presupposes elements in the germ which can vary slowly and consistently in a definite direction without causing any change in the rest of the germ.

This state of affairs gives to the phyletic evolution of the workers a great theoretical significance, for it proves that positive as well as negative variations of the most diverse parts of the body, that simultaneous and correlative variations of many parts, can take place in the course of the phylogeny, without the co-operation of the Lamarckian factor. I have not hitherto laid any special emphasis upon the degree of differences occurring between workers and queens; but I must now add that this may far exceed the degree that we are familiar with in our common indigenous ants, both in regard to instinct and to bodily form. Even in the red Amazon ant of Western Switzerland, Polyergus rufescens, we find quite a new instinct[17], that of carrying off the pupæ of other species of ants, not to devour, but to introduce them to their own nest and thus secure 'slaves.' For these workers of a strange species, which emerge in a strange nest, naturally regard the place of their birth as their home, and do there what instinct impels them, and what they would have done in the nest of their parents: they feed the larvæ, fetch food, collect building material, and so on. The domestic activity of the workers of the master-species thus becomes superfluous, and they have ceased to exercise it, and have now entirely lost the power of caring for their brood, searching for food, and keeping up the nest. They have even forgotten how to take food themselves, because they are always fed by the 'slaves.' Forel informs us—and I have myself repeated the experiment—that Polyergus workers, which are shut up with a drop of honey on the floor of their prison, will leave it, their favourite food, untouched, and finally starve, unless one of their 'slaves' be shut up with them. As soon as this happens, and the slave perceives the honey, it partakes of it, and then the 'mistress' comes and strokes the 'slave' with her antennæ to signify her desires, whereupon the 'slave' proceeds to feed her from its own crop.

But while the Polyergus workers have forgotten their domestic habits, and have even ceased to be able to recognize their food, remarkable changes have taken place in their jaws; these have lost the blunt teeth on the inner margin, which, in other species, serve for masticating the food, for seizing building material, and for other domestic occupations, and have become sharp weapons, bent in the form of a sabre, very well suited for piercing the head of an enemy, but also well adapted for carrying off the pupæ, because they can seize them without doing them any injury.

No one will doubt that the predatory expeditions of the Amazon ants, and the slave-making habit, can only have developed after the habit of living in large companies had long existed, and this case proves that variations of instinct, as well as of bodily structure, can take place even after the workers have long been sterile. The case is the more instructive that it seems as if it were due to the transmission of a newly acquired and inherited habit of life, while in point of fact these Amazon-workers can transmit nothing, because they bear no offspring. But if old instincts can be lost, and new ones acquired, when all possibility of inheritance is excluded, we see that Nature has no need of the Lamarckian factor of modification for her transformations and new adaptations.

If we wish to understand clearly that, in these changes, we have to do not merely with the alteration of a single part, but of many parts which all work together, we have only to think of the still more striking physical modifications which have taken place in many tropical ants, and which have led to a dimorphism of the workers. In many species, certainly, the only difference is in size, so that one can distinguish between large workers and small, and the former are sometimes five times as big as the latter. But even in the South European Pheidole megalocephala, which is abundant in Italy, the larger workers are also different in structure from the smaller, for they have an enormous head with powerful jaws. They are usually known as 'soldiers,' and are entrusted with the defence of the colony. Emery directly observed in regard to Colobopsis truncata, an ant which lives in the trunks of trees, that the soldiers, with their enormous heads, occupied all the entrances to the nest, ready to seize any intruder with their powerful jaws. In the Sauba ant (Œcodoma cephalotes) Bates described three different types of worker, differing in size, and although he was not able to determine with certainty what the particular function of each was, there can be no doubt that they have special offices, and that the differences in their structure are adaptations to the differences in their functions. The same is true of the Indian ant, Pheidologeton diversus, depicted in Fig. 106, whose three forms of workers I owe to the kindness of Professor August Forel.

If the increase in the size of the head and jaws must bring with it an increase in the thickness of the skeleton of these parts, as well as a strengthening of the musculature of the head, it follows that the strain on the body must be greater, just as in the case of the increase in the weight of the stag's antlers, so that the skeleton of the thorax must likewise have become thicker and heavier, the muscles and nerves of the legs stronger, the articulations of the joints capable of greater resistance; in short, a whole series of variations of other parts must have taken place simultaneously, if the primary variation was to be of use, and not to lead to the destruction of its possessor. Here again we have a proof that the co-adaptation of many parts can take place without any intervention of the Lamarckian principle, and that there must be some other factor which brings this about.