LECTURE XXIV
OBJECTIONS TO THE THESIS THAT FUNCTIONAL
MODIFICATIONS ARE NOT TRANSMITTED
Giant stag as an example of co-adaptation or 'harmonious adaptation'—This occurs even in passively functioning parts—Skeleton of Arthropods—Stridulating organ of ants and crickets—Limbs of the mole-cricket—Wing-venation—Colorations which form mimetic pictures—Harmonious adaptations in worker-bees and ants—Degeneration of their wings and ovaries—The quality of food acts as a liberating stimulus—Vom Rath's case of drones fed with royal food—Transition-forms between females and workers—Wasmann's explanation of these—The Amazon ants—Two kinds of workers—Appendix: Zehnder on the case of ants—On the skeleton of Arthropods—Hering's interpretation of Ehrlich's Ricin experiments—Hering's position in regard to the transmission of functional modifications.
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.
Fig. 91 (repeated). Hind-leg of a
Grasshopper (Stenobothrus protorma), after
Graber. fe, femur. ti, tibia. ta, tarsal
joints. schr, the stridulating ridge.
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.
Fig. 102. Brush and comb on the
leg of a Bee (Nomada). tib, end of
the tibia. t1, first tarsal joint with the
brush and its comb (tak). Between
these and the tibial spine (tisp) with
its lappet (L) the cross-section of an
antenna (At) is indicated. Drawn from
a preparation by Dr. Petrunkewitsch.
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.
Fig. 103. Claw (Sch) on the leg of a 'Beach-fly,' an
Amphipod Crustacean (Orchestia). I, II, the two first
joints. uA, the lower blade of the claw, a non-mobile
prolongation of the penultimate joint. oA, the upper
blade of the claw, the movable last joint; the tubercles
and indentations of the two blades fit one another. After
F. Müller.
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.
Fig. 104. Digging leg
of the Mole-cricket
(Gryllotalpa). cox, coxa
attaching the limb to
the thorax. fe, the short
broad femur. tib, the
tibia forming a broad
spade with six large
sharp teeth. tars, the
tarsal joints, which are
turned upwards and
cannot be used in locomotion.
After Rösel.
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].
[14] Aeussere Einflüsse als Entwickelungsreize, Jena, 1894.
Fig. 105. Ovary of a fertile Queen-Ant and
ovaries of a Worker. Od, oviduct. A, one ovary
of Myrmica lævinodis with many ovarian-tubes,
in each of which there is an almost ripe egg
(Ei) and a younger egg (Ei´). B, the ovaries of a
Worker of Lasius fuliginosus; each ovary has only
one ovarian-tube, and no ripening egg-cells. After
Elizabeth Bickford.
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.
[15] Since completing my manuscript I note that the point was settled three years ago, when Koshewnikow had the opportunity of investigating drone-pupæ which were abnormally reared in royal cells, and therefore fed with royal food. He found their sexual organs perfectly normal, and agrees with me that the abnormalities in Vom Rath's case must have been due to some other cause. (See the report by Von Adelung on the Russian paper in Zool. Centralblatt, Sept. 10, 1901.)
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.
[16] There are different kinds of 'mixed forms' among ants, which may owe their origin to a variety of conditions, as Forel, Wasmann, and Emery have shown in detail.
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.
[17] 'New' in this sense, that the instinct is not exhibited by most worker-ants, that it did not occur in the primaeval ancestors of modern ants. It is, however, exhibited by a number of modern forms, and even by some German species.
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.
Fig. 106. Three workers of the same species of Indian Ant (Pheidologeton diversus), drawn from specimens supplied by Prof. August Forel. A, the largest, B, the intermediate, C, the smallest form.
Where, then, shall we look for this other factor, if not in the processes of selection, in the selecting of the most suitable variations among all those which occur? We are confronted with the alternative of either working out a sufficient explanation with this factor, or of giving up the attempt at explanation altogether. Yet the application of the principle of selection in relation to the neuters of colony-forming insects is by no means simple, for, as the workers are sterile, a modification of them through processes of breeding cannot begin directly with themselves. The workers which exhibit the most suitable variations cannot be selected for breeding, but only their parents, the sexual animals, and these according to whether they produce better workers or worse. This is how Darwin looked at the matter, and his view receives support from one peculiarity in the composition of these animal colonies, whose significance becomes apparent in relation to this problem. It has long been known that in a bee-hive there are from 10,000 to 20,000 workers, but only one true female, the so-called queen, and the meaning of this remarkable arrangement probably is, that the adaptation of the workers through natural selection becomes much more easily possible, since the whole number are the children of a single pair. It is not the individual workers, but the whole colony, that is, the whole progeny of the queen, which is selected, according to the greater or less degree of effectiveness displayed by the workers. Strictly speaking, it is the single queen that is selected in relation to her power of producing superior or inferior workers. A colony whose queen was unsatisfactory in this respect could not hold its own in the struggle for existence, and only the best colonies and the best hives would survive, that is, through their descendants. If the hive contained a hundred queens instead of a single queen the process of selection would be much more complex and less clear, and it is even quite conceivable that the production of specially modified workers, adapted to their functions, or of two or three different kinds of workers, would not have been possible at all. For it would not have helped much if one out of a hundred females had produced workers of better structure; only a majority of such females could give the colony any advantage as compared with other colonies.
It has not been definitely established whether, among ants, a single female is in all cases the founder of the whole colony, but it is certain that there are only a few females. In the tropical Termites we know that the ovaries of the female attain to such a colossal size that one female must certainly suffice for the necessities of the largest colonies. Grassi has shown, indeed, that, as far as the South European Termites are concerned, not only are there several females present, but that even the workers frequently reproduce; but the Termites in general are inhabitants of warm countries, and the few European species probably hardly represent the original composition of these animal colonies. But of the tropical species, which have as yet not been sufficiently studied, we know at least the extraordinary dimensions of the body, and the corresponding fertility of the queens, and we conclude from this that only a few can be present in each termitary[18].
[18] Ingwe Sjostedt has recently established in Africa that it is usually a single queen and a single king that found a termitary (Schwed. Akad. Abh. Bd. 34, 1902).
Now that we have discussed all these facts it will not be out of place to summarize the results, in as far as they have any relation to the acceptance or rejection of the theory of the inheritance of acquired characters.
No direct proof of such transmission could be found; on the contrary, it has been shown that all that has hitherto been advanced as such will not stand the test of close examination; an inheritance of wounds and mutilations does not exist, the transmission of traumatically induced epilepsy is not only doubtful as regards its causes, but cannot even be considered as the transmission of a particular morphological lesion.
We may regard as indirect proofs such facts as can only be explained on the assumption of this mode of inheritance, and in this connexion our opponents have cited especially the correspondence between modifications acquired through use in the individual lifetime, and worked out through histonal selection, with the phyletic transformations of the same parts. But it has been shown that a number of parts which do not function actively at all, but only passively, and thus cannot be caused to change through use, like the hard skeletal parts of the Arthropods, vary phyletically in the same certain and direct course as those which function actively, so that we have every ground for assuming that there are other factors operating in the transformation of the active as well as of the passive parts. Finally, we discussed the last and strongest argument which has been put forward in favour of the Lamarckian principle, that of co-adaptation, that is, the simultaneous adaptation of many parts co-operating in a common action, and we were able to controvert this altogether by showing that exactly similar phenomena of co-adaptation occur in systems of passively functioning parts, and further, that they occur also among the workers of ants and bees, that is, in animals which do not reproduce, and which, therefore, cannot transmit the acquired results of exercise during their life.
We therefore reject—and are compelled to reject—the Lamarckian principle, not only on the ground that it cannot be proved correct, but also because the phenomena, to explain which it is used, occur also under circumstances which absolutely exclude any possibility of the co-operation of this principle.
Supplementary note on the Transmissibility of Acquired
Functional Modifications.
I cannot conclude this section without some reference to the utterances of some naturalists who have quite recently attempted to represent the inheritance of functional modifications as a conceivable and even a necessary assumption.
I may name first Ludwig Zehnder, a physicist who has wandered into the domain of biology. In regard to the very facts which I have adduced as evidence against the existence of such inheritance, he has endeavoured to show how we might conceive of them as having by this very means arisen[19].
[19] Zehnder, Die Entstehung des Lebens, Freiburg-i.-Br., 1899.
He deals with the case of ants, that is to say, with the differentiation of the sterile workers into several castes, in the following interesting manner.
The task of the workers is to procure all the food necessary for all the individuals of the colony in quantity and quality corresponding to the demand; failing this the whole colony would perish. Now the different persons of the colony need different food, according to their constitution and their functions. Soldiers, for instance, are more powerful than ordinary workers, since they are adapted for fighting, and they therefore require a different kind of food from the weaker workers who are adapted only for other duties. Since the soldiers have evolved from the latter by selection, what we may, for the sake of brevity, call the soldier-food in the common stores of the ants would be drawn upon more lavishly than before, and would therefore disappear more quickly, and, whenever this occurred, those workers which had already brought in this kind of food would be impelled to bring more and more to satisfy the demand for it. But in order to do this they would require to exert themselves more, and would therefore require a larger quantity of food—not of course soldier-food, but the particular kind which their particular qualities demanded. Probably this second importation of food was undertaken by a second kind of worker, for, according to Zehnder, each worker does not carry all the kinds of food; they are divided into legions, each of which has its particular task of food-collecting to fulfil.
In the end the storehouse of the ant-colony must contain a provision in which the different kinds of food are in exact proportion to the necessities of the different kinds of persons in the colony. It must alter in its composition again as soon as, in the course of time, one or other kind of person acquires new characters, for these presuppose a new kind of diet.
But how are these acquired characters to be transmitted since neither soldiers nor workers reproduce? Zehnder answers this by pointing out that the sexual animals eat all the kinds of nourishment which are accumulated in the stores, that is to say, all the different kinds of food exactly in the proportion in which they have been imported—the proportion in which the different kinds of persons are represented in the colony. Thus the kinds of nourishment which caused the appearance of the newly acquired characters in the non-sexual animals also reached the sexual animals and their sex-cells, and there gave rise to substances which evoke the relevant qualities in their descendants, for instance, in the soldiers, or in the still more modified workers, and so on; and thus we have an 'inheritance of acquired characters.'
This is certainly ingeniously and cleverly thought out, and it reads even better and more smoothly in the original than in my brief summary, but it will hardly be regarded as a refutation of my position; the hypotheses are all too daring for that. We have no knowledge that particular modifications in form can be produced and conditioned by particular kinds of food, and, indeed, the contrary has been proved, namely, that the two or three different castes of polymorphic species have precisely similar diet. I need only recall the six forms of female in Papilio merope, of which at least three have been obtained from the same set of eggs, and by feeding with the same plant.
It is true that there are ants which lay in stores of nourishment, but these consist, for the most part, of one kind of seeds, or of honey, not of different substances, and we have no knowledge that the different persons use different food, or even that there is any diversity in the mode of feeding the helpless larvæ. The feeding in some species takes place from mouth to mouth, and therefore cannot be precisely investigated, and we can only suppose from analogy with bees that the larvæ of the males and females frequently receive not only more abundant, but qualitatively different food. They are fed from the crop unless the food consists of the pith of a tree in which the larvæ are imbedded, as Dahl informs us is the case with some tropical ants of the Bismarck Archipelago.
But even if we assume that the soldiers take different food from the ordinary workers, and different again from that of the sexual animals, is it by virtue of the quality of their food that they have become what they are? Have our breeds of pigeons or hens been produced by different diet, or do we know anything in the whole range of animal life of such a parallelism between food and bodily structure as Zehnder here assumes? And if, in reality, let us say, the breeds of pigeon had arisen through specific dieting, and we were to feed one pair with the specific food-stuffs of three different breeds, would the descendants of this pair exhibit the form of these three breeds? Or would they exhibit them in precisely the proportion in which the food-stuffs had been mixed? It seems to me that Zehnder's assumptions diverge so far from what we are accustomed to regard as solid ground in biology that they hardly require consideration, and yet he not only uses them for the explanation of the case of the ants, but bases upon them the whole of his theory of the inheritance of acquired characters.
He considers that the results of use (that is, increased function) are generally transmitted, because the increase in the organ which is functioning more strongly changes the composition of the blood, by withdrawing from it in a greater degree the specific substances which the organ in question—a muscle, for instance—requires for its activity. All parts of the animal are thereby affected and modified, but especially those smallest vital units or 'fistellæ' (corresponding to biophors) which preside over digestion, and of which there are several sorts. Among them those work most arduously which have to produce the specific substances which serve for the nutrition of the muscles with increased function, because these are needed in larger quantities. This kind of digestive 'fistella' therefore multiplies, while other kinds, whose products are not required and therefore not used up, cease to be so active, diminish in number, and in course of time disappear. In this way the composition of the blood is altered, and with it to a greater or less degree all the characters of the whole organism. Of course the reproductive cells are also under the influence of this change in the composition of the blood, because the different nutritive substances are accumulated within them in an altered proportion corresponding to the changed composition of the blood, the nutritive substances for the muscles with increased function being contained in it in a larger quantity, and thus the greater development of the muscle will repeat itself in the progeny, that is to say, the acquired character is transmitted.
It is obvious that this is precisely the same line of argument as that used in reference to the origin of the worker and soldier ants. The different kinds of 'digestive fistellæ' correspond to the different food-carrying workers, and the blood to the assumed storehouse from which soldiers and workers select the food suitable for their respective needs, while the sex-cells in the one case, the sexual animals in the other, partake of all kinds exactly in the proportion in which they are stored, and thus the organ which functions most vigorously must be stronger in the offspring.
How the minute quantity of nutritive material contained in the ovum, still less in the sperm, is to effect the strengthening of the particular muscles in the descendants is not stated; moreover, such minimal quantities of food must soon be exhausted, and cannot possibly increase. It would seem as if the muscles could not even begin by being stronger, much less that they should remain so, if they were not exercised equally vigorously by the descendants. If the specific nutritive stuffs were 'fistellæ,' that is to say, were living units capable of multiplication, one could understand it. But there can, of course, be no possibility of a production of living units through digestion; that can only give rise to digested substances. Or if the alteration in the composition of the blood produced in the determinant system of the germ-plasm just those variations requisite to bring about a strengthening of the muscular system, it would remain to be shown how this could happen, for the gist of the problem lies in this. For muscles do not lie in the germ-plasm as miniature models of the subsequent muscular system, and even if they did, would not all the muscles, and not merely those which were no longer exercised, decrease hereditarily when a particular group, like the muscles of the ear in man, degenerates? Zehnder replies to this with the hypothesis that the muscles are not all chemically alike, but that each possesses a particular chemical formula, though they may all be very similar, and that, therefore, the nutritive materials required by each must be slightly different. In that case there would require to be, in the ovum and sperm of man, in order that functional modifications might be transmitted, as many special nutritive substances as there are muscles, and, in addition to these, innumerable hosts of other kinds of specific nutritive substances for all the other parts of the body, since all of them can be strengthened by exercise and weakened by disuse. And even if we suppose that all these millions of specific nutritive substances are accommodated within the germ-cells, as Zehnder's theory requires, they could not perform what Zehnder ascribes to them, for, as we have already said, they cannot multiply in the manner of living units, and so control the growing organism. The different specific nutritive materials contained in the blood are just as powerless to perform the task ascribed to them by Zehnder as the specific kinds of food in the hypothetical storehouse of the ants are to give rise to the different persons of the ant-colony.
Zehnder also attempts to overthrow the arguments against the Lamarckian principle which I based on the skeleton of Arthropods.
It does not seem to him probable that the chitinous coat of mail can be an absolutely dead structure, and he supposes that very delicate nerve-fibrils penetrate into all its most minute parts, and so are stimulated by 'every pressure and every strain' exerted on the chitinous skeleton. They 'work' when they are stimulated, and in doing so they use up 'their specific food-stuffs.' At places which are frequently stimulated the corresponding nerves develop more than elsewhere. The necessary specific food-stuffs for these particular nerves therefore increase proportionately within the body, and also in the reproductive cells. Accordingly, in the germ-cells there is an increase of the aforesaid nervous substances, which in the offspring become associated with the relevant part of the chitinous covering, and induce in development the secretion of chitin at this part. At this particular spot, then, the chitin will be specially thick.
This clearly implies that each particular part of the skin has its specific nutritive substances, necessitated by the nerves which traverse it! Thus there must be as many nerve-nutritive substances as there are skin-nerves, specific chemical combinations for every part of the body which is capable of heritable variation. This is so extraordinarily improbable that I need say nothing more about it. If the Lamarckian principle requires this kind of hypothesis to bolster it up, it is undoubtedly doomed.
If we disregard altogether the positive aspect of Zehnder's hypothesis, and assume that the skin-nerves are really stimulated through the chitin by every strain and pressure to which a spot of skin is exposed, and that they cause a correspondingly greater secretion of chitin, which would then, according to the Lamarckian principle, be hereditary, does this harmonize with what actually occurs in the development of the skeleton as we know it in the case of Insects and Crustaceans? Not at all! Can we suppose that the carapace of a crab or the enormously hard wing-covers of a water-beetle are exposed to a continual pounding and pressing and pushing? Exactly the contrary is the case. Every assailant takes care not to grasp the animal where it is so well protected, and seeks out the most vulnerable parts for its attacks. It may be answered that, while this is certainly the case now, the animals were badly protected when the ancestral forms were evolving. But that they could not have become hard by dint of being frequently bitten or otherwise wounded should be obvious from the fact that the whole of the wing-covers and the whole of the carapace is uniformly covered with thick chitin, while each wound would only stimulate particular spots; and we should also have to admit that, since these parts of the skin which are now so well protected are no longer seized and stimulated, they would long ago have become thin again, according to the principle of the degeneration of parts no longer used, or, in this case, no longer stimulated. But there is no need for wasting time over such quibbles, since there is a fact which absolutely contradicts Zehnder's hypothesis. I mean the degeneration of the chitinous skeleton in those Crustaceans and Insects which protect the abdomen within a shelter like the hermit-crabs, the caddis-flies (Phryganidæ), (Fig. 107) and the sack-carrying caterpillars of the Psychidæ among Lepidoptera. The hermit-crabs, as is well known, squeeze their abdomen into a usually spirally-coiled Gasteropod shell, and they always choose houses which are wide enough to conceal the whole body up to the hard claws when necessity arises. In this case there is surely a continual pressure on the abdomen, which, being soft, must be squeezed very tightly every time the animal retreats into its shell. One of my opponents has described the disappearance of the tough integumentary skeleton from the abdomen of these animals as an inherited result of this pressure, and another regards it as the inherited result of the degeneration of the muscles in this part of the body. But, according to Zehnder, this continuous pressure, and the frequent rubbing up and down of the abdomen on the inner surface of the Gasteropod's shell, would undoubtedly have a stimulating effect on the skin-nerves, and would therefore bring about a thickening of the chitinous cuticle. In regard to the larval Phryganidæ and Psychidæ, the case would be the same, though perhaps hardly to the same degree, for while these larvæ make their own houses, and will therefore at least make them big enough to begin with, the pressure and friction must increase with the growth of the animal.
Fig. 107. Larva of a Caddis-fly, after Rösel. A, removed from its case, showing the hooks (h) which attach it thereto, and the whitish abdomen, covered only by a thin cuticle. B, the same larva, moving about with its case.
If the regulation of the strength of the integumentary skeleton be referred to selection, we see at once why carapace and wing-covers should be of equal thickness throughout their whole extent, and why they do not disappear, although they do not function actively, and are less stimulated than any other parts of the skeleton; and we also understand why the abdomen of hermit-crabs and of larval Phryganidæ and Psychidæ has become soft, whether it be exposed to pressure or friction in a greater or a less degree. It no longer requires to be hard, because it is protected by the house, and in the case of the Pagurids it must not be hard because it could not then be readily squeezed into the hard-walled and narrow recesses of the Gasteropod shell; in this case there has therefore been positive selection. I have not yet referred to the fact that the chitinous covering is certainly not living, though it is not exactly dead; it is a secretion of the epidermic cells, not a tissue, and we cannot suppose that there are any nerve-endings in it. It is almost superfluous to say that the fact that the skin is cast is in itself enough to make such an assumption untenable, for the whole of the assumed delicate nervous network would be shed at every moult and torn away from the nerves which lead to it. As far as my knowledge goes, nothing of this kind occurs anywhere in the whole range of the animal kingdom.
Even if we assume, for the benefit of Zehnder's hypothesis, that although there are no nerves in the chitin itself yet irritations affecting the chitinous coat may be transmitted through it to the delicate nerve-endings lying beneath it, this should take place in a greater degree at the thin places of the skeleton than at the thick parts! But this interpretation is again fallacious, for we see that the tactile organs of Arthropods always break through the chitinous cuticle and protrude beyond it in the form of setæ.
Of the many other opponents of my views in regard to the transmissibility of acquired functional modifications, I need only deal in detail with Oscar Hertwig.
He seeks for direct proofs of an inheritance of acquired characters, and believes that he has found these in the hereditary transmission of acquired immunity from certain diseases. He reminds us of Ehrlich's well-known experiments on mice with ricin and abrin.
Even small doses of these two poisons kill mice, but they are tolerated in very minute doses, and if their administration be continued for some time in such minute doses, the animals gradually acquire a high degree of insensitiveness to these poisons; they become immune to ricin and abrin.
This immunity is transmitted from mother to young, but it only lasts for a short time, about six to eight weeks after birth. Yet this is regarded by Hertwig as an illustration of the transmission of an acquired character, as an acquired modification of the cells of the body, for he explains the immunity on the assumption that all the cells of the body undergo a particular variation due to the influence of the poison, and are thus, to a certain extent, modified in their nature, and that the ovum also undergoes this variation and transmits it to the young animal. The immunization might certainly come under the category of functional modifications, and it might be thought that we have here a case of transmission of such an acquired character.
Against this, however, we have to put the fact that the acquired immunity is not transmitted from the father to the descendants. Hertwig attempts to explain this by saying that the short duration of the experiments has only allowed the poison to affect the cell-substance (cytoplasm) and not the nucleus, that is, the hereditary substance of the sperm-cells, an assumption which has little probability considering the intimate nutritive relations between the cell-nucleus and the cytoplasm. I should be rather inclined to conclude from the difference in the transmitting power of the sperm and of the ovum, that this 'inheritance of immunity' does not depend, as Hertwig supposes, on a modification of the cells to 'ricin immunity,' but, as Ehrlich and the bacteriologists believe, on the production of so-called 'anti-toxins,' and that these anti-toxins are handed on to the embryo not by the ovum itself, but by the interchange of blood between mother and offspring which lasts throughout the whole embryonic period. It is then self-evident why no transmission of immunity through the father occurs.
But it would lead me too far were I to attempt to refute all the attempts that have been made to interpret individual cases as due to the inheritance of acquired characters. I should, however, like to say something as to the theoretical possibility of such an assumption.
When we try to conceive how experiences and their consequences can be entailed, how new acquirements of the 'personal part can have representative effects on the germinal part,' we find ourselves confronted with almost, if not entirely, insuperable difficulties. How could it happen that the constant exercise of memory throughout a lifetime, as, for instance, in the case of an actor, could influence the germ-cells in such a way that in the offspring the same brain-cells which preside over memory will likewise be more highly developed—that is, capable of greater functional activity? We know what Zehnder's answer to such a question would be; he would make the blood the intermediary between the brain-cells and the germ-cells, but we have seen that specific food-stuffs for each specific cell-group cannot be assumed, and that, even if they could, they would not meet the necessities of the case. Yet every one who does not regard the germ-plasm as composed of determinants is constrained to make some such assumption. But if we take our stand upon the theory of determinants, it would be necessary to a transmission of acquired strength of memory that the states of these brain-cells should be communicated by the telegraphic path of the nerve-cells to the germ-cells, and should there modify only the determinants of the brain-cells, and should do so in such a way that, in the subsequent development of an embryo from the germ-cell, the corresponding brain-cells should turn out to be capable of increased functional activity. But as the determinants are not miniature brain-cells, but only groups of biophors of unknown constitution, and are assuredly different from those cells; as they are not 'seed-grains' of the brain-cells, but only living germ-units which, in co-operation with the rest exercise a decisive influence on the memory-cells of the brain, I can only compare the assumption of the transmission of the results of memory-exercise to the telegraphing of a poem, which is handed in in German, but at the place of arrival appears on the paper translated into Chinese.
Nevertheless, as I have said before, I do not disagree with those who say, with Oscar Hertwig, that the impossibility of forming a conception of the physiological nexus involved in the assumed transmission does not ipso facto constrain us to conclude that the transmission does not occur. I cannot, however, agree with Hertwig that the case is exactly the same as in the 'converse process,' that is, 'in the development of the given invisible primary constituents in the inheritance of the cell into the visible characters of the personal part.' Certainly no one can state with any definiteness how the germ goes to work, so that from it there arises an eye or a brain with its millionfold intricacies of nerve-paths, but although the process cannot be understood in detail, it can in principle, and this is just what is impossible in regard to the communication of functional modifications to the germ. Moreover, in addition to this, there is the very important difference that, in the one case, we know with certainty that the process actually takes place, although we cannot understand its mechanical sequence in detail, while in the other we cannot even prove that the supposed process is a real one at all. From the fact that we are unable to form clear conceptions of a hypothetical process, we are not justified, it seems to me, in assuming it to be real, even though we are aware of many other processes in nature which we are unable to understand.
Nor does Hertwig take up this position, for he is at pains to show the mechanical possibility of the process of inheritance which he assumes, and he bases this upon the suggestions made by Hering in his famous work Ueber das Gedächtniss als eine allgemeine Function der organisirten Materie [On memory as a general function of organized matter], 1870. As this essay probably contains the best that can be said in favour of a transmission of functional modifications, and as it also includes some indisputable truths, we may consider it in some detail.
Hering is undoubtedly right in regarding 'the phenomena of consciousness as functions of the material changes of organic substance, and conversely.' That is, he believes that every sensation, every perception, every act of will arises from material changes in the relevant nerve-substances. But we know that 'whole groups of impressions, which our brain has received through the sense-organs, are stored up in it, as if resting, and below the margin of consciousness, to be reproduced when occasion arises, in correct order of space and time, and with such vividness, that we may be deceived into regarding as a present reality what has long ceased to be present.' There must therefore remain in the nerve-substance a 'material impact,' a modification of the molecular or atomic structure, which enables it 'to ring out to-day the note that it gave forth yesterday if only it be rightly struck.'
Hering attributes a similar power of memory and reproduction to the germ-substance; he believes that he is justified in making the assumption that acquired characters can be inherited, although he admits that it 'appears to him puzzling in the highest degree' how characters which developed in the most diverse organs of the mother-being can exert any influence on the germ. That he may be able to assume this he points to the interconnexion of all organs by means of the nervous system; it is this that makes it possible that 'the fate of one reverberates in the other, and that, when excitement takes place at any point, some echo of it, however dull, penetrates to the remotest parts.' To the delicate-winged communication by means of the nervous system, which unites all parts among themselves, must be added the general communication by means of the circulation of the fluids of the body. According to Hering's view, the germ experiences, in some degree, in itself all that befalls the rest of the organs and parts of the organism, and these experiences stamp themselves more or less upon its substance, just as sense-impressions or perceptions stamp themselves upon the nerve-substance of the brain, and these experiences are reproduced during the development of the germ, just as the brain brings memory-pictures back to consciousness. He says, 'If something in the mother-organism has so changed its nature, through long habit or exercise repeated a thousand times, that the germ-cell resting in it is also penetrated by it in however weakened a fashion, when the latter begins a new existence, expands, and increases to a new being whose individual parts are still itself and flesh of its flesh, it reproduces what it experienced as part of a great whole. This is just as wonderful as when an old man suddenly remembers his earliest childhood, but it is not more wonderful than this.'
But I think it is more wonderful. There exist demonstrably in the brain thousands upon thousands of nerve-elements, whose activity is a definite and limited one, because each particular visual impression, for instance, only excites to activity certain definite nerve-elements, and can leave memory-pictures in these alone. According to my conception of it, the germ-plasm is quite as complex in its composition, and does not consist of homogeneous elements, but of innumerable different kinds, which are not related to the parts of the complete organism indiscriminately, but only to particular parts. But is it allowable to assume that there are invisible nerve-connexions, not only to every germ-cell, but also within the germ-plasm, to every determinant, like the nerve-paths which lead from the eye to the nerve-cells in the optic-area of the brain? For if it were otherwise, how could we conceive of the modification of an organ—as, for instance, the ear-muscles in Man—communicating itself to the precise determinants of these muscles in the germ-plasm? I have often been met with the reproach that my conception of the composition of the germ-plasm is much too complex—but the complexity of Hering's suggestion seems to me to go a long way beyond mine.
Hering's ideas, which are not only ingenious but very stimulating, might be accepted as the first indication of an understanding of the assumed inheritance of functional modifications, if it could be proved that such inheritance is a fact; but, as we have seen, that is not the case. The assumption might be permitted, perhaps, if it could be shown that certain groups of phenomena left no other possibility of explanation open except this assumption, but that also, as far as I can see, is not the case. Of course, others hold a different opinion, but chiefly because they have rejected without much reflection the sole explanation which presents itself for numerous phenomena—I mean the processes which we are about to study under the name of 'germinal selection.' But, in any case, Hering's ideas seem to me very valuable, because they make it apparent that, however much we know of the organism, we only know it in a general way, and that numberless delicate processes go on in it which leave no trace for our microscope, and that we can only recognize the final results of numerous invisible and often, in their subtlety, also unimaginable factors. This ought to be taken to heart, especially by all those who speak of simplicity in reference to the germ-plasm. So much at least is certain: If there were any inheritance of functional modifications, we should have another proof that the germ-plasm is composed of determinants, for without them there could be no possibility that the 'experiences' of an individual organ would be transmitted to the germ in the way that the Lamarckian principle implies. Something, and that something material, must be modified in the germ-plasm if the vigorous use of a group of muscles, or of a gland, or of a nerve-cell, is to be communicated to the germ, and not to the whole germ-plasm, but only to so much of it as is necessary to cause variation in the corresponding group of cells in the child. It may perhaps be said that this still does not necessitate the assumption of special determinants for these cell-groups, and that one might, with Herbert Spencer, conceive of the germ-plasm as consisting of homogeneous units which vary in the development in accordance with the diverse regularly alternating influences to which it is exposed from step to step, and that, therefore, in each of these units of very complex structure only a single molecule, or perhaps only a single atom, would need to vary in order that, in the course of development, the resulting cell-group should appear in the rudiment in somewhat altered strength.
But I do not believe that a chemical molecule, still less an atom, is sufficient for this, for reasons which I have already stated—yet we need not go into this now, but rather deduce the consequences of this admission. It follows that the 'unit' is made up of numerous 'molecules' or 'atoms,' of which each, by dint of changes it has undergone, causes particular parts of the body to vary in a definite manner; in other words, we have here again a theory of determinants, only they are on a much smaller scale, since each invisible little vital particle or 'unit' contains all the determinants within itself, while in my theory it is only the id, that is, the visible chromosome, which includes the determinant complex. Such a theory would be far from a simplification of mine, it would rather complicate it enormously, and that without anything being gained. At most it would be made more evident how inconceivably complex the nerve-paths must be which lead from the part that has been modified by exercise to the germ-plasm, and must also lead to all the innumerable 'molecules or atoms' of the individual 'units.' But even on my theory of the composition of the ids as aggregates of living determinants, such nervous transmission of qualities would be a monstrosity which no one would accept, and I think on this account that my argument as to the impossibility of conceiving of the transmission of the modifications of the personal part to the germinal part retains its force, notwithstanding Hering's interesting analogy.
If the transmission of functional modifications were an indisputable fact, I repeat, we should have to give in, and then we might regard the 'memory of organized material' as affording a hint of the possibility of the unimaginable process. But as long as the occurrence of this transmission cannot be proved either directly or indirectly, such a vague possibility of explanation need not induce us to assume an unproved process.