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].

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].

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.

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.