August Weismann

“ Many facts indicate that the differentiation of sex can also within certain limits be regarded from a similar point of view. The primary constituents for the characters of both sexes are included in the same egg, and in many instances it appears that a stimulus decides as to which group of them shall undergo development—whether the male or the female. Unfortunately in only very few cases can we as yet determine the stimulus with certainty, but we are at any rate certain that it is not the same in every instance. ”

“ These influences, moreover, are by no means purely of an external kind, but are to a great extent exercised by one part of the organism on another, by cell on cell, by tissue on tissue, by organ on organ. If I am not mistaken, the phenomenon which Darwin described as correlation, and justly regarded as an important factor in evolution, is for the most part an effect of intra-selection, which has great influence on phylogeny as well as upon ontogeny, even though its results are not transmitted. ”

“ In the case of human beings it appears very obvious that in a certain sense sex may be hereditary:—I mean that the determination of sex may be dependent on the composition of the germ. The fact that many families consist chiefly either of male or of female children could hardly be accounted for in connection with any external influences. If, however, we conceive of the germ-plasm as composed of 'male' and 'female' ids, the numerical relations of these will usually be as 1:1. ”

“ Thus it would not be inconceivable that the caterpillars of a species which produces two generations in a year should have become adapted in respect of protective colouring to two different and alternating food-plants; and in this case too, the periodic change of colour would apparently depend on the direct influence of the summer and autumn climate, while really due to the presence of double primary constituents in the germ, which by some external influence—perhaps warmth or perhaps the quality of the light falling on the young caterpillar-would only undergo development alternately. ”

“ It is certainly very remarkable that this adaptation of the larval organism to the determining stimulus of a specific mode of nourishment should be so perfectly similar in two such different groups of insects as the bees and termites: for there can of course be no doubt that it has arisen independently in each group. But it does not seem to me surprising that natural selection has in both these cases coupled the development of the caste primary constituents with stimuli dependent on nutrition. ”

“ It would not only show how marked and constant the differences between the two generations are, but would also prove the biological value of these differences; for whenever dimorphism depends on processes of selection, each of the forms is an adaptation, and must have a biological significance. For instance, if the colouration of the underside of the summer-generation were of no importance—that is, if the pattern might equally well be leaf-like—it would be difficult to see why the adaptation of the autumn-generation to dried leaves should not have extended to the other generation as well. ”

“ I have frequently been charged with exaggerating the sphere of activity of natural selection and with putting the direct effects of extemal influences in the background. But I hope to have shown today in one instance that there is sometimes a tendency to go to the other extreme, and to make extemal influences responsible for a metamorphosis in which they can have had no part. ”

“ The facts here brought forward, as well as the interpretation of them at which I have arrived, confirm me more strongly than ever in the belief that selection is the all-sufficient principle on which the development of the organic world has been guided on its course. ”

“ Various degrees of difference may exist amongst these forms; they may refer merely to the colouration and marking—as in cases of the adaptive seasonal dimorphism which I have attempted to show probably occurs in caterpillars and butterflies,—or they may be so considerable as those which exist between the castes of state-forming insects.
External influences are in none of these cases the actual cause of the differences; they merely play the part of the stimulus that decides which of the primary constituents shall undergo development. ”

“ Although, in my opinion, we are now quite justified in denying that evolution has taken place owing to purely internal causes, it can by no means be said that we are yet quite clear as to the way in which external influences have formed and transformed organisms. ”

“ The great significance of intra-selection appears to me not to depend on its producing structures that are directly transmissible,—it cannot do that,—but rather consists in its causing a development of the germ-structure, acquired by the selection of individuals, which will be suitable to varying conditions. Intra-selection effects the special adaptation of the tissues to special conditions of development in each individual. ”

“ It is often assumed, without much proof that a certain variation of a living being is the direct consequence of an external influence simply because the variation in question is, in fact, in some causal connexion with a definite external influence: such an assumption is, however, founded on a totally false idea as to the interconnexion of the phenomena. ”

“ How would they have become adapted in so wonderful a degree to these stimuli if they had not themselves helped in any way to bring about the result? And as every vital manifestation is a reaction to stimulus, there is hardly anything left for a developmental force to do. ”

“ When a tree is crowded by other trees on one side, and is thus cut off from the light, it grows less rapidly on this side, but develops all the more luxuriantly on the other: the hereditary so-called 'molecular' constitution of its shoots, which we speak of as positive heliotropism, is the cause of this. Similarly, when a fractured bone heals out of the straight, the plates of the spongy portion again become set in the line of greatest tension and pressure, the cause, being the hereditary molecular sensitiveness of the connective tissue-matrix of the bone. ”

“ And how wonderfully fine it can become we see by reference to the various special activities of the elements of our own body, as well as to the numerous processes of all kinds which occur both in plants and animals. For example, we know that as a rule self-fertilization is avoided among Phanerogams, that numerous complicated arrangements exist in the flowers to prevent its occurrence, and that even the very existence of many flowers depends on the fact that cross-fertilization by the agency of insects is an advantage. ”

“ It is clear that this statement is incorrect, and that not the cold, but the peculiar organization of the marmot causes the reaction of hibernation: cold cannot throw a dog or a bird into a state of slumber for the winter. We are here, therefore, concerned with a special adaptation of the organism to a stimulus—cold—which affects it in such a manner that it escapes from what would otherwise be a destructive influence. ”

“ Recent advance in our knowledge of the physiology of plants has, however, resulted in showing that much commoner phenomena of plant—life must depend upon adaptation, and are not mere products of the general constitution of the plants. ”

“ In my experiments with flies, at any rate, those which were poorly fed showed no less appetite as imagines than their sisters which had been well fed as larvae. Or ought we to suppose that the habit dates from ancient times, when all the females instinctively took but little food? In this case they would not have reproduced, as the workers at the present day cannot mature the scanty egg-cells present in their few egg-tubes. And we must not forget the double, or—in the case of some ants and the termites—three-fold forms of the workers, to which I referred in this connection in my essay! ”

“ Even supposing that here and there a variation of a few spongy plates happened to arise by chance, how could these give the individual any advantage in the struggle for existence, when hundreds and thousands of them are required to make the bone better adapted for its work, so as to give to the variation a selective value? In reply I should like to ask if there is no possibility of assuming that the primary constituents (Anlagen) of a tissue occurring in many parts of the body might be improved by natural selection acting on the germ alone. ”

“ I refer to the neuters or workers of state-forming insects—bees, ants and termites. As is well known, these workers do not originate from special eggs, but it has been demonstrated in bees and termites, and shown to be very probable in the case of ants, that there is only one kind of egg, from which queens and workers as well as males arise. When female larvae are supplied with a very rich and nourishing diet, they give rise to queens; but workers are developed if the larvae receive scantier and less nourishing food. ”

“ But I have lingered too long on the state-forming insects, and must now return to the point in connexion with which I first referred to them. I wished to show that when it is important to regulate different possibilities of development, nature makes use of external influences as stimuli. But it is not always an external stimulus that originates a change of form;—it may be an internal one. ”

“ The gradually decreasing number of egg-tubes in the ovaries of the workers in the different species proves, on the contrary, that these egg-tubes are valueless for the maintenance of the species in the struggle for existence. ”

“ The ovary of the queen-bee consists of from 180 to 200 egg-tubes, and numerous eggs may become mature in one of these. The queen is accordingly able in the course of her life to produce an immense number of eggs,—more than a hundred thousand. The ovary of the worker on the other hand contains from two to six egg-tubes only, and no matter how rich the food may be, no more can be formed in the imaginal state. The workers certainly produce eggs occasionally, when they have received exceptionally rich nourishment in the imaginal-stage ”

“ The struggle of parts unequal in strength—that is, unequal in susceptibility to stimulus—must be the cause which brings about the mingling of parental primary constituents, and prevents the occurrence of monsters with parts ill adapted to one another.
Intra-selection apparently also renders it possible for a harmonious organism to live and undergo development from an only moderately harmonious mass of primary constituents, such as I imagine constitute the germ-plasm. ”

“ The matter is, if possible, still clearer in the case of geotropism, for there is no doubt that the habit of the plant to react in some particular way to the force of gravity cannot be a general and primary peculiarity of plants. A Volvox colony, rotating freely in water, is in a sense a living klinostat, uninfluenced by the attraction of the earth. The geotropic sensitiveness of plants can only have arisen when they became attached to the ground, and may therefore, even in its most general form, be regarded as an adaptation. ”

“ It is difficult to disbelieve in the potency of external influences when one sees how invariably all the vital manifestations of animals and plants are ultimately reactions to such influences, and how both animal and plant are comparable to machines so constructed that stimuli from the outer world cause them to act in the most purposeful manner for their own maintenance. ”

“ The 'male' ids—if I may be allowed the expression—become active in the absence of fertilization, the 'female' ids on the occurrence of fertilization, and the kind of food supplies the stimulus for the worker-ids or queen-ids. Slow processes of selection have gradually changed the 'female' ids in two directions, and finally led to the establishment of two perfectly distinct forms of females. ”

“ It might, however, be said that flies and bees are very different organisms, and therefore react differently to external influences. This is quite true, and is exactly what I wish to be acknowledged. My experiments with the flies were merely meant to show that all insects, even though they may resemble bees in some respects, do not react in a similar way to the bee to meagreness of nutrition, and that accordingly this mode if reaction is a characteristic of bees:—it is and was not possessed by the ancestors of these insects. ”

“ How exceedingly important, too, are the consequences of the sensitiveness of plants to light;—what infinitely fine gradations in this sensibility occur in the reactions of the parts of plants, and how exactly is it adapted to their needs! ”