The Evolution Theory, Vol. 2 of 2

by August Weismann

But it is quite otherwise with purely quantitative differences in nutrition. That meagre diet influences individuals unfavourably is indubitable, and we are certainly justified in considering whether this may not have an effect on the germ-cells, and one which will correspond to the changes induced on the body, so that if the poor nutrition should last through many generations an hereditary degeneration of the species would occur, which would not at once disappear though the animals were transferred to more favourable conditions.

We certainly know nothing of how far the minuteness of the determinants of the germ-plasm, the whole quantity of the germ-plasm, or the reduced size of the germ-cell, may bear an internal relation to the smallness of the animal which develops therefrom, but it surely cannot be regarded as absurd to suppose that there is some such relation. There are no experiments known to me which prove that meagre diet brings about a progressive decrease in the size of the body. Carl von Voit has observed that dogs of the same litter grew to very different sizes of body according as they received abundant or scanty food, but it would be difficult to make animals small through scantiness of food and at the same time to keep them capable of reproduction, and thus proofs of the inheritance of the dwarfing are lacking. Moreover, the experiments which Nature herself has made are never quite convincing, because we never can definitely exclude the indirect effect of altered circumstances. The case of the feral horses of the Falkland Islands, so often cited since the time of Darwin, which have become small 'through the damp climate and scanty food,' seems to me, of all known cases of the kind, the one we should most readily attribute to the direct effect of continued scanty diet; but even here we cannot altogether exclude the possibility of the co-operation of adaptations of some kind to the very peculiar conditions of life in these islands, as far as the feral horses are concerned. I have not been able to find any record of more modern exact investigations either regarding these feral horses, or in regard to the others which are reared in the Falklands under conditions of domestication. Darwin himself, however, in the Journal of his famous voyage tells us much that is interesting in regard to the mammals of the Falkland Islands. Cattle and horses were brought there in 1764 by the French, and have increased greatly in numbers since that time; they roam about wild in large herds, and the cattle are strikingly large and strong, while the horses both wild and tame are rather small, and have lost so much of their original strength that they cannot be used for catching wild cattle with the lasso, and horses have to be imported from La Plata for this purpose. From this contrast between the horses and the cattle we may at least conclude that it cannot be 'scanty food' alone which causes the horses to become smaller, but that the climatic conditions as a whole are concerned in the matter. Whether the total amount of variation which has taken place in the horses which have lived wild there for a hundred years would take place in the course of a single life, or whether it is a cumulative phenomenon, has still to be decided.

Similar statements, for the most part still more uncertain, are made in regard to changes in the hair of goats, sheep, cattle, cats, and sheep-dogs, which are referred to climatic influence. The raw climate of many highlands, like Tibet and Angora, is said to have directly produced the long and fine-haired breeds. But there is a lack of proof that adaptation or artificial selection did not also play a part, and the fact that similar long-haired breeds have arisen among rabbits and guinea-pigs in quite different places and under quite different climatic conditions, but under the directing care of man, speaks in favour of our supposition. But, on the other hand, it does not seem impossible that the climate may have a variational influence upon certain determinants of the germ-plasm, for we have already seen that the influence of cultivation may incite plants and animals to hereditary variations, and that slowly increasing disturbances in the equilibrium of the determinant system may thereby be produced, which may suddenly find marked expression as 'mutations.' But there is little probability that adaptations, that is, transformations corresponding to the altered climate, can arise in this way. The thick fur of the Arctic mammals is assuredly not a direct effect of the cold, although it has developed in all Arctic animals, not only in the modern polar bears, foxes, and hares of the polar regions, but also in the shaggy-haired mammoth of diluvial Siberia, whose tropical relatives of to-day, the elephants, have an almost naked skin. Another interesting case, recently brought to light, shows that a group of animals which, in correspondence with their otherwise exclusively tropical distribution, have only a moderately developed coat of hair, may, on migrating to a cold country, grow as good a fur as the members of other families. I refer to one of the higher apes, Rhinopithecus roxellanæ, which live in companies in the forest on the high mountains of Tibet, notwithstanding that the snow lies there for six months[26].

[26] See Milne-Edwards, Recherches pour servir à l'histoire nat. d. mammifères, Paris, 1868-74.

But we should assuredly make a mistake if we were to regard the thick fur of these apes as a direct reaction of the organism to the cold. We see at once that this cannot be the case if we compare them with marine mammals, which differ just as much from one another in this respect and yet are exposed to the same low temperature. The whale and the dolphin are quite naked, absolutely hairless, but the seals possess a thick hairy coat. This striking difference is obviously connected with the mode of life; the whales remain always in the water, the seals leave it often and therefore require the hairy coat, especially in colder climates, since otherwise they would be too rapidly cooled by the evaporation of the water from their bodies. For the whales, on the other hand, even a very thick hairy coat would not have sufficed as a protection against cold, since water is a much better conductor of heat than air, and so it was necessary for them to become enveloped with the well-known thick layer of blubber, a deposit of fat lying under the skin, and this—after it was once developed—made the hairy coat superfluous, so that it disappeared. The seals certainly also possess a layer of fat under the skin, but it is only in the largest of them that it affords sufficient protection against the cooling effect of evaporation when they go upon land or on the ice, and it is therefore only in these larger ones that the hairy coat has markedly degenerated, as, for instance, in the walrus and the sea-lion; in all the smaller seals, in which the mass of the body is much less, the hairy coat is necessarily very thick and protected from soaking by being very oily, because the layer of fat under the skin would not be sufficient to prevent excessive cooling when on land. But the thick coat of hair is no more produced by the cold than is the layer of fat. As Kükenthal has shown, all these characters are adaptations, and may depend here as elsewhere upon natural selection and upon the 'fluctuating' variations of the germ-plasm upon which that process is based. They are directed by personal selection because there is the need for them, and they are produced and augmented by germinal selection.

In all these cases the direct effect of external influences has nothing to do with the matter, but in other cases that alone brings about the whole change, which is then limited to the individual and does not affect the species as a whole at all.

Plant-galls afford striking illustration of the extraordinary changes that may be brought about in an organism or in its parts by external influence in the course of the individual life. All possibility of adaptation on the part of the plant is excluded in this case. The gall can only depend upon the direct influence of a stimulus, which is exercised by the young animal, the larva, upon the cells which surround it; and yet these cells vary to a considerable extent, become filled with starch or form a woody layer, secrete special substances, such as tannic acid, in large quantities, or develop hairs, moss-like growths, pigments, and so on, which do not otherwise occur in that particular part of the plant. Since Adler and Beyerinck have proved that it is not a poison conveyed by the mother animal into the leaf or bud when laying the eggs, which gives rise to the gall-formation, the matter has become rather clearer. We can now understand that different stimuli in succession affect the cells which enclose the larva, and that the ordered succession of these and the exactly graded stimulation incite the cells to activity in various ways, whether to mere growth and multiplication in a given direction, or to the secretion of tannic acid, or to the formation of wood, or to the deposition of reserve material, and so on. Even the feeble movements of the young larva may form a stimulus that increases with its growth; then the movements made by the larva in feeding, and not least the different secretions emanating from the salivary glands of the animal, which must contain some substances capable of acting as stimuli and probably changing in character as time goes on. All these factors must act as specific stimuli to the plant-cells, influencing and modifying their processes of growth and metabolism in one direction or another. In principle at least, if not in detail, we understand the possibility that through the ordered succession and exact balancing of these different cell-stimuli the really marvellous structure of the gall may be brought about as the product of the direct influence, exercised only once, of the gall-insect upon the plant's parts. But the animal's power of exercising such a succession of finely graded stimuli upon the plant must be referred to long-continued processes of selection, and the structure of the gall, which is adapted to its purpose down to the minutest details, can thus be understood. The assumption of substances which can act even in minute quantities as specific cell-stimuli, which we require to make in this attempt to explain galls, is no longer without corroboration since we find analogies in the Iodothyrin of Baumann, the specific secretions of the thymus and the supra-renal bodies in the higher animals, not to speak of the 'anti-toxins' of the pathogenic bacteria, which are only known by their effects.

The case of plant-galls is thus of great theoretical interest because we can exclude all preparation of the plant-cells for the stimuli exercised by the animal, since the gall is quite useless for the plant, though many have endeavoured to discover some utility. We have therefore here a clear case of modification due to the effect, exercised once only, of external influences, an adaptation of the animal to the mode of reaction of particular plant-tissues.

It might be supposed that if any inheritance of somatogenic modifications, any transmission of the acquirements of the personal part to the germinal part, were possible at all, it would occur in this case, for many species of gall-insects attack plants, particularly oaks, in great numbers every year. It has actually been maintained that galls may arise spontaneously, that is without the presence of a gall-insect. But no proof of this has ever been found, and the fact that no one has paid any attention to the assertion probably implies an unconscious condemnation of the hypothesis of the transmissibility of acquired characters.

It has been proved by Nägeli's often discussed experiments on hawkweeds (Hieracium) that much less specialized external influences can give rise to changes which are not hereditary. The Alpine species of hawkweed varied considerably in their whole habit in the rich soil of the Botanic Gardens at Munich, but their descendants, when transferred to a poor flinty soil, returned to the habit of the Alpine species. The changes which occurred in garden soil were therefore somatic and, as I have called them, 'transient,' and they did not depend upon variations of the germ-plasm. It may be objected in regard to these experiments that they were not continued long enough to prove that hereditary variations would not also have cropped up in consequence of the altered conditions. But in any case they prove that marked changes in the whole body of the plant may occur without any obvious variation of the germ-plasm. This does not mean, however, that the possibility of variations of the germ-plasm through such direct external influences is disputed. We must assume the occurrence of these on a priori grounds, if we refer—as we have done—individual hereditary variation to fluctuations in the nutrition of the individual determinants of the germ-plasm. It is probable that many general nutritive variations or climatic factors affect the germ-plasm as well as the soma, and it is by no means inconceivable that it is not all, but only certain definite determinants that are caused to vary.