The Evolution Theory, Vol. 2 of 2

by August Weismann

But it happens sometimes, though more rarely, that not the whole plant but individual shoots may exhibit the variation. To this class belong the 'bud-variations' of our forest trees, the copper-beeches, copper-oaks, and copper-hazels, the various fasciated varieties of oak, beech, maple, and birch, and the 'weeping trees'; also the numerous varieties of potato, plantain, and sugar-cane. It seems that only a few of these breed true when reproduced from seed, or in other words, they usually exhibit reversions to the ancestral form: on the other hand, in the weeping oak for instance, nearly all the seedlings exhibit the character of the new variety, though 'in varying degrees.' The records as to the transmissibility of bud-variations through seed are probably not all to be relied upon, and new investigations are much to be desired, but the fact that in many cases they may be propagated not only by means of layers and cuttings but by seed also, is most important in our present discussion, for it proves that here too the varied determinants must be contained in a majority of ids. As it is only a single shoot that exhibits the saltatory variation, only the germ-plasm which was contained in the cells of this one shoot can have varied, and it must have done so in so many ids that the variation prevailed and found expression. But that, in this case also, the variation does not appear in all, but only in a small majority of ids, is proved by the frequent reversion of bud-varieties to the ancestral form. I have already reported a case of this kind shown to me by Professor Strasburger in the Botanic Gardens in Bonn, where a hornbeam with deeply indented 'oak-leaves' had one branch which bore quite normal hornbeam leaves. In my own garden there is an oak shrub of the 'fern-leaved' variety, whose branches bear some leaves of the ordinary form; variegated maples with almost white leaves often exhibit in individual branches a reversion to the fresh green leaves of the ancestral form. We see from this that what is so energetically disputed by many must in reality occur—namely, differential or non-equivalent nuclear division—for otherwise it would be unintelligible how the ids of the new variety, if they once attain a majority in the tree, could give place in an individual branch to a majority of the ancestral ids. Only differential nuclear division, in the manner of a reducing division, can be the cause of this. Of course this implies only a dissimilar or differential distribution of the ids between the two daughter-nuclei, not a splitting up of the individual ids into non-equivalents.

That in free Nature bud-variations left to themselves can ever become permanent varieties is probably an unlikely assumption, because of the inconstancy of their seeds which only breed true in rare cases; nor is it likely that such variations as the copper-beech, the weeping ash, and so on could hold their own in the struggle for existence with the older species; but there is certainly nothing to prevent our assuming that, in certain circumstances, saltatory variations, when they have a germinal origin, may become persistent varieties and may even lead to a splitting of the species. This may happen, for instance, when the variations remain outside the limits of good and bad, and thus are neither of advantage to the existence of the species nor a drawback thereto. In the next chapter we shall discuss the influence of isolation upon the formation of species, and it will be seen that in certain conditions even indifferent variations may be preserved, and that saltatory variations, as for instance in the evolution of species of land-snails or butterflies, may have materially contributed to bring this about.

I should like to emphasize still more the part played by saltatory variations arising from germinal selection in the origin of secondary sexual characters. As soon as personal selection, whether sexual or ordinary, prefers as useful in any sense a saltatory variation, it is not only preserved and becomes a character of a variety, but it may increase, and we have to ask whether such sudden variations are frequently of a useful kind, especially when not individual characters alone, but whole combinations of them are implicated. If we may judge from the sports of the flowers and the leaves of plants, transformations useful to the species as a whole rarely occur suddenly, that is, they occur only in a few out of very numerous sports; they are much more frequently indifferent, although quite visible and often conspicuous variations.

For this reason I am disposed to attribute to saltatory variations a considerable share in the production of distinctive sexual characters. From saltatory variations in flowers, fruits, and leaves we know that these may be conspicuous enough even on their first appearance, and so we are justified in finding in such variations the first beginnings of many of the decorative distinguishing characters which occur in the males of so many animals, especially butterflies and birds. As soon as it is admitted that variations of considerable amount, which have been slowly prepared in the germ-plasm by means of germinal selection, can suddenly attain to expression, one of the objections against sexual selection is disposed of, for conspicuous variations are necessary for the operation of this kind of selection, since the changes in question must attract the attention of the females if they are to be preferred. Without such preference, even though it be not quite strict and consistent, a long-continued augmentation of the decorative characters is inconceivable.

But as intra-germinal disturbances of the position of equilibrium in the determinant system is at the root of the saltatory variations of our cultivated plants, it must also have played a large share in the evolution of breeds among our domesticated animals, which is therefore by no means wholly due to artificial selection operating upon the variation of individual characters. In all breeds in the formation of which the production of more than a single definite character was concerned, as, for instance, in the broad-nosed breeds of dog—bull-dog and pug-dog—we may refer the peculiar variation of many parts to disturbances of the equilibrium of the determinant system, which bring to light, not suddenly as in the case of saltatory variations, but gradually and increasingly, the curious complex of characters. Darwin referred such transformations of the whole animal facies, where a single varying character is deliberately selected, to correlation, and by this he understood the mutual influence of the parts of an animal upon one another. Such correlation certainly exists, as we have already seen in discussing histonal selection, but here we have rather to do with the correlation of the parts of the germ-plasm, with the effects of germinal selection, which, affected by the artificial selection of particular characters, gradually brings about a more marked disturbance in the whole determinant system.

In the evolution of our breeds of domesticated animals, germinal selection in the negative sense must also have played a part—I mean through the weakening and degeneration of individual determinants. Only in this way, it seems to me, can we explain the tameness of our domestic animals, dogs, cats, horses, &c., in which all the instincts of wildness, fleeing from Man, the inclination to bite, and to attack, have at least partly disappeared. It is, of course, very difficult to estimate how much of this is to be ascribed to acquired habitude during the individual lifetime. The case of the elephant might be cited in evidence of tameness which arises in the individual lifetime, for all tame elephants are caught wild, but it seems that captured young beasts of prey, such as the fox, wolf, and wild cat, not to speak of lions and tigers, never attain to the degree of tameness exhibited by many of our domesticated dogs and cats. The very considerable differences in the degree of tameness of dogs and cats go to show that the case is one of instincts varying in different degree.

If this be so, then the instinct of wildness, if I may express myself so for the sake of brevity, has degenerated in consequence of its superfluity, and through the process of germinal selection, which allowed the determinants of the brain-parts concerned to set out on a path of downward variation upon which they met with no resistance on the part of personal selection.

Herbert Spencer adduced against my position the case of the reduction in the size of the jaws in many breeds of dog, especially in pugs and other lap-dogs, which he regarded as evidence of the inheritance of acquired characters. But this and analogous cases of the degeneration of an organ during a long period in which the animal had been withdrawn from the conditions of natural life is intelligible enough on the assumption of persistent germinal selection aided by panmixia. The jaws and teeth in these spoilt pets no longer require to be maintained at the level of strength and sharpness essential to their ancestors which depended on these characters, and so they fell below it, became smaller and weaker, but could not disappear altogether, for the process of degeneration was brought, or is being brought, to a standstill by the intervention of personal selection.

Even the lower jaw in Man is declared by many authors to be degenerate. Collins found that the lower jaw of the modern Englishman was one-ninth smaller than that of the ancient Briton, and one-half smaller than that of the Australians; Flower showed that we are a microdont race like the Egyptians, while the Chinese, Indians, Malays, and Negroes are mesodont, and the Andamanese, Melanese, Australians, and Tasmanians are macrodont. This does not of itself imply that we exhibit a degeneration of dentition, though this conclusion is hinted at by other facts, such as the variability of the wisdom-teeth. It need not surprise us, indeed, that a retrogressive variation tendency should have started in this case, for, with higher culture and more refined methods of eating, the claims which personal selection was obliged to make on the dentition have been greatly diminished, and germinal selection would thus intervene.

Every one knows how the quality of human teeth has deteriorated with culture, and this not in the higher classes only, but even among the peasantry, as Ammon has observed. The time is past when raw flesh was a dainty, and when bad teeth meant poor nutrition, if not actual starvation. Even nowadays famine plays a terrible and periodically recurrent rôle as an eliminator among some negroid races.