The Evolution Theory, Vol. 2 of 2

by August Weismann

We must not forget, however, that natural selection works essentially upon a basis of sexual reproduction, which with its reduction of the ids and its continually repeated mingling of germ-plasms, combines the existing variational tendencies, and thus diffuses them more and more uniformly among the individuals of a whole area of occupation. Sexual reproduction, continual intermingling of the individuals selected for breeding, is thus a very effective and important, if not an indispensable, factor in the evolution of the specific type.

But it is not only in the case of species transformations due to new adaptations that sexual mingling operates; it does so also in the case of variations due to purely intra-germinal causes. We have already seen in discussing Isolation that isolated colonies may come to have a peculiar character somewhat different from that of the parent form, because they were dominated by some germinal variational tendency which occurred only rarely in the home of the parent form, and therefore never found expression there. On the isolated area this would indeed be mingled with the rest of the existing germinal variational tendencies, but the result of this mingling would be different, and the further development of the tendency in question would probably not be suppressed.

We need not wonder, therefore, that specific types occur in such varying degrees of definiteness. If a species is distributed over a wide connected territory, sporadically, not uniformly, it will depend partly upon the mutual degree of isolation of the sporadic areas whether the individual colonies will exhibit the same specific type or will diverge from one another. If the animal in question is a slow-moving one like a snail, the intermingling from neighbouring sporadic colonies will be much slower than in the case of a resident bird such as a woodpecker. Many interesting results would undoubtedly be gained if the numerous careful investigations into the geographical distributions of species and their local races were studied with special reference to this point, and much light would undoubtedly be thrown upon the evolution of the specific type. But it would be absolutely necessary to study carefully all the biological relations of the animals concerned, to trace back the history of the species as far as possible, and to decide the period of immigration, the mode and direction of distribution, and so on.

Nothing shows more plainly the enormous duration of the period of constancy in species than the wide distribution of the same specific type on scattered areas or even over different areas absolutely isolated from one another. If, as we saw, the same diurnal butterflies live in the Alps and the far North, they must have remained unvaried since the Glacial period, for it was the close of that period that brought them to their present habitats, and while other diurnal butterflies now living on the Alps differ from their relatives in the Arctic zone (Lapland, Siberia, and Labrador) in some unimportant spot or line, and must therefore have diverged from one another in the course of the long period since the Glacial epoch, they have done so only to a minimal degree, and in characters which possibly depend solely upon germinal selection and can hardly be regarded as adaptations.

I should like, however, to cite one of the few cases known to me in which a slight deviation from the specific type undoubtedly depending upon adaptation has occurred on an isolated region. The nut-jay (Caryocatactes nucifraga) lives not only upon our Alps and in the Black Forest, but also in the forests of Siberia, and the birds there differ from those with us in small peculiarities of the bill, which is longer and thinner in them, shorter and more powerful in ours. Ornithologists associate this difference with the fact that in this country the birds feed chiefly on hard hazel nuts, which they break open with their bill, and on acorns, beechmast, and, in the Alps, on cembra-cones, while in Siberia, where there are no hazel nuts, they feed chiefly upon the seeds of the Siberian cedar, which are concealed deep down in the cones. Thus we find that in Siberia the bill is slender, and that the upper jaw protrudes awl-like beyond the lower, for about 2.5 mm., and probably serves chiefly to pick out the cedar nuts from behind the cone-scales. In the Alps the birds (var. pachyrhynchus) break up the whole cone of the cembra-pine with their thick, hard bill, and in the Upper Engadine, where the nut-jay is abundant, I have often seen the ground underneath the cembra-pines covered with the débris of its meal. In addition to these differences between the two races, the Alpine form is stronger in build, the Siberian form is daintier; in the former the white terminal band on the tail is narrow (about 18 mm.), in the Siberian form it is broader (about 27 mm.).

Such cases of variation of individual parts in different areas seem to me very important theoretically, because they furnish us with an answer to the view which represents the species as a 'life-crystal,' which must be as it is or not at all, and which therefore cannot vary as regards its individual parts. The case of the nut-jay has the further interest that it is one of the few in which we find the new adaptation of a single character without variation of most of the other characters.

It is only in an essentially different sense that we can compare the species, like any other vital unit, to a crystal, in so far as its parts are harmoniously related one to another, or, as I expressed myself years ago, are in a state of equilibrium, which must be brought about by means of intra-selection. This analogy, however, only applies to the actual adjustment of the parts to a whole, and not to their casual adjustment. Species are variable crystals; the constancy of a species in all its parts must be regarded as something quite relative, which may vary at any time, and which is sure to vary at some time in the course of a long period. But the longer the adaptation of a species to new conditions persists the more constant, ceteris paribus, and the more slowly variable will it become, and this for two reasons: first, because the determinants which are varying in a suitable direction are being more and more strictly selected, more and more precisely adapted, and are thus becoming more like each other; and secondly, because, according to our theory, the homologous determinants of all the ids do not vary in the required direction, and a portion of the unvaried ancestral ids is always carried on through the course of the phylogeny, and only gradually set aside by the chances of reducing division. But the more completely these unvaried ids are eliminated from the germ-plasm the less likely will they be to find expression in reversions or in impurities of the new specific characters. I may recall the reversions of the various breeds of pigeons to the rock-dove, those of the white species of Datura to the blue form, and the Hipparion-like three-toed horse of Julius Cæsar, and so on. The unvaried ancestral ids, which in these cases find only quite exceptional expression, will make the new 'specific character' fluctuating, as long as they are contained in the germ-plasm in considerable numbers, but they must become more and more infrequent in the germ-plasm as successive generations are passed through the sieve of natural selection, and the oftener these germ-plasms, to which the chances of reducing division and amphimixis have assigned a majority of the old determinants, are expelled from the ranks of the species by personal selection. The oftener this has occurred in a species the less frequently will it recur, and the more constant, ceteris paribus, will the 'type' of the species become.

If we add to this idea the fact that adaptations take place very slowly, and that every variation of the germ-plasm in an appropriate direction has time to spread over countless hosts of individuals, we gain some idea of the way in which new adaptations gradually bring about the evolution of a more and more sharply-defined specific type.

So far, however, we have only explained the morphological aspect of the problem of the nature of species, but there is also a physiological side, and for a long time this played an important part in the definition of the conception of species. Until the time of Darwin it was regarded as certain that species do not intermingle in the natural state, and that, though they could be crossed in rare cases, the progeny would be infertile.

Although we now know that these statements are only relatively correct, and that in particular there are many higher plants which yield perfectly fertile hybrids, it is nevertheless a striking phenomenon that among the higher animals, mammals, and birds the old law holds good, and hybrids between two species are very rarely fertile. The two products of crossing between the horse and the ass, the mule and the hinny, are never fertile inter se, and very rarely with a member of the parent stock.