The Evolution Theory, Vol. 2 of 2

by August Weismann

When we have thus recognized that the origin of a variation in a definite direction results as inevitably when it is called forth by the indirect influence of conditions, that is, through the need for a new adaptation, as when it is induced in the germ-plasm by direct causes such as those of climate, we shall not be disposed to estimate very highly the part played by mutual sterility in the origin of species. We shall rather be inclined to assign it a rôle at a later stage, after the separation of the forms has taken place, and this view is supported by the fact of the mutual sterility of most nearly related species, and by the theoretical consideration that the frequency of hybrids, even if these are always eliminated in the struggle for existence, must signify a loss for both the parent species. But no certain conclusion can be based upon either of these arguments—not upon the theoretical one, because here again we are unable to estimate the extent of this loss; and not upon the argument from fact, because the results of experiments in crossing animals have generally been overestimated, since we are apt to regard the most nearly related animals that are at our disposal as being very closely related. Thus, for instance, horse and ass, horse and zebra are undoubtedly rightly included within the same genus, but the fact that there are several species of zebra in Africa gives us an idea of the number of transition stages that may have existed between the horse and the zebra. Entomologists have sometimes reared hybrids between the most nearly related indigenous species of hawk-moth of the genus Smerinthus—hybrids of Smerinthus ocellata, the eyed hawk-moth, and Smerinthus populi, the poplar hawk-moth. I have myself made many experiments of this kind, and have often succeeded in getting the two species to pair and even to deposit eggs, but I have never seen a caterpillar emerge from them. The hybrids do occur, however, and they have been repeatedly obtained by Standfuss. In external appearance they are intermediate between the parent forms, but with marked divergences, thus, for instance, the beautiful blue eye on the posterior wing of S. ocellata ([Fig. 5], vol. i. p. 69) may have almost disappeared or be only indicated. They are sterile. But we know three species of Smerinthus in North America, which are all much nearer to S. ocellata than S. populi is, for they all possess the eye-spot referred to, although it is less well developed. The proof that the most nearly related species do not yield fertile descendants should be sought for by crossing Smerinthus ocellata with one of these American species if it is to have any decisive value.

Experiments of the same kind have been made by Standfuss with different species of indigenous Saturnia, and these have shown not only that crossing is possible, but that the hybrids are fertile in their turn. These results are to be valued the more highly because it is well known that Lepidoptera, and even the usually prolific silk-moths, do not readily reproduce in captivity, even within the same species. We have in Saturnia pyri, spini, and carpini three well-marked distinct species with no intermediate forms in nature, and with quite different colouring in the caterpillars. That these should have been successfully combined in a triple hybrid proves at least that sexual alienation cannot have advanced far in this case.

We must beware, however, of attributing too much to the constant mutual crossing which occurs in a species living on a connected area and of regarding its influence as irresistible. Undoubtedly it must go far towards securing the uniformity of individuals, but not only is it unable to achieve this, but it cannot successfully resist the stronger influences making for variation which may be exerted upon a part of the area of the species. We have already seen that it is quite erroneous to suppose that every new adaptation must be lost sight of again because of the continual crossing with other members of the species upon the same area. Other things being equal, this depends entirely upon the importance of the adaptation in question. Just as climatic influences may be so strong that they entirely overcome the influence of crossing, and give rise to a local race notwithstanding imperfect geographical isolation, so the same may happen in the case of adaptations. It is quite conceivable that the polar hare of Scandinavia may have evolved a whole series of races, each of which is adapted to the duration of the snow in its geographical range, although a crossing of these quick-footed animals must frequently occur in the course of time, even as regards forms from widely separated areas, and although the whole region is inhabited without a break by the species, so that a 'mingling' of the hares of all regions from south to north, and conversely, may take place, and indeed must be continually taking place, though of course very slowly.

It is precisely this extreme slowness with which the intermingling of racial characters take place that seems to me essential for the production of local or, as in this case, regional races. It is not difficult to calculate the rate of 'blood-distribution' if we assume that the conditions for a rapid dissemination are as favourable as possible. Let us assume that it takes place along a certain line—in this case from south to north—and that the numerical strength of the species remains constant, each pair of hares yielding a pair of surviving offspring, which will attain to reproduction. Let us suppose that one of these hares moves his home northwards to the extent of his range, that is, as far as a hare is accustomed to range from his head quarters, and that he pairs with one of the descendants on the next stretch.

Let us further suppose that this stretch is ten kilometres in extent, and that the change of quarters take place once in each year, then the blood of a South Scandinavian hare would have extended ten kilometres further north in ten years, and in a hundred years 100 kilometres; it would not, however, be quite pure, but mixed and thinned by crossing with a hundred mates of different individual bloods, that is, thinned to the extent of 2 to the 100^th power, that is, to less than a millionth part. Thus even with these much too favourable assumptions the influence of a region of hares 100 kilometres distant would be actually nil upon the inhabitants of a region which was in process of new adaptation. That the assumptions are too favourable is quite obvious, since every surviving hare would not be likely to move his home, and probably the majority would remain in the old quarters and find mates there. The blood-mingling would therefore take place much more rarely, perhaps only once in ten years, and the wandering descendants of the second generation might move southward, and so neutralize the previous blood-mingling, and so on. But let us keep to our favourable assumptions, and attempt to determine how strong the assimilating influence of the blood-mingling from south to north would be upon a point A. The blood of the nearest stretch diluted to a half would affect the inhabitants of A once in each year; the second stretch would only contribute blood of ¼ strength, the third of 1/8, the fourth of 1/16, and the blood of the tenth would be diluted to 1/1024. A region B, extending over twenty such stretches, or 200 kilometres, would thus shelter within it a hare population of which the centre would only be influenced from the periphery in vanishing proportions. If the winter were of equal length over the whole area of B, all the inhabitants would be tending to vary the period for which the winter dress was worn in correspondence with the length of the winter, and the centre of the region would be the less impeded in this process because the more peripheral areas would also be approximating to the same adaptation. But since even the admixture of 1/32 of strange blood could have no hindering influence upon a variation, there would remain a region of 2 × 5 = 10 stretches upon which the influence of the non-varying regions would be without effect. There would therefore arise a new race in relation to the duration of the winter dress, and this would not cease abruptly, but would gradually pass over into the neighbouring regions, which however would be pure at their centre, just as is probably the case in reality, if we regard B as any point in the line of distribution from south to north.

The harmony of the individuals within a species will therefore depend in part upon the mingling of hereditary primary constituents associated with reproduction, but in greater part upon adaptation to the same conditions; it is a similarity of adaptation, and the strongest influence which sexual reproduction exerts lies not in the mingling of these hereditary constituents alone, but above all in the reduction in the germ-plasm of the two parental hereditary contributions—a reduction which results from and through the sexual intermingling. It is only this that prevents these primary constituents from varying at too unequal a rate in the transformations of species, and causes them ultimately to resemble each other closely again.

But while mutual sterility is not an absolutely necessary condition in the separation of species, it would be going too far in the opposite direction to regard mutual fertility as something general, or to attribute to it a rôle in the origination of new species.

Certain botanists, like Kerner von Marilaun, regard the mingling of species as a means of forming new species with better adaptations; they suppose that fertile hybrids may, in certain circumstances, crowd out the parent species, and themselves become new species. It will be admitted that such cases do occur, that, for instance, in the north of Europe the hybrid between the large and the small water-lily, Nuphar luteum and Nuphar pumilum, to which the name Nuphar intermedium has been given, has driven both the parent species from the field, because its seeds mature earlier, and it is therefore better adapted to the short vegetative period of the north, but nevertheless we must maintain that the evolution of species on the whole does not take place through hybridization. Such cases are probably nothing more than rare exceptions. This is corroborated by the entire insignificance of hybridization in animals, among which species appear in the same way as they do in plants, and where the mingling of two species occurs only sporadically and in a few species, never to any very great extent.

If species are complexes of adaptations, based in each case on the given physical constitution of the parent species, then we can readily understand the fact that they are in our experience not fixed or eternal, but that they change in the course of the earth's history. The numerous fossil remains in the various strata of the earth's crust prove that this is true in a high degree, that in almost every one of the more important geological strata new species occur, and that not only species and genera, but families, orders, indeed whole classes of animals, which lived at one time, have now completely disappeared from the face of the earth. We can understand this phenomenon when we reflect that the conditions of life have also been slowly changing through the course of the earth's history, so that the old species had only the alternative of dying out, or of becoming transformed into new species.

But simple as this conclusion is, it can hardly be deduced with certainty from the occurrence and succession of the fossil species alone. For instance, we should strive in vain to recognize the cause which led one of those regularly arranged snail-species of the Steinheim lake basin to become transformed into one or two new species at a particular time, or to find the cause which moved those curious tripartite Crustaceans of primitive times, the Trilobites, which peopled the Silurian seas with such a wealth of forms, to become suddenly scarce towards the end of the Silurian period, and to disappear altogether in the succeeding period, the Devonian. The famous geologist Neumayr sought to refer this striking phenomenon to the fact that just at that time the Cephalopods, 'the most formidable and savage marauders among the invertebrate marine fauna,' gained the ascendancy, and it is quite possible that he was right in his surmise, but who is to prove it? Can we decide even in the case of animals now living whether the losses inflicted on a much persecuted species by an abundant and greedy persecutor exceed the numbers of progeny, and are therefore driving the species gradually towards extermination? Probable as such a supposition appears, it cannot be accepted as proven.