The Evolution Theory, Vol. 2 of 2

by August Weismann

The simplest and clearest cases are again found among plant-hybrids. It may happen, for instance, that a hybrid between two species, when dusted with its own pollen, gives rise to descendants, some of which resemble only one of the ancestral forms: thus we have reversion to one of the grandparents. The explanation of this lies in the different modes in which the reducing divisions are effected; if they take place in such a manner that all the paternal ids of the hybrid are separated from the maternal ids, then the result is germ-cells which are like those of the grandparents, that is, those of the parent species, and these, if they happen to combine in amphimixis, must give rise to a pure seedling of one or other of the two ancestral species. This case occurs less rarely than was formerly supposed, and than it could do if absolutely free combination of the idants took place at reduction. If combination were quite unrestricted, all other possible combinations would be likely to occur as frequently as these. But recent experiments have shown that, in many plant-hybrids, the germ-cells of the hybrids which are fertilized by their own pollen are either purely paternal or purely maternal. There cannot, therefore, be free combination of the idants at the reducing divisions; the idants of the two parent-forms separate from one another and do not combine. It is doubtful, however, whether the same thing occurs within a race, for instance, in the case of reproduction within a human race.

In Man reversion to a grandparent occurs not infrequently, and we may explain it thus: the id-group which controlled the type of the grandfather was also contained in the germ-cell which gave rise to the existence of the father, but it did not dominate the type in that case because a more powerful id-group was opposed to it in the germ-cell of the grandmother. When, later on, at the reducing divisions of the germ-cells of the father, this id-group again arrived in the sperm-cells of the father, it would predominately control the type of the child, that is, of the third generation, provided that the egg-cell with which it combined contained a weaker id-group.

In the case of ordinary plant-hybrids what are designated reversions can only be called so in a wide sense, for the ancestral characters are contained visibly in the parent, although mingled with those of the other parent. In human families, however, there are undoubted cases in which one or more characters of the grandparent reappear in the child which were not in any visible way expressed in the parent, and must therefore have been contained in the parent's germ-plasm in a latent state. And there are both in animals and plants reversions to ancestors lying much further back, to characters and groups of characters which have not been visible for many generations, and the occurrence of these can only be explained on the assumption that certain groups of ancestral determinants have been carried on in the germ-plasm in too small a number to be able to give rise ordinarily to the relevant character. Such isolated determinants may, however, in certain circumstances be strengthened by the amphimixis of two germ-cells both containing small groups of them, and thus augmented they may gain a controlling influence. In this case the chances of the reducing divisions have a part to play, since they bring together the old unvaried ancestral determinants which, as we have seen, may persist in the germ-plasm of any species through a long series of generations. This would, of course, only suffice to bring about a reversion if the determinants of the ancestral species were still contained in the germ-plasm in comparative abundance. If this is no longer the case, something more is necessary, and that is the relative weakness of the more modern determinants.

If two white-flowered species of thorn-apple, Datura ferox and Datura lævis, be crossed, there arises a hybrid with bluish violet flowers and brown stalks instead of green. This was interpreted by Darwin as a reversion to violet-flowering ancestral species on both sides, for there are even now a great number of species of Datura with violet flowers and brown stalks. When the two white forms are crossed the reversion takes place every time, not merely in some cases, and we may conclude from this that in both these species there is still such a strong admixture of the same unvaried ancestral ids that they always excel the ids of the two modern species crossed, in strength though certainly not in number. And this superiority must again depend on the fact that similar determinants of the same part are cumulative in their effect, while dissimilars are not.

For this reason reversions to remote ancestors occur readily when species and breeds are crossed, while they are rare in the normal inbreeding of a species. The reversions of the breeds of pigeon to their wild ancestral form, the slaty-blue rock-pigeon, never result, as Darwin showed, and as we have already noticed, from pure breeding of one race, but only when two or more breeds are repeatedly crossed with one another. Even then it occurs by no means always, but only now and again. The germ-plasm of the breeds must therefore still contain ids of the rock-pigeon, but in a small number, varying from individual to individual. If by fortunate reducing divisions and the meeting together of a sperm-cell rich in ancestral ids with a similarly endowed egg-cell the number of ancestral ids be raised to such a point that it exceeds the number of modern-breed ids contained in each one of the conjugating germ-cells, the ancestral ids control the development and reversion occurs, for the ancestral ids together have a cumulative effect while the ids of the two parent breeds are different and therefore, as far as they are so, cannot be co-operative in their influence. But it must be understood that they need not be different as far as all their determinants are concerned, but usually only as regards some groups, and thus it happens that reversion does not occur in regard to all, but only in regard to particular characters—thus in the Datura hybrids, chiefly in regard to the colour of the flowers and the stem, and in the hybrids between different breeds of pigeon mainly in regard to the colour and marking of the plumage.

The reversions of the horse and ass to striped ancestors, which Darwin has made famous, go much further back into the ancestral history of the species, for while we know the ancestral form of the domestic pigeon in the still living rock-dove (Columba livia), the ancestral form common to the horse and the ass is extinct, and we can only suppose that it was striped like a zebra, because such striping occasionally occurs in pure horses and pure asses, at least in their youth, although now only on the legs, and because this striping is often more marked in the hybrid between the horse and the ass, the mule. In Italy, where one sees hundreds of mules, the striping is not exactly frequent, but it may occur in about two per cent., while in America it is said to be much more frequent. The germ-plasm of the horse and the ass must therefore contain, in varying numbers, ids whose skin-colour determinants represent in part still unmodified ancestral characters. When two germ-cells chance to meet in fertilization, both of which have received, through a favourable reducing division, a relatively large number of such ids, a relative majority of these in the fertilized ovum is opposed to the dissimilar and therefore mutually neutralizing homologous determinants of horse and ass, and reversion to the ancestral form occurs.

These cases of reversion are enough to show us that the old unmodified ancestral determinants may persist in the germ-plasm through long series of generations. But an even deeper glimpse into the dim ancient history of our modern species of horses is afforded by the occurrence of three-toed horses, references to a small number of which the palæontologist Marsh was able to discover in literature, and one of which he was able to observe in life. Julius Caesar possessed a horse whose three-toed feet represented a reversion to the horses of Tertiary times, Mesohippus, Miohippus, and Protohippus or Hipparion; for all these genera possessed, in addition to the strong middle toe, two weaker and shorter lateral toes.

In the germ-plasm of our modern horses there must still persist in certain ids the determinants of the ancestral foot, which, after a long succession of favourable reducing divisions combined with favourable chances of fertilization, may come to be in a majority, and may thus be able to induce a reappearance of a character which has long been hidden under the surface of the present-day type of the species.

I do not propose to enter further on the discussion of the phenomena of inheritance. A more detailed investigation of the phenomena of reversion is to be found in 'The Germ-plasm' published ten years ago; and the discussion could not be resumed here without a critical consideration of a relatively large series of newly acquired facts not always harmonizing, and, as yet, not even fully available. The year 1900 has given us the investigations of three botanists, De Vries, Correns, and Tschermak, who have sought by experiments in hybridization between different sorts of peas, beans, maize, and other plants, to throw light on the phenomena of inheritance, and thus on the actual processes which occur in the germ-plasm at the reducing division. This led to the discovery that similar experiments had been published as far back as 1866 by the Abbot of Brünn, Gregor Mendel, and that these had been formulated as a law which is now called Mendel's law. Correns showed, however, that this law, though correct in certain cases, did not by any means hold good in all, and we must thus postpone the working of this new material into our theory until a very much wider basis of facts has been supplied by the botanists. There is less to be hoped for from the zoologists in regard to this problem owing to the almost insuperable difficulties in the way of a long series of experiments in hybridization in animals. I myself have repeatedly attempted experiments in this direction, and have always had to abandon them, either because the crossing succeeded too rarely, or because the hybrids did not reproduce among themselves, or did so defectively, or because the distinguishing characters of the crossed breeds proved insufficiently tenacious or diagnostic. But it would be a fine task for zoological gardens to undertake such experiments from the point of view of the germ-plasm theory, and their success would afford material for the criticism of the theory, the more valuable because it is apparent from the experiments on plants that the processes of heredity are manifold, and are far from being uniform in different domains[10].