The Evolution Theory, Vol. 2 of 2

by August Weismann

[10] Castle and Allen have recently published the results of experiments in crossing white mice with grey, and these confirm Mendel's Law.

I have assumed for my theory that the reducing division took place according to the laws of chance, and that thus every combination of ids occurred with equal frequency. This assumption seems to be confirmed, by the experiments of the botanists I have mentioned, only in so far that in the crossing of hybrids with one another every combination of distinctive characters occurred with equal frequency. But, on the other hand, the splitting of the germ-plasm at the reducing division seems, as I said before, in many cases to take place in such a way that the id-groups of the two parents are discretely separated from one another; this was so in the stocks, peas, beans, and other hybrids. But even if this were always the case in these, we could hardly infer that it must be the same everywhere; we should rather expect that the relationship of the two parents and their ids would bring into play the finer attractions and repulsions between the ids of the germ-plasm, and would thus determine their arrangement and grouping. Further investigations may clear up this point; in the meantime we can only say that already—even among hybrids—many deviations from Mendel's Law have been established, for instance, by Bateson and Saunders (1902).

Before I conclude this lecture I should like to refer briefly to a phenomenon which Darwin was acquainted with and sought to explain through his theory of Pangenesis, but which at a later date was regarded as not sufficiently authenticated to justify any attempt at a theoretical explanation, since it seemed to contradict all our conceptions of hereditary substance and its operations. I refer to the phenomenon to which the botanists have given the pretty name of Xenia (guest-gifts), and which consists in the fact that in crosses of two different plants the characters of the male may appear not only in the young plant but even in the seed, so that a transference of paternal characters seems to take place from the pollen-tube to the mother, to the 'tissue of the maternal ovary.' In heads of yellow-grained maize (Zea) it is said that, after dusting with pollen from a blue-seeded variety, blue seeds appear among the yellow, and similar observations on other cultivated plants have been on record for more than half a century. Thus dusting the stigma of green varieties of grape with the pollen of a dark blue kind is said frequently to give rise to dark blue fruits.

Darwin accepted these observations as correct, and endeavoured to explain them as due to a migration of his 'gemmules' from the fertilized ovum into the surrounding tissue of the mother-plant. His explanation was not correct, we can say with confidence now, but he was right so far, for the phenomena of Xenia do occur; they are not illusory as most modern botanists seem inclined to believe. I myself was at first inclined to wait for further facts in proof that the phenomena of Xenia really occurred before attempting to bring them into harmony with my theory, and this will not be found fault with when it is remembered that these cases of Xenia seem to stand in direct contradiction to the fundamental postulates of the germ-plasm theory. For this depends essentially on a definite stable structure of the germ-substance, which lies within the nucleus in the form of chromosomes, and which cannot pass from one cell to another in any other way than by cell-division and division of the nucleus; how then could it pass from the fertilized ovum to the cells of the endosperm which do not derive their origin from it at all, but from other cells of the embryo-sac? In point of fact some of my opponents have cited Xenia as an actual refutation of my theory.

Fig. 82. Fertilization in the Lily (Lilium martagon), after Guignard. A, the embryo-sac before fertilization; sy, synergidæ; eiz, ovum; op and up, upper and lower 'polar nuclei'; ap, antipodal cells. B, the upper part of the embryo-sac, into which the pollen-tube (pschl) has penetrated with the male sex-nucleus (♂k) and its centrosphere; below that is the ovum-nucleus (♀k) with its (also doubled) centrosphere (csph). C, remains of the pollen-tube (pschl); the two sex-nuclei are closely apposed. Highly magnified.

That cases of Xenia really do occur is now established by the comprehensive and at the same time exceedingly careful experiments recently made by C. Correns with Zea Mais; it is only necessary to look through the beautiful figures with which his work is adorned to be convinced that heads of maize whose blossoms have been dusted with the pollen of a different kind produce more or less numerous seeds of the paternal kind, usually mingled with those of the maternal. Thus heads of the variety Zea alba resulting from fertilization with Z. cyanea exhibited a majority of white grains, but among them a smaller number of blue; and the converse experiment, of dusting Z. cyanea with the pollen of Z. alba, yielded heads in which a minority of white grains appears among a majority of blue. But it is always only the nutritive layer surrounding the embryo—the endosperm—which exhibits the character of the paternal species, and even the capsule surrounding the seed shows nothing of it, but is purely maternal. Thus the heads of different species with pale-yellow capsule, when dusted with the pollen of Z. rubra, never have red seeds like those of Z. rubra, but always seeds with a pale-yellow skin, while, in the converse experiment, dusting of the red-skinned species Z. rubra with pollen from Z. vulgata, all the seeds are red, like those of the maternal species, and the influence of the paternal species only shows when the strong red skin has been removed, so that the intense yellow colour of the endosperm, which in the pure maternal species is white, is exposed to view. Thus the mysterious influence of the pollen never goes beyond the endosperm, and the riddle of this influence is solved in the most unexpected manner, indeed was solved even before Correns had securely established the genuine occurrence of Xenia. The explanation is due to recent disclosures in regard to the processes of fertilization in flowering plants.

It had long been known that the pollen-tube contains not merely one generative nucleus but two, which arise from one by division. But what had till recently remained unknown was that not only one of these penetrated into the embryo-sac to enter into amphimixis with the egg-cell, but that the other also makes its way in, and there fuses with the two nuclei which had long been designated the upper and lower polar-nuclei ([Fig. 82, op. cit.]). Nawaschin and Guignard demonstrated that these two nuclei fuse with the second male nucleus; thus two acts of fertilization are accomplished in the embryo-sac, and one of these gives rise to the embryo, while the second becomes nothing less than the endosperm, the nutritive layer which surrounds the embryo, whose origin from the polar nuclei had been previously recognized.

Thus the riddle of Xenia is essentially solved. We understand how paternal primary constituents may find their way into the endosperm, and indeed must do so regularly; we understand also how the paternal influence never goes beyond the endosperm. The riddle is thus not only solved, but at the same time the view which assumes a fixed germ-plasm, and believes it to lie in the nuclear substance of the germ-cells, receives further confirmation, if it should need any, for if facts which are apparently contradictory to a theory can be naturally brought into harmony with it, this affords a stronger argument for the correctness of the theory than the power of explaining the facts which were used in building it up.

There is much more to be said in regard to Xenia, and I am sure that much that is of interest will be brought to light by deeper investigation; theoretical difficulties will still have to be overcome, and I have already pointed out one of these in my 'Germ-plasm,' but I must here rest satisfied with what has been already said.