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]. [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.