SHARE OF THE PARENTS IN BUILDING UP THE OFFSPRING 57 



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 Brlinn, 

 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 j)ostpone 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 insuificiently tenacious or 

 diagnostic. But it would be a line 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 experi- 

 ments on plants that the processes of heredity are manifold, and are 

 far from being uniform in different domains ^. 



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 con- 

 firmed, by the experiments of the botanists I have mentioned, only in 

 so far that in the crossing of hybrids with one another every com- 

 bination of distinctive characters occurred with equal frequency. 

 But, on the other hand, the splitting of the germ-plasm at the reduc- 

 ing 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 



' Castle and Allen have recently published the results of experiments in crossing 

 white mice with grey, and these confirm Mendel's Law. 



