HEREDITY 1 83 



blood relationship of animals. Nuttall found among others that if 

 dog's serum was injected into a rabbit, the serum of this rabbit after- 

 ward gave a precipitation with the blood of eight various canides, 

 but with the blood of no other group of animals ! These experiments 

 may also explain why the bastards between the sea urchin and starfish 

 show a much greater mortality than the pure breed. In my experi- 

 ments the hybrids between starfish and sea urchin died in large 

 numbers after they reached the blastula or gastrula stage.* It seems as 

 if the spermatozoon of the starfish, in addition to a developmental sub- 

 stance, also carries something else into the sea urchin's egg which poisons 

 the latter. It remains for further experiments to decide how far the 

 physicochemical incompatibiHty of heterogeneous species which Landois 

 and his successors discovered, restricts heterogeneous hybridization. It 

 is, however, already obvious enough that ultimately the problem of 

 hybridization and heredity must be transferred from the morphological 

 to the chemical or physicochemical field. 



We may now continue the discussion of the problem of heredity. 

 The man whose work marks with that of Landois the beginning of a 

 real theory of heredity had a fate similar to that of Landois. Gregor 

 Mendel was a teacher of physics in Graz, and evidently the writings of 

 Darwin induced him to investigate the laws of heredity; but he went 

 at the problem in a spirit so entirely different from that of the biologists, 

 and at the same time in a way which was so superior, that his discoveries 

 were entirely overlooked for over thirty years, until De Vries discovered 

 the same facts, and also discovered accidentally Mendel's paper. At 

 almost the same time Correns and Tschermak also called attention to 

 Mendel's work. Mendel f carried on experiments on the hybridization 

 of varieties of peas which he selected so that they differed in only one 

 characteristic. It was his intention to find out what became of that 

 discriminating or critical characteristic in the offspring. He found that 

 the children of such parents — the first generation of hybrids — did 

 not occupy an intermediate position between the two parents in regard 

 to the discriminating characteristic, but were all pure breeds, inasmuch 

 as the discriminating characteristic of one parent was transmitted to 

 all of the children, while the characteristic of the other parent was ap- 

 parently not transmitted. The discriminating characteristic of the peas 

 used by Mendel for his experiments were, e.g., the difference in the shape 

 of the ripe seeds, whether they were spherical or angular. When he 

 crossed two forms of peas which were identical in every respect, except 



* Loeb, loc. cit. 



t Gregor Mendel, Versuche uber Pflan%enhybride. Ostwald, Klassiker der Naturwissen- 

 schaflen, Vol. 121. De Vries, Die Mutaiionsiheorie, Leipzig, 1901. 



