c 4 o PROBLEMS OF GENETICS 



compatible pairs of plants. Whether I was deceived by the 

 consequences of apogamy, or whether the pollen of certain plants 

 may belong to more than one class I do not know. The results 

 were confused in various ways. Usually the self-fertilised plants 

 set little or nothing, and cross-fertilised they set fully with such 

 uniformity that the few failures could plausibly be attributed to 

 mistakes in manipulation or to other extraneous causes. Later 

 de Vries announced 6 (without giving particulars) that he had 

 proved the existence of such classes in Linaria vulgaris; but on 

 making experiments with that species I again got no positive 

 results, and I came to the conclusion that in spite of inherent 

 improbability the conventional belief must be substantially true. 

 At last, however, the work of Correns, lately published, 7 does 

 definitely show that in one species, Cardamine pratensis, classes of 

 individuals exist such that individuals of the same class are 

 incapable of fertilising themselves or each other, but fertilisation 

 made between the classes is usually completely effective. Many 

 complications were encountered and some contradictory evidence 

 is recorded, but the general bearing of the results was positive 

 and indubitable. 



We know far too little of this phenomenon as yet to be able 

 to understand its significance, but I suppose we may anticipate 

 with some confidence that it will be found to be a manifestation 

 of dissimilarity between the male and female gametes of the 

 same individual, comparable with that first seen in the Stocks 

 (Matthiola) which throw doubles a state of things in all likeli- 

 hood to be found widely spread among hermaphrodite organisms. 

 Whether the incompatibility between species is to be associated 

 with that of the self-steriles also cannot be positively asserted, 

 though it seems not unreasonable to expect that such an associa- 

 tion will be discovered. 



The case of the apple and the pear is an impressive illustration 

 of this possibility. The two species are of course exceedingly 

 alike in all outward respects, but nevertheless the pollen of each 

 is entirely without effect on the other. Presumably we should 



6 Species and Varieties, 1905, p. 471. 



7 Correns, Festschr. med.-nat. Ges. zur 84 VersammL Deutsch. Naturf. u. Aertze. 

 Munster i. W., 1912. 



