are male and female gametes of both types, but certain 

 mutative changes are sex limited. In the following dis- 

 cussion I shall designate the two types of gametes as a 

 and p gametes. The former are those which bear the 

 most distinctive specific characters of the various forms, 

 whereas the latter bear the more general characters. The 

 known facts seem to be accounted for if we assume that in 

 fertilization the conjugation of an a with a p gamete ordi- 

 narily takes place, but not the conjugation of two p gam- 

 etes. In certain cases it seems that fertilization takes 

 place by the fusion of two a gametes and it appears likely, 

 also, that some species produce no p gametes. Some spe- 

 cies produce a and p gametes of both sexes. Others do 

 not seem to do so. It sometimes seems to be the case that 

 the female gametes are all a. When a mutation takes 

 place the modified character is perhaps Mendelian if it is 

 borne by both a and p gametes, but non-Mendelian if it 

 affects only the a gametes of a species in which fertiliza- 

 tion takes place by the fusion of an a with a p gamete. 



This conception of non-equivalent gametes has been 

 highly developed by de Vries, in a somewhat different way 

 from that outlined above. It has many obvious advan- 

 tages in explaining the (Enothera situation. It explains 

 seed sterility as well if not better than the Mendelian 

 hypotheses of .Renner and of Heribert-Nilsson, hypotheses 

 which are based of course upon the idea of gametic equiv- 

 alence. It explains why certain reciprocal crosses are 

 alike, and others unlike, why some of them breed true, 

 whereas others show segregation, why certain crosses 

 yield twin hybrids, and why the twins are, respectively, 

 matroclinic and patroclinic. It also explains other com- 

 plications which are quite unintelligible from a Mendelian 

 standpoint. I would by no means give the impression that 

 there are not many phenomena which remain obscure, but 

 I do wish to emphasize very strongly that a flood of light 

 is thrown upon the (Enothera situation by the conception 

 of non-equivalent gametes. 



By way of illustration, let us consider for a few mo- 

 ments the phenomenon which I have called mass mutation. 



