THE AMERICAN NATURALIST 



[Vol. L 



with a certain amount of seed sterility indicate the probability 

 that other classes of gametes are eliminated or fail to func- 

 tion and that possibly certain types of zygotes may be formed 

 which are unable to live. CEnothera neo-Lamarckiana therefore 

 shows itself to be impure or heterozygous because it develops 

 different types of gametes even though the plant when selfed 

 reproduces itself in a fairly large proportion of its progeny. 

 This behavior seems to me quite the same in principle as that of 

 De Vries's Lamarckiana, the only difference being that the total 

 number of individual variants thrown by Lamarckiana is much 

 smaller than those at present thrown by neo-Lamarckiana. How- 

 ever, as has been noted, the seed sterility of Lamarckiana is very 

 much higher than that of neo-Lamarckiana in the F 4 generation, 

 and it is my working hypothesis that this fact is at least partly 

 responsible for the smaller numbers of variants produced by the 

 former. 



CEnothera Lamarckiana of De Vries's cultures seems to me 

 best interpreted as an impure species producing regularly be- 

 cause of its heterozygous or hybrid nature a number of classes 

 of gametes relatively few of which, because of the extensive 

 sterility, both gametic and zygotic, are able to form viable seeds 

 different from those that reproduce the species. CEnothera 

 Lamarckiana breeds true in its high degree because only the 

 gametic combinations that reproduce Lamarckiana survive the 

 mortality visited on most of the gametes and zygotes. By this 

 view Lamarckiana is very much the reverse of a representative 

 pure species which De Vries has assumed it to be and its "mutat- 

 ing habit" is the result of its hybrid origin and heterozygous 

 nature rather than a spontaneous expression of homozygous germ 

 plasm. The fact that the same types of "mutants" from La- 

 marckiana are produced by successive generations in fairly stable 

 proportions indicates that their differentiation lies in the mechan- 

 ism of segregation in heterozygous germ plasm rather than in a 

 sporting tendency (mutation) which would be expected to ex- 

 press itself in ever-varying ways and degrees. 



It is worth noting how different is the conception, here ex-, 

 pressed, of the constitution of a pure species from the view 

 formerly and probably now very generally held. Formerly a 

 species was considered pur» if it bred true. Now we believe that 

 a species may be impure and still breed very largely or even 

 wholly true if a degree of sterility is present sufficient to render 



