342 THE AMERICAN NATURALIST [Vol. XLV 



fowls do not carry definite genes for definite degrees of 

 fecundity. 



We touch here upon an important point; namely, the 

 relation of the mode of reproduction to the mode of in- 

 heritance. As one reflects upon the matter it becomes 

 clear that it is only in the sense of a reproductive line 

 that we can not, by definition, have pure lines in organ- 

 isms where the sexes are separate. It is perfectly pos- 

 sible to have a line of such organisms in which all the 

 individuals are gametically pure with reference to any 

 particular character. For example, it is the simplest of 

 matters to establish a line of horses pure in respect to 

 chestnut coat color. Any individual in such a line 

 mated to any other will never produce anything but 

 chestnut offspring. So similarly with any other char- 

 acter, it is only necessary to obtain homozygous individ- 

 uals in respect to any character in order to form a 

 gametically pure strain with reference to that character. 



It must further be kept clearly in mind that a repro- 

 ductive "pure line" (in the sense of Johannsen's defi- 

 nition) may be made up of individuals not gametically 

 pure (i e., homozygous). Thus suppose one crosses a 

 yellow and a green pea and then takes an F 2 heterozy- 

 gote individual seed which originated from a self- 

 fertilized F, individual as the "single, self -fertilized 

 individual" with which to start a line. The individ- 

 ual which starts such a line arose by self-fertilization 

 and is selfed to produce progeny and would thus fulfil 

 every requirement of a reproductive "pure line" as de- 

 fined by Johannsen. Yet it would produce both yellow 

 and green offspring. On the other hand, as already 

 pointed out, a line which is not, and from the nature of 

 its mode of reproduction never can be, reproductively 

 "pure" may be gametically so (*. e., have none but 

 homozygous individuals with respect to any character). 



We then see that the fact that in fowls the sexes are 

 separate and we therefore can not have reproductive 

 "pure lines" gives, per se, no reason to suppose that fe- 



