FERTILITY 215 



If they are mated to any other male of a different strain, 

 they are fertilized. The males, too, are capable of fer- 

 tilizing the eggs of other strains, in fact, are quite 

 fertile. 



The factor that makes the rudimentary winged 

 fly is of such a sort that it carries infertility along with 

 it in the sense of self-infertility. This result has 

 nothing to do with inbreeding, and the stigma cannot 

 be removed by crossing out and extracting. 



A somewhat similar factor, though less marked, is 

 found by Hyde in certain of his inbred stock to which 

 I have referred. As his experiments show, the infer- 

 tility in this case is not due to lack of eggs or sperm, but 

 to a sort of incompatibility between them so that not 

 more than 20 per cent of the eggs can be fertilized by 

 males of the same strain. 



In the flowering plants where the two sexes are often 

 combined in the same individual, it has long been known 

 that there are cases in which self-fertilization will not 

 take place. The pollen of a flower of this kind if placed 

 on the stigma of the same flower or of any other flower 

 on the same plant will not fertilize the ovules. Yet the 

 pollen will fertilize other plants and the ovules may be 

 fertilized by foreign pollen. 



Correns has recently studied that problem and has 

 arrived at some important conclusions. He worked 

 with a common plant, Cardamine pratensis. In this 

 plant self-fertilization is ineffectual. He crossed plant 

 B with plant G, and reared their offspring. He tested 

 these with each other and also crossed each of them back 

 to its parents that had been kept alive for this pur- 

 pose. The latter experiment is simple and more in- 



