FERTILITY 211 



But even amongst the four lines descended from the 

 same ex-con jugant marked differences exist. These 

 differences cannot be attributed to constitutional dif- 

 ferences unless a segregation of factors takes place 

 after conjugation or unless it can be shown that these 

 differences are not significant. In the light of these 

 conflicting results on paramcecium it may seem unsafe 

 to draw any far-reaching conclusions concerning the 

 nature of sexual reproduction in general from the evi- 

 dence derived from these forms. In the higher animals, 

 however, the evidence that segregation takes place 

 prior to fertilization and that recombinations result 

 can scarcely be doubted. 



THEORIES OF FERTILITY 



Let us now try to sum up the evidence in regard to 

 the influence of cross-fertilization. This can best be 

 done by considering the three most important hypoth- 

 eses that have been brought forward to explain how 

 crossing gives greater vigor. 



Shull and East explain the vigor of the hybrid by 

 the assumption that it contains a greater number of dif- 

 ferent factors in its make-up than either of its parents. 

 They support the view by an appeal to the next (F2) 

 generation from such hybrids that shows a lower 

 range of vigor, because, while a few individuals of this 

 generation will be as mixed as the hybrid (Fi), and 

 therefore hke it, most of them will be simpler in com- 

 position. This interpretation is also supported by the 

 evidence that when pure lines (but not necessarily, 

 however, homozygous lines) are obtained by self-fer- 

 tilizing the offspring of successive generations from 



