Nos. 618-619] THE ROLE OF REPRODUCTION 



2*7 



zygotic individual in competition with its fellows can not 

 be gainsaid. 



The investigations of Shull and of the writer on the 

 effects of cross- and self-fertilization have brought to 

 light another series of facts with a bearing on the problem 

 under discussion. It has been shown that the apparent 

 deterioration of cross-bred species when self-fertilized is 

 in large measure and perhaps wholly due to the loss of 

 hybrid vigor 4 through the formation of homozygotic 

 Mendelian recombinations and not an effect of inbreeding 

 per se because of the union of like germplasms. This is 

 a plausible argument against Darwin's idea that con- 

 tinued inbreeding is abhorrent to Nature. It may even 

 be said to be a valid reason for declining to accept 

 Maupas's belief in the impossibility of continued asexual 

 reproduction, for there is no very good reason for dis- 

 tinguishing between continued asexual propagation and 

 continued self-fertilization. Inbreeding simply brings 

 about the opposite effect from crossing, and we can see 

 no reason for the comparative failure of naturally inbred 

 types in the wild other than the lack of chances for 

 progress. The one is the conservative manufacturer who 

 continues the original type of his article, the other is the 

 progressive who makes changes here and there without 

 discouragement until the acceptable improvement is 

 found. In fact, if this argument be overlooked, the in- 

 bred types which have persisted hold some advantages 

 over the cross-bred types. The self-fertilized species are 

 inherently strong and vigorous, witness tobacco and 

 wheat. They stand or fall on their own merits. They 

 are unable, as are cross-bred species, to cover up in- 

 herent weakness by the vigor of heterozygosis. Cross- 

 fertilized maize has become the king of cultivated plants 

 because of its variability, but many of our best varieties 

 carry recessive characters very disadvantageous to the 

 species. 



The next secondary advantage of sexual reproduction is 



4 Accepting the view that the vigor of the first hybrid generation is due 

 to dominant characters meeting makes this argument even more forcible. 



