MODE OF REPRODUCTION IN RELATION TO BREEDING 47 



From these and other results (Jones, 1918) it is apparent that 

 selfing in maize produces: 



1. Strains which can not be perpetuated. 



2. Strains which can be perpetuated only with difficulty. 



3. Strains which exhibit normal development but vary in amount of 

 growth attained. 



EXPLANATION OF HYBRID VIGOR 1 



The studies of the early hybridizers, Koelreuter, Gartner, 

 Knight, and others, gave results which can be summed up in a 

 single sentence as follows (East and Hayes, 1912) : 



" Crosses between varieties or between species often give hybrids with 

 a greater vegetative vigor than is possessed by either parent." 



Darwin made extended and careful studies of the effects of 

 cross- and self-fertilization in plants. He conclusively proved 

 that in general there is an advantage in cross-fertilization. While 

 he noted some self-fertilized families he believed these would 

 eventually perish. Lacking as he did a knowledge of Mendelian 

 phenomena it was impossible for Darwin to develop as logical 

 an explanation of these results as we now have. Darwin thought 

 the results could best be explained by the nature of the sexual 

 elements rather than in the act of crossing. 



Several explanations of hybrid vigor have been advanced 

 since the rediscovery of Mendel's law. In all cases hete'rozygosis 

 has received a major place in the explanation. The results of 

 these studies have been summed up as follows (East and Hayes, 

 1912): 



"1. The decrease in vigor due to inbreeding naturally cross-fertilized 

 species and the increase in vigor due to crossing naturally self-fertilized 

 species are manifestations of one phenomenon. This phenomenon is 

 heterozygosis. Crossing produces heterozygosis in all characters by 

 which the parent plants differ. Inbreeding tends to produce homozy- 

 gosis automatically. 



" 2. The phenomenon exists and is in fact widespread in the vegetable 

 kingdom. 



"3. Inbreeding is not injurious in itself, but weak types kept in 



1 A recent monograph by EAST and JONES (1919) presents in a clear and 

 concise way the effects of inbreeding and cross-breeding in the light of 

 modern theories of genetics. This publication has been used very freely 

 in this section. 



