Again the seeds from cross-fertilized flowers on plants 

 that came from self-fertilized flowers were planted. The 



results were: 



Average height of six plants from first set = 10.31 



inches. 



Average height of six plants from second set = 12.56 



inches. 



Moreover, there is a great range in the productivity of 



hybrids— from absolute sterility i to complete fertility. 



In this regard also hybrids of closely related parents are, as 



a rule, more fertile than those from widely different parents. 



Species-hybrids have been divided into three classes: 



1. Where the hybrid has vigor and fertility equal to or 

 greater than the parents, e. g. Nicotiana alata x N. 

 langsdorffi. 



2. Where the vigor is equal to or greater than that of 

 the parents but the fertility is much reduced, e. g. 



, Raphanus sativus x Brassica oleracea; Bison americanus 

 X Bos tauTus. 



3. Where both size and vigor have become much re- 

 duced and there is complete sterility, e. g. Nicotiana 

 tabacum x N. paniculata; N. rustico x A'', alata. 

 Hybrids between nearly related but distifict'species are 



frequently observed to have characters intertnediate be- 

 tween the unlike characters of the parents; they are vigorous 

 and stable, but more or less sterile. On the other hand, 

 hybrids between doubtfully distinct species, or between a 

 species and a variety, do not have interfnediate characters 

 but, as a rule, act in a Mendelian manner. 



The large number of stable species in the Onagraceae, 

 Solanacese, Rosacea, Compositae, etc. is accounted for by 

 some investigators by the formation of natural hybrids of 

 distinct species. 



Jeffrey, making pollen sterility a criterion of hybridi- 

 zation of species, is of the opinion that DeVries' Oe- 

 lamarckiana is a hybrid and not a true species. 



East and Shull discovered an interesting point regard- 

 ing the influence of continued self-fertilization and crossing. 

 They found that although corn will lose vigor for several 

 generations when self- fertilized, the loss does not continue 

 at the same rate — decreasing in successive generations until 

 a condition of constancy is reached. In this condition 

 cross-fertilization with a plant of the same strain does not 

 mcrease the vigor of the progeny. 



(1)— Infertile hybrids may be of great commercial valua if thev can 

 be reproduced by cuttings, grafts, tubers, etc. 



78 



