Other Kinds of Hybridizing 159 



De Vries has found that some of the elementary species of the 

 evening primrose also show a certain degree of infertiUty when 

 crossed ; and there can be little doubt that infertility may begin 

 with the appearance of elementary species, and increase in pro- 

 portion to every new change in the germ that takes place. 

 Whether infertility is a general rule for elementary species, may 

 be questioned. 



Many cases of crossing between wild species of animals and 

 plants have been recorded as occurring in nature, and many 

 more cases have been experimentally brought about, especially 

 in plants, with wild forms kept under domestication. The re- 

 sults are different in different cases, but it is a generally accepted 

 opinion that the species-cross is generally intermediate be- 

 tween the parents. This conclusion needs, perhaps, careful 

 revision in the light of the results of recent years on crossing types 

 that differ in only one character, where in many cases discontinu- 

 ous inheritance is the rule. Darwin was so impressed with the 

 difference in the results of crossing Linnaean species and sports 

 that he concluded that wild species could not have arisen as 

 sports, since the latter when crossed show discontinuous inheri- 

 tance, while wild species give intermediate forms. Since Dar- 

 win's time our knowledge of the results of hybridizing has greatly 

 increased, and his argument seems less conclusive, because, in 

 the first place, a single mutation may show incipient infertility, 

 as in de Vries's Oenotheras; in the second place, because the 

 results of crossing elementary varieties and elementary species 

 with the parent forms or with each other do not always show 

 discontinuous inheritance ; thirdly, because wild species have 

 undergone so many changes of different kinds that the results 

 are too complicated for an analysis of single characters; and, 

 fourthly, because discontinuous inheritance may sometimes oc- 

 cur between wild species, if unit characters rather than the 

 ensemble of characters is considered. It is the failure to recog- 

 nize this last point that has probably led to an exaggerated idea 

 of the difference between the inheritance of single variations and 

 of complex variations that characterize Linnaean species. 



