PART III. THE FERTILITY OF THE PARENT SPECIES 



AND HYBRIDS. 



18. INTRODUCTORY DISCUSSION. 



When the wild Brazilian male cslvj, Cavia rufescens, was crossed 

 with the tame domestic female guinea-pig, Cavia porcellus, the hybrids 

 were fertile females and sterile males. At least three problems were 

 immediately self-apparent : for how many generations would the hj^brid 

 females have to be crossed back to the parent males before producing 

 fertile hybrid males ; what proportion of sterile males would the more 

 dilute wild hybrid females produce; and when fertile hybrid males 

 were produced, would their offspring be fertile in both sexes if these 

 males were mated with their hybrid sisters or with guinea-pig females. 



Sterility is a common phenomenon in the hybrids obtained by cross- 

 ing individuals belonging to distantly related groups or types, both in 

 animals and in plants. In fact, there is a tacit understanding among 

 biologists that members of the same species produce fertile offspring; 

 but a successful cross between members of different species or genera 

 may result in sterility of the hybrids, in one or both sexes. In case 

 both sexes in a species cross are sterile, a continuation of the genetic 

 investigation becomes impossible. If one sex alone is sterile, then the 

 fertile sex can be crossed back to either parent species, and it becomes 

 possible to study the inheritance of various other characters as well as 

 their fertility and sterility. In the experiments recorded in this paper, 

 wdld C. rufescens males were mated with the tame guinea-pig females 

 and produced fertile female and sterile male hybrids. The fertile 

 hybrid females w^ere crossed back to the males of both parent species. 

 The back-cross to the wild C. rufescens males succeeded in so few cases 

 (four offspring were produced) that this class of matings had to be 

 abandoned. The back-cross to the guinea-pig males was entirely suc- 

 cessful. The \ wild females alone were fertile, and a second back-cross 

 to the guinea-pig produced the | wild. In this manner there were 

 produced ten generations of hybrids, by repeatedly crossing female 

 hybrids of one generation back to guinea-pigs to obtain the next more 

 dilute wild-blooded generation. The results of these crosses have been 

 studied with regard to coat, color, growth, size, and morphological 

 characters and recorded in Parts I and II of this paper. The same 

 animals were used in studies on fertility and sterility. 



Bateson (1913), in his review of " Mendehan segregation and species," 

 is inclined to the view "that successful investigation of the nature even 

 of sterility consequent on crossing, the most obscure of all genetic 

 phenomena, may become one of the possibilities of Mendelian research." 

 The material presented in this part of the series of studies in a mam- 



