82 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



which came under my observation, there were only two which failed 

 to breed. When the contents of the epididymis were examined it was 

 found that they had an abundance of live, motile spermatozoa. Their 

 impotence may have been due to sluggishness and a failure to copulate 

 rather than to innate sterility. Female guinea-pigs in good condition 

 are rarely sterile. 



In view of the foregoing facts it would seem that the problem of 

 sterility in the male hybrids in these crosses was fundamentally a 

 problem of physiology and heredity, and not one of environment. The 

 facts may be summarized as follows: 



(1) The wild cavy species was fertile in both sexes in captivity. 



(2) The tame domestic species was likewise fertile under the same 

 conditions. 



(3) The hybrids resulting from a cross between these two species 

 were not like either parent, for they were sterile males and fertile 

 females. Nevertheless these hybrids were very vigorous, as was shown 

 in Part II. 



(4) The peculiar sterility of the males persisted in later, more dilute 

 wild generations in a manner which will be described subsequently. 

 These later hybrids, however, could not be distinguished from the 

 tame guinea-pig in shape, size, growth, mental traits, or any other 

 characters, except their peculiar sterility. Therefore, since the wild 

 were difficult to raise in captivity, but were fertile, and since their less- 

 wild hybrid sons were easily raised in captivity but were sterile, it 

 would appear that their sterility is not due to captivity or environment. 

 If the facts have been correctly interpreted, some sort of consistent 

 explanation should be found, based on heredity. The cross resulted in 

 a definite disturbance in fertility such as did not obtain in either parent 

 species when kept under the same conditions. 



Many species crosses have been made in both plants and animals. 

 In most cases the crosses were made by those who were merely inter- 

 ested in the sheer possibility of a cross, but not for the purpose of an 

 extended genetic study. Much of the literature deals with the subject 

 of sterility from a taxonomic point of view, for the fertility or sterility 

 of the hybrids is considered a criterion of the close or distant relation- 

 ship between the parents. From time to time compilers have given 

 lists of species crosses with brief mention of the partial or complete 

 dominance of one parent and the fertility of the hybrids when known. 

 As in most other genetic studies, the botanists have led the way, and 

 the studies of the early plant hybridists include many accounts of 

 species crosses, or at least what were regarded as "species" crosses. 

 Very complete summaries of species crosses in plants were made by 

 Gartner (1849) and Focke (1881). Numerous crosses have been made 

 since, but in all the crosses between varieties or between species but 

 few of them deal with the inheritance of fertility and sterility. 



