80 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



malian species cross deals mainly with sterility in the male sex, conse- 

 quent on crossing. 



That such complicated physiological phenomena as fertility and 

 sterility in all kinds of crosses and under all conditions can be discussed 

 or treated solely as problems in heredity is out of the question. Prob- 

 ably no one would insist that fertility or degrees of fertility always 

 depend upon ''factors" or ''germinal determiners." However, it does 

 not follow that in certain crosses factors may not be transmitted in 

 Mendelian fashion which influence the fertihty of the hybrids. On a 

 priori grounds we have no reason to suppose that all cases of varying 

 fertility and sterility are due to environmental conditions; for, although 

 environment undoubtedly influences fertility, there are unquestionable 

 instances in which the results may be ascribed to other causes. 



There ^ems to be little doubt that environmental conditions may 

 affect the fertility of one or both sexes, and this should be carefufly con- 

 sidered when we are dealing with the inheritance of the same. Marshal 

 (1910) states: "it is well known that wild animals, when removed from 

 their natural conditions and brought into captivity, often become 

 partly or completely sterile." He cites cases from different groups of 

 mammals and birds. Darwin (1876) also drew attention to this fact. 

 Both of these investigators recognized that animals differ widely in 

 this respect. The Indian elephant, chetahs, some carnivores, some 

 rodents, monkeys, hawks, finches, parrots, and many other cases show 

 sterility ; but one can not generalize hastily and infer that all changes 

 from a wild state to captivity result in a lowered fertility, for it is also 

 known that certain gallinaceous birds, ostriches, pigeons, ducks, geese, 

 and gulls, and some mammals like the skunk, ferret, mink, and Cavia 

 aperea will breed readily in captivity. It is often asserted that wild 

 animals in captivity are sterile because of change in diet, temperature, 

 surroundings, lack of exercise, and the like; but none of these factors 

 necessarily causes sterility, for one can always cite contradictory 

 evidence. 



It is no easy task to differentiate between the effect of environmental 

 factors and hereditary factors, particularly when the influence of the 

 different factors is small and their number is large. In any comparison 

 between the fertility of the wild C. rufescens, the domestic guinea-pig, 

 and the various hybrids, a number of environmental factors should 

 be given careful consideration, since it may be supposed that the wild 

 species underwent a great change when transferred from its native 

 habitat in Brazil to the laboratory of the Bussey Institution. All of 

 the causes which are cited as disturbing fertility appeared to be of 

 little or no consequence in these crosses, for it will be shown that the 

 wild were apparently quite fertile inter se; and the wild males were 

 surely fertile in crosses on tame females. The change from a wild 

 habitat with the concomitant changes in diet, temperature, surround- 



