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THE AMERICAN NATURALIST [Vol. XLIX 



been made on such a scale as the experiments reported by Det- 

 lefsen. The causes of sterility are very obscure and but little 

 understood. A change of environment and consequent lack of 

 exercise or difference of diet may be a contributing cause of ster- 

 ility in birds and wild animals in captivity, but none of these 

 influences were operative in Detlef sen's experiments, because the 

 wild cavies from Brazil bred inter se under laboratory condi- 

 tions. Sterility is frequent in hybrids of species not closely re- 

 lated, and it is an axiom among biologists that crosses between 

 different species or genera produce sterile hybrids in one or 

 both sexes. As stated above, all F 1 hybrid males from a cross 

 between a wild cavy male and a guinea-pig female were sterile. 

 However, the F l females were fertile and were crossed back to 

 guinea-pig males. These likewise gave in F 2 sterile males and 

 fertile females. Repeated back crosses of fertile females with 

 guinea-pig males produced fertile males in increasing numbers 

 with each generation. 



In order to test the fertility of the hybrid males two methods 

 were used: (a) breeding tests; (b) microscopic examination of 

 spermatozoa obtained by transecting several tubules from the 

 epididymis on one side of the animal. Such an individual could 

 be bred subsequently. In all, 483 males were tested by one or 

 both methods: ">0 by breeding alone: 331 by microscopic exami- 

 nation alone; and 102 by both methods. The following table, 

 giving the results of combined microscopic and breeding tests, 



the fertility of males: 



It will be noted that of the 58 males with many motile sperma- 

 tozoa, 14 proved to be sterile upon breeding. Among these 

 males, Detlefsen attributes the sterility of 9 to external causes, 

 without specifying them, but he can assiun no reason for the im- 

 potency of the remaining 5. He therefore concludes that 

 the number and motility of the sperm are not the only essentials for a 

 real fertility, inasmuch as real fertility in the last analysis may mean 



