FETILTILY OF PARENT SPECIES AND HYBRIDS. 91 



but all were motile. A ■priori, one might expect "over half, 4," "all, 

 3," and "all, 4" individuals to be fertile in breeding. In the tables, all 

 males recorded with "many motile sperm" were of the grade "all, 4." 

 Any manifest departure from this condition is recorded in tables as 

 having "few motile sperm." This will make clear that our records 

 were more discriminating than our tables. 



Using the term "few motile sperm" in the tables to mean any con- 

 dition of number or motility plainly below that of a normal guinea- 

 pig, we may say that 9 individuals out of a total of 10 were sterile in 

 breeding. The exception was a \ wild male ( cr469) recorded as "over 

 half, 2." This male had "over half" the usual number of sperm; but 

 only half of these were motile. He was bred continuously for 9 months 

 and sired one male. Possibly all males with any motile sperm what- 

 ever might have fertilized eggs had we increased their chances by 

 using large numbers of females and long periods of mating. 



(4) Hybrid males with many motile spermatozoa. — As, previously 

 stated, hybrids classified this way in the tables were as nearly like a 

 normal guinea-pig as one could judge by examination of the contents 

 of the epididymis. I expected they would prove to be just as fertile in 

 breeding; but this was not the case, for some of them sired no young 

 after a thorough breeding test. There were 7 males of this class among 

 the \ wild; and all but one were successful sires. This exceptional 

 male (cf 721), large and vigorous, produced no young, although con- 

 tinually with fertile females for many months. Among the ^V ^W 

 there were 22 males with many motile sperm, but only 16 of these 

 were successful sires. The reason why the remaining 6 individuals 

 were impotent is not clear; their weights and growth curves gave 

 every indication of vigor; 3 of the 6 males were bred for the mini- 

 mum time reasonably required to show fertility, and it is barely pos- 

 sible that the cause lay there; but this still fails to account for the 

 remaining 3. 



Likewise among the -^ ^W, 6 males out of 24 had many motile 

 sperm but failed to breed. Here again no evident reason, such as lack 

 of vigor or early death, could be assigned to at least one of these cases. 

 Of the ^ wild males, 2 sired young, while one failed to — ^in all proba- 

 bility because of poor condition. Summarizing the results, there were 

 58 males with many motile spermatozoa, and 44 of these were successful 

 sires. The remaining 14 individuals were sterile in breeding; of these 

 14 it is just barely possible that because of external causes 9 may have 

 been sterile in spite of their abundance of motile sperm; but there 

 was surely no patent cause for the sterility of the remaining 5 males. 

 In other words, of 49 males (68 nainus 9) which gave every indication 

 of being fertile by a microscopic test and had opportunity to prove 

 themselves so in breeding, there were only 44 which actually impreg- 

 nated females. To state it differently, 89.8 per cent of the male 



