98 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



from those with no sperm to those fertile in breeding. The f wild 

 males, bred to | Wild females, gave one male with many motile sperm 

 (fertile in breeding also) out of 7 tested. The tV wild males, bred to 

 their sisters in blood, gave 8 males with many motile sperm out of 14 

 tested. The F5 males bred to Fs females gave 5 males with many 

 motile sperm out of 8 tested. One F7 male bred to an F7 female gave 

 one male, and he had many motile sperm. The other 9 matings 

 correspond to these, for irrespective of what generation the fertile 

 male sires were they gave a preponderance of sterile male offspring 

 when bred to intense wild-blooded female hybrids, but increasing signs 

 of fertility in their sons when bred to females of later generations. For 

 example, the Fs and Fe males, bred to Fi and F2 females respectively, 

 gave entirely sterile sons; but one F5 male gave sons with many motile 

 sperm when mated to F4 females, while two F4 males, bred to F5 females, 

 gave sons with many motUe sperm also. 



If the hypothesis advanced is correct, and a fertile male hybrid 

 represented the same combination of factors for fertility as a guinea- 

 pig male, then from matmg fertile male hybrids with female hybrids 

 we should expect about the same results that were obtained by mating 

 guinea-pigs to similar female hybrids. We have already shown that 

 when guinea-pigis were mated to the different generations of female 

 hybrids, increasing signs of fertile males came with each back-cross. 

 The hypothesis implies that more and more females were being 

 obtained which lacked the disturbing factors and failed to transmit 

 such. The results in the sons of fertile male hybrids bred to female 

 hybrids are consistent with this hypothesis, for the intense wild females 

 gave more sterile sons than the dilute wild females in this class of 

 matings, just as they did when mated to guinea-pigs. The two series of 

 percentages of males with many motile sperm produced in these mat- 

 ings are given in table 79. The number of sons from female hybrids 

 and male hybrids is far too small for broad generalizations; but the 

 results indicate that sterility is transmitted in the same manner by the 

 female hybrids crossed with male hybrids as when crossed with guinea- 

 pigs. The percentages of sons with many motile sperm in both sorts 

 of crosses in the different generations from the Fi to the F7 are as 

 follows: 



0.0 0.0 14.3 68.8 63.6 .... 100 . with fertile hybrid sires. 

 0.0 0.0 14.3 33.3 60.7 69.4 73 . 3 with guinea-pig sires. 



For further details and numbers involved, see table 79. 



Fertile Htbrid Males in Crosses with Guinea-pigs. 



A total of 22 sons from this sort of mating was tested, all having a 

 microscopic test only. The fertile hybrid sires belonged to the F3, F4, 

 Fg, and Fe generations (see tables 72 to 76). To test the hypothesis 

 that the fertile hybrid sires, with many motile sperm, had segregated 



