102 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



that selective fertilization takes place, or that unequal viability of the 

 two sexes during early development accounts for the discrepancies. 



Guyer (1909), compiling the proportion of sexes in hybrid birds, 

 stated : "When due allowance is made for all errors, the facts still indicate 

 that there is a marked tendency for the hybrids, especially those from 

 widely separated parents, to be male." Since the female is supposed 

 to be favored by increased nutrition, he thought the excess of males 

 might be due to default in metabolic processes because of incompati- 

 bilities between dissimilar germ plasms, such incompatibilities being 

 especially inimical to the production of females. 



King (1911), tabulating the sex ratios of hybrids between wild and 

 albino rats, stated: "It appears, therefore, that hybridizing alters the 

 sex ratio by producing a marked increase in the relative proportion of 

 males. This conclusion is in essential agreement with that reached by 

 Buffon, by R. and M. Pearl, and by Guyer." 



King found 231 males to 194 females in the totals of the first three 

 hybrid generations, this being a ratio of 119.07 males to 100 females. 



Minot (1891) crossed guinea-pigs inter se and obtained 223 males to 

 187 females, or a ratio of 119.2 males to 100 females. 



The results in the hybrids between C. rufescens and C. porcellus did 

 not show an excess of males, but, to the contrary, a significant excess of 

 females (see table 81). The wild parent bred in captivity gave 20 

 males, 25 females, and 1 of unknown sex. The | wild hybrids gave 

 14 males and 23 females, or a ratio of 60.87 males to 100 females. 

 There were 2 young of unknown sex, having died prematurely. If 

 we call them males, the ratio is 69.57 males to 100 females. The F2, or 

 I wild, gave 31 males and 52 females, or a ratio of 59.62 males to 100 

 females. The F3, or | wild, gave 101 males to 116 females, or a ratio 

 of 87.07 males to 100 females. It is apparent that, as the generations 

 became less hybrid in nature, the sexes were gradually approaching 

 equality. 



After the | wild, the sexes were more nearly equal, for the next four 

 generations gave a total of 406 males to 409 females, practically an 

 equality of sexes, for the ratio is 99.24 males to 100 females. This is 

 strikingly different from the total of the first three generations, in which 

 there were 146 males to 191 females, or a ratio of 76.44 males to 100 

 females. The total results of all hybrids were 552 males and 600 

 females, or a ratio of 92 males to 100 females. These ratios do not 

 confirm the results shown by Guyer or King. 



Previous data have shown that sterility was common in the males 

 of the early hybrid generations, for there were disturbances in sperma- 

 togenesis. It is shown here that the early generations also gave a 

 deficiency of males. May it not be possible that the same incompati- 

 bilities between dissimilar germ plasms which gave rise to sterility in 

 gametogenesis also caused disturbances in fertilization. Possibly male 



