194 THE BREEDING OF ANIMALS 



seem to indicate some peculiar influence of the season in 

 favor of the males. In 1871, however, the bulls were 

 largely in excess of the cow-calves, and there was quite 

 as decided a preponderance of females among the sheep." 



185. Sex cannot be controlled by external conditions. 

 The most recent investigations of sex determination 

 lead to the belief that sex is predetermined in the germ- 

 cell. It is not subject to change through any change in 

 the environment, as nutrition or temperature. The 

 germ-cell contains a determiner for sex as it contains 

 determiners for other characters. 



Castle concludes, l " If, as has been suggested, the deter- 

 mination of sex in general depends upon the inheritance 

 of a Mendelian factor differentiating the sexes, it is highly 

 improbable that the breeder will ever be able to control 

 sex. Male and female zygotes should forever continue 

 to be produced in approximate equality, and consistent 

 inequality of male and female births could result only from 

 greater mortality on the part of one sort of zygote than 

 of the other." 



The same idea is similarly expounded by Morgan. 2 

 " If these observations are confirmed, they show that 

 in man, as in so many other animals, an internal mechan- 

 ism exists by which sex is determined. It is futile, then, 

 to search for environmental changes that might determine 

 sex. At best the environment may slightly disturb the 

 regular working out of the two possible combinations 

 that give male or female. Such disturbances may affect 

 the sex ratio but have nothing to do with sex-determina- 

 tion. 



1 Castle, "Heredity," 1911, p. 180. 



2 Morgan, " Heredity and Sex," p. 248. 



