156 HEREDITY AND ENVIRONMENT 



the quantity or kind of food influences sex 

 determination. 



Extensive statistics show that in many ani- 

 mals including man more males are born than 

 females whereas according to the chromosome 

 theory of sex determination as many female- 

 producing spermatozoa are formed as male- 

 producing ones. It is possible to explain such 

 departure from the 1 : 1 ratio of males and fe- 

 males in conformity with the chromosome 

 theory if one class of spermatozoa are more ac- 

 tive or have greater vitality than the other 

 class, or if after fertilization one sex is more 

 likely to live than the other. In the human 

 species it is known that mortality is greater 

 in male babies before and after birth than in 

 female babies, and if before fertilization the 

 activity or vitality of male-producing sper- 

 matozoa is greater than of female-producing 

 ones it would offer a possible explanation of 

 the greater number of males than of females 

 at the time of birth. In certain insects it is 

 known that only females develop from ferti- 

 lized eggs, and in one of these cases, viz., 

 Phylloxera, Morgan has discovered that this 



