150 HEREDITY AND ENVIRONMENT 



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 

 is due to the fact that all the male-producing 

 spermatozoa degenerate and that only female- 

 producing spermatozoa become functional. 

 Possibly experimental alterations of the sex 

 ratio, such as Hertwig, King and others have 

 brought about may be explained in a similar 

 way. At least the chromosomal theory of sex 

 determination is so well supported in so many 

 cases and has been found to apply in so many 

 instances where at first it seemed impossible of 



