1 66 Heredity and Environment 



spermatozoa are formed as male-producing ones. It is possible 

 to explain such departure from the I : I ratio of males to fe- 

 males in conformity with the chromosome theory if one class of 

 spermatozoa are more active 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, but if before fertilization the activity or vitality of male- 

 producing spermatozoa is greater than that 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 fertilized 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, Whitney and others have brought about 

 may be explained as due to a differential action of the modified 

 egg cells or of the environment upon the two types of sperma- 

 tozoa. At least the chromosomal theory of sex determination is 

 so well supported in so many instances where at first it seemed 

 impossible of application, that it ought not to be abandoned until 

 unmistakable evidence can be adduced against it. 



Herma-phrodites and Intersexes. Finally a number of cases 

 have been brought to light which indicate the necessity of dis- 

 tinguishing between the hereditary determination of sex and its 

 ontogenetic development- It has been known for a long time 

 that in bees and ants the workers are imperfect females, while 

 the queens ar.e perfect females, and that the kind or amount of 

 food which is fed to the larvae determines whether they will be 

 workers or queens (see p. 222). Again in many animals the 

 development of " male or female characters is dependent upon 

 internal secretions or hormones from the sex glands or other 

 organs (see pp. 224-225). In these cases it is evident that sex 

 was determined at an early stage, probably at fertilization, but 



