400 



SAMUEL F. HILDEBRAND. 



fish are maturing, as in July and August when the greatest dis- 

 parity of the sexes occurs. The uncertainty of the sex determina- 

 tion from an external examination of some of the fish, classed as 

 females in Table I., therefore, accounts for a part of the differ- 

 ence in sex ratio shown but probably only for a small part of it. 

 It is believed that ample evidence has been produced by the data 

 in Table II. to show that a one to one sex ratio exists among 

 young Gambusia. It would appear also from these data that this 

 one to one ratio obtains at least until the fish become sexually 

 mature. It is evident from Table I., however, that a " thinning 

 out " process takes place among adult males which is very much in 

 excess of the decrease among the adult females. A discussion of 

 this interesting phenomenon is offered in the following section. 



TABLE II. 

 THE SEX IN YOUNG Gambusia. 



LARGER IMMATURE Gambusia. 



A DIFFERENTIAL DEATH RATE OF THE SEXES IN Gambusia. 



This subject is discussed at some length by Geiser (1924) who 

 says, " The males are much less resistent to harmful environ- 

 mental factors than females, and hence have a lower survival 

 value." This author then continues to say that he has found 



