SPECIAL CASES OF SEX-INHERITANCE 231 



the mammals, there is an excess of males at birth, as 

 the next table shows. 



Males Females 



Horse 98.31 100 (Busing) 



Cattle 107.3 100 (Wilchens) 



Sheep 97.7 100 (Irwin) 



Pig 111.8 100 (Wilchens) 



Rat 105.0 100 (Cuenot) 



Dove 105.0 100 (Cuenot) 



Hen 94.7 100 (Darwin) 



A little later I shall bring forward the evidence that 

 makes probable the view that in man the mechanism 

 for sex-determination is like that in other animals, 

 where two classes of sperm are produced, male- and 

 female-producing. How then can we account in the 

 human race for the excess of eggs that are fertilized 

 by male-producing spermatozoa ? At present we do not 

 know, but we can at least offer certain suggestions that 

 seem plausible. 



In mammals the fertilization occurs in the upper 

 parts of the oviduct. In order to reach these parts 

 the sperm by their own activity must traverse a dis- 

 tance relatively great for such small organisms. If 

 the rate of travel is ever so slightly different for the two 

 classes of sperm, a differential sex ratio will occur. 



Again, if from any cause, such as disease or alcoholism, 

 one class of sperm is more affected than the other, a 

 disturbance in the sex ratio would be expected. 



At present these are only conjectures, but I see 

 no ground for seizing upon any disturbance of the 

 ratio in order to formulate far-reaching conclusions 

 in regard to sex-determination itself. As I pointed 

 out in the beginning of this chapter, we may go 



