THEORIES OF HEREDITY 91 



other hand, the more vigorous or superior parent is held 

 by some to determine the sex of the children. But, while 

 it is difficult to define what is exactly meant by vigour or 

 superiority, we have at the same time statements exactly 

 the reverse, maintaining that the exhausted soldiers 

 returning from a war have a tendency to beget males. 



(c) Internal Conditions of the Germ. 



In some animals, as Rotifers and Insects, two kinds of 

 eggs exist, the one giving rise to males, the other to females. 

 Often the female egg is distinguished by its larger bulk, in 

 accordance with the general rule that the female, being 

 more sedative and anabolic, is of larger size than the male, 

 as is exemplified also in the great contrast of size between 

 ovum and spermatozoon. 



According to the latest researches, there occur in certain 

 insects two kinds of spermatozoa, half their number having 

 an odd " accessory " chromosome, while the other half is 

 without it. The ova, on the other hand, all contain the 

 odd chromosome. E. B. Wilson has been able to show 

 that the fertilization of an ovum by a spermatozoon with 

 the accessory chromosome leads to the production of a 

 female, while the union of an ovum with a spermatozoon 

 without the odd chromosome produces a male.* 



In the bee the act of fertilization determines the sex, for 

 the drones spring from unfertilized eggs, while the queen 

 and workers develop from those eggs which are fertilized. 

 In other cases the age of the germ is supposed to have an 

 influence. Thus, K. Diising argued that young ova have 

 a tendency to produce females, young spermatozoa males, 

 while old germs have the opposite effect. He found herein 

 the cause for the automatic regulation of sex-proportion in 

 the race ; for if there is a scarcity of males, they will 

 fertilize often, their spermatozoa will always be fresh, and 



* A Mendelian explanation has been attempted for tliese cases 

 (see W. Bateson, Mendel's Principles of Heredity). 



