90 THE FIRST PRINCIPLES OF HEREDITY 



other, but each alone is capable of producing a complete 

 being. 



The real cause of sex determination has so far not been 

 found, and we can only give a brief review of the many 

 theories advanced. In this it is best to follow Professor 

 J. A. Thomson's arrangement, dividing the different factors 

 adduced into (a) influences of food and other agencies ; 

 (b) influence of parents ; and (c) internal conditions of the 

 germ. 



(a) INFLUENCES OF FOOD, ETC. 



In the various species the ratio of male to female in- 

 dividuals is fairly constant : thus, in man the proportion 

 of males to females born is about 106 to 100. Now, it has 

 been shown that in many cases the usual proportion of the 

 sexes can be appreciably altered by varying the external 

 conditions of the animals. Thus, in tadpoles, where the 

 sex is determined relatively late, the percentage of females 

 can be raised from 57 to 92 by giving various kinds of flesh 

 diet. The production of female generations can be pro- 

 longed in plant-lice by giving them abundant food and 

 warmth. The case of the bee is well known : the larva 

 destined to become a queen is fed with a special diet. If 

 a queen dies, one of the worker larvae can still be turned 

 into a queen by appropriate feeding. In the little Rotifer 

 Hydatina males or females can be reared at will by either 

 raising or lowering the temperature. In man, though 

 many suggestions have been made, no influence of dieting 

 the mother has been found to be of any decided effect. 



(b) INFLUENCE OF PARENTS. 



The age of the respective parents is said to have some- 

 thing to do with the sex of the children. Thus, according 

 to some statistics, in marriages where the husband is the 

 older the offspring are supposed to have a tendency to be 

 preponderately of the male sex, and vice versa. On the 



