CLASSIFICATION OF THE THEORIES 481 



tion of the germ-cells as such, there being female-producing and 

 male-producing germ-cells, predetermined from the beginning 

 and arising independently of environmental influence. 



(d) That maleness and femaleness are Mendelian characters. 



(e) That environmental and functional influences, operating 

 through the parent's body, may alter the proportion of effective 

 female-producing and male-producing germ-cells. 



It will be seen that these five theories are not in a strict 

 way mutually exclusive. Even if we conclude that there are. 



f 



FIG. 46. Winged male and wingless female of Pneumora, a kind of 

 grasshopper. (From Darwin.) 



for instance, two kinds of ova in the ovary, one set predestined 

 to develop into males and the other set predestined to develop 

 into females, it does not follow that the relative numbers of 

 these may not be changed as life goes on, e.g. by the diet of the 

 parent. And even if we conclude that there are two kinds of ova 

 predestined from the start, it does not follow that the predestina- 

 tion need be quite unalterable by the conditions of fertilisation 

 and of development. 



Another preliminary caution must be noted. One must be 

 careful in arguing from one set of organisms to another. What 

 determines sex in frogs may not hold true for cattle ; wha< 

 determines sex in Rotifers may not apply to birds. Nature 



