38 THE EVOLUTION OF SEX. 



sex, nor its determination in individual cases, did Darwin see 

 further than his contemporaries. He refers to the current 

 theories of the influence of age, period of impregnation, and 

 the hke ; and further contributes a great body of statistics on 

 the numerical proportions of the sexes, and the supposed 

 influence of polygamy. "There is reason," he says, "to 

 suspect that in some cases man has by selection indirectly 

 influenced his own sex-producing powers." He falls back 

 upon the unanalysed "belief that the tendency to produce 

 either sex would be inherited like almost every other peculiarity, 

 for instance, that of producing twins." " In no case, as far as 

 we can see, would an inherited tendency to produce both sexes 

 in equal numbers, or to produce one sex in excess, be a direct 

 advantage or disadvantage to certain individuals more than to 

 others ; . . . and therefore a tendency of this kind could 

 not be gained through natural selection." " I formerly thought 

 that when a tendency to produce the two sexes in equal 

 numbers was advantageous to the species, it would follow from 

 natural selection, but I now see that the w^hole problem is so 

 intricate that it is safer to leave its solution for the future." 

 Any other hints that Darwin threw out, have been so well 

 elaborated by Diising's work on the advantageous self-regula- 

 tion of the sex-proportions, that reference to the latter is more 

 profitable. 



§ 10. D using on the Proportioiis of the Sexes, and the 

 Regulation of these. — In an important work, Diising has 

 recently treated the w^hole subject with some synthetic result. 

 He recognises that the fates or factors determining the sex are 

 manifold, and operate at different periods. ]\Iuch is determined 

 by the condition of the reproductive elements, i.e., by the con- 

 stitution and habits of the parents ; much depends also on the 

 period of fertilisation ; while again the nutrition of the embryo 

 may be of moment. Diising has collected a great body of 

 facts, from both plants and animals, in favour of his conclusions; 

 but the copious summary of his work, given in the article "Sex" 

 already referred to, need not here be repeated, while some of 

 his experimental results will be included in the next chapter. 



Diising's memoir is very important, however, for this special 

 reason, that he analyses what may be termed the mechanism 

 by which the proportion of the sexes is regulated. Instead of 

 vaguely referring the whole matter to natural selection, he 

 shows in detail how the numbers are in a sense self-regulating, 



