THE DETERMINATION OF SEX. 33 



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

 unanalyzed ' ' 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 whole 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-regulation of the sex-proportions, that 

 reference to the latter is more profitable. 



X. Dusing on the Proportions of the Sexes, and the 

 Regulation of these. — In an important work, Dusing has recently 

 treated the whole subject with some synthetic result. He recognizes 

 that the fates or factors determining the sex are manifold, and operate V s 

 at different periods. Much is determined by the condition of the 

 reproductive elements, that is, by the constitution and habits of the 

 parents; much depends also on the period of fertilization; while again 

 the nutrition of the embryo may be of moment. Dusing has collected 



a great body of facts, from both plants and animals, in favor 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 analyzes 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, how there is always produced 

 a majority of the sex that is wanted. That is to say, if one sex be in the 

 decided minority, or under conditions which come to the same thing, 

 then a majority of that sex will be produced. If there be, for instance, 

 a great majority of males, there is the greater likelihood of the ova 

 being fertilized early, but that means a probable preponderance of 

 female offspring, and thus the balance is restored. It would be rash 

 to say that in every case he makes out his contention, but his general 

 argument that disturbances in the proportion of the sexes bring about 

 their own compensation is carefully and convincingly worked out. 



XI. Sex of Twins. — It sometimes happens among many 

 different classes of animals that from one ovum two organisms 



