286 THE PRINCIPLES OF Part II. 



that variations which first appear in either sex at a late 

 period of life, tend to be developed in the same sex 

 alone ; whilst variations which first appear early in life 

 in either sex tend to be developed in both sexes. I am, 

 however, far from supposing that this is the sole de- 

 termining cause. As I have not elsewhere discussed 

 this subject, and as it has an important bearing on 

 sexual selection, I must here enter into lengthy and 

 somewhat intricate details. 



It is in itself probable that any character appearing 

 at an early age would tend to be inherited equally by 

 both sexes, for the sexes do not differ much in constitu- 

 tion, before the power of reproduction is gained. On 

 the other hand, after this power has been gained and the 

 sexes have come to differ in constitution, the gemmules 

 (if I may again use the language of pangenesis) which 

 are cast off from each varying part in the one sex would 

 be much more likely to possess the proper affinities 

 for uniting with the tissues of the same sex, and thus 

 becoming developed, than with those of the opposite 

 sex. 



I was first led to infer that a relation of this kind 

 exists, from the fact that whenever and in whatever 

 manner the adult male has come to differ from the 

 adult female, he differs in the same manner from the 

 young of both sexes. The generality of this fact is quite 

 remarkable : it holds good with almost all mammals, 

 birds, amphibians, and fishes ; also with many crus- 

 taceans, spiders and some few insects, namely certain 

 orthoptera and libellulae. In all these cases the varia- 

 tions, through the accumulation of which the male ac- 

 quired his proper masculine characters, must have oc- 

 curred at a somewhat late period of life ; otherwise the 

 young males would have been similarly characterised ; 

 and conformably with our rule, they are transmitted to 



