286 
THE PRINCIPLES OF 
Part IL 
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 pow 7 er 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 w r ould 
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 libellulse. 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 
