S06 THE DESCENT OF MAN 



should, with rare .exceptions, be developed in the female 

 alone. The very same character, such as deficient or super- 

 numerary digits, color-blindness, etc., may with mankind 

 be inherited by the males alone of one family, and in 

 another family by the females alone, though in both cases 

 transmitted through the opposite as well as through the 

 same sex. '° Although we are thus ignorant, the two follow- 

 ing rules seem often to hold good — that variations which 

 iirst appear in either sex at a late period of life tend to be 

 developed in the same sex alone, while 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 determining cause. As I have not elsewhere dis- 

 cussed this subject, and as it has an important bearing on 

 sexual selection, I must here enter into lengthy and some- 

 what 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 constitution 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 differs from the ■ adult female, he differs in 

 the same manner from the young of both sexes. The gen- 

 erality of this fact is quite remarkable: it holds good with 

 almost all mammals, birds, amphibians, and fishes; also 

 with many crustaceans, spiders, and some few insects, such 

 as certain orthoptera and libellulse. In all these cases the 



'* Eeferences are given in my "Variation of Animals under Domestication," 

 VoL ii. p. 72. 



