SEXUAL SELECTION. 263 



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 following rules seem often to hold good that varia- 

 tions which first appear in either sex at a late period of life 

 tend to be developed in the same sex alone; while varia- 

 tions which first appear early in life in either sex tend to 

 be developed in both sexes. I am, however, far from sup- 

 posing that this is the sole determining cause. As I have 

 not elsewhere discussed this subject, and as it has an impor- 

 tant 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 constitution before the 

 power of reproduction is gained. On the other hand, after 

 this power has been gained and the sexes have come to 

 diifer 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 generality 

 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 libellulae. In all these cases the variations, 

 through the accumulation of which the male acquired his 

 proper masculine characters, must have occurred at a some- 

 what late period of life; otherwise the young males would 

 have been similarly characterized; and conformably with 

 our rule, the variations are transmitted to and developed in 

 the adult males alone. When, on the other hand, the adult 

 male closely resembles the young of both sexes (these, with 

 rare exceptions, being alike), he generally resembles the 



* References are given in my " Variation of Animals under Domes- 

 tication," vol. ii, p. 73. 



