iyi The Descent of Man. Part II. 



On the Relation betuten the Period of Development of a Character 

 and its Transmission to one Sex or to both Sexes. — Why certain 

 characters should be inherited by both sexes, and other charac- 

 ters by one sex alone, namely by that sex in which the character 

 first appeared, is in most cases quite unknown. We cannot even 

 conjecture why with certain sub-breeds of the pigeon, black 

 striae, though transmitted through the female, should be deve- 

 loped in the male alone, whilst every other character is equally 

 transferred to both sexes. Why, again, with cats, the tortoise- 

 shell colour should, with rare exceptions, be developed in the 

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

 pernumerary digits, colour-blindness, &c, 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. 38 Although we are 

 thus ignorant, the two following rules seem often to hold good— 

 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 determining 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 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 generality of this fact is quite 

 remarkable: it holds good with almost all mammals, birds, 



with this character, he says : " It is <k facts that I have related ; but it 



" a singular circumstance that Mr. " is remarkable how very closely 



" Darwin should have suggested the " he suggested the right method of 



" possibility of modifying the sexual " procedure." 



" colours of birds by a course of 3S References are given in my 



" artificial selection. When he did Variation of Animals under Domes« 



u so, he was in ignorance of these tication,' vol. ii. p. 72. 



