SEXUAL SELECTION. 227 



On the Belation between the Period of Development of a Character 

 ancl its Transmission to one Sex or to both Sexes, — Why cer- 

 tain characters should be inherited by both sexes, and other char- 

 acters 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 developed in the 

 male alone, whilst every other character is equally transferred to 

 both sexes. Why, again, with cats, the tortoise-shell color should, 

 with rare exceptions, be developed in the female alone. I'he very 

 same character, such as deficient or supernumerary digits, color- 

 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.'* 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 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 reproduc- 

 tion is gained. On the other hand, after this power has been 

 gained 'and the sexes have come to differ in constitution, the gem- 

 mules (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 afiinities 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 re- 

 markable: it holds good with almost all mammals, birds, amphib- 

 ians, and fishes; also with many crustaceans, spiders, and some 

 few insects, such as certain orthoptera and libellulse. In all these 



"should have suggested the possibility of modifying the sexual colors 

 "of birds by a course of artificial selection. When he did so, he was 

 "in ignorance of these facts that I have related; but it Is remarkable 

 "how very closely he suggested the right method of procedure." 



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

 tication," vol. ii. p. 72. 



