27G . THE PRINCIPLES OF [Pakt II. 



first sexually limited in its development, it would be easy 

 to make a breed with the females alone thus character- 

 ized ; but if the variation was not thus originally limited, 

 the process would be extremely difficult, perhaps impos- 

 sible. 



On the Relation between 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 characters 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 stria?, 

 though transmitted through the female, should be de- 

 veloped in the male alone, while 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. The very same characters, 

 such as deficient or supernumerary 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 the same sex. 23 Although we are thus ignorant, 

 two rules often hold good, namely, that variations which 

 first 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 sup- 

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

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

 portant bearing on sexual selection, I must here enter 

 into lengthy and somewhat intricate details. 



It is in itself probable that any character appearing at 



23 References are given in my ' Variation of Animals under Domesti- 

 cation,' vol. ii. p. 72. 



