The Four Kinds of Mutations 



It is our belief that sexual dimorphism arises 

 frequently, if not invariably, as a mutation. 

 Mutations may be of four different kinds. 



Those which appear only, or especially, in con- 

 junction with the male organs, for example, 

 whiteness in domesticated geese allowed to 

 breed indiscriminately. 



Those which appear only, or especially, in con- 

 junction with the female organs ; mutations of 

 this description appear to be very rare, but it 

 may be noted that in fowls allowed to breed 

 indiscriminately, as in India, completely black 

 hens are common, but completely black cocks 

 are rarely, if ever, seen. This indicates an 

 association between blackness and femininity. 



Those which appear in the same manner in 

 both sexes. The great majority of mutations 

 appear to be of this kind. 



Lastly, those that appear in both sexes but 

 take a different form in the case of the two 

 sexes ; thus in cats a mutation has given rise 

 to sandy males and tortoise-shell females. The 

 mutation which has produced the black-winged 

 peacock shows itself in the form of a black wing 

 in the cock, while it causes the plumage of the 

 hen to be grizzly white. 



We shall deal with the phenomenon of correla- 

 tion at some length in the next chapter. It is a 

 subject to which sufficient attention has not been 

 paid. Even as certain characters are correlated 



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