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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