The Making of Species 
sparrow of a large part of Africa (Passer 
swainsonz) both sexes are very plain, like the 
hen of the house-sparrow; in this species (P. 
domesticus), as every one knows, the cock, though 
by no means brilliant, is noticeably handsomer 
than his mate; while in the Tree-sparrow (P. 
montanus) both sexes have a plumage of mascu- 
line type, much like that of the cock house- 
sparrow. 
If we consider in conjunction with one another 
the various facts we have cited above, we begin 
to grasp the nature of the phenomena of sexual 
dimorphism. 
Let us consider an imaginary case of a defence- 
less little bird which builds an open nest. Let 
us suppose that it is inconspicuously plumaged. 
Now suppose that a mutation of the first kind 
shows itself, a mutation which affects the cock 
only and makes him more conspicuous. Let 
us further suppose that the cock does not 
share in the duties of incubation. It is quite 
possible that, in spite of this apparently unfavour- 
able mutation, the species may survive, for, as 
we have seen, it does not affect the hen, and 
she, since she alone incubates, stands the most 
in need of protective colouring. Moreover, as 
Stolzmann has suggested, the species can pos- 
sibly afford to lose a few males. But suppose 
that both cock and hen share in the duties of 
incubation, it is then quite likely that the muta- 
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