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- 



342 



