CHAP. X.] SEXUAL SELECTION. 317 



of the males cannot be accounted for, as Mr. Wallace believes 

 to be the case in birds, by the exposure to danger of the 

 females during incubation &quot; (vol. ii. p. 37). 



But if in these cold-blooded classes we have this sexual 

 difference developed by an innate law, why may not a similar 

 cause produce the phenomena in question in the class of 

 birds also ? 



We may indeed well ask this question, since varieties arise 

 from time to time possessing sexually distinct plumage which 

 obviously cannot be due to any protecting action of female 

 sobriety of colour. Thus &quot; there are breeds of the pigeon in 

 Belgium in which the males alone are marked with black 

 strice,&quot; and &quot; in the case of the fowl variations of colour 

 limited in their transmission to the male sex habitually occur&quot; 

 (vol. ii. p. 157). Certainly the large crop and wattles of 

 male pigeons are not due to the cause assigned by Mr. 

 Wallace, nor indeed to sexual selection either, &quot; for fanciers 

 have not selected one sex more than the other, and have had 

 no wish that these characters should be more strongly dis 

 played in the male than in the female, yet this is the case 

 with both breeds.&quot; But even more may be said ; for, as Mr. 

 Darwin justly remarks, if the brightly-coloured females had 

 been continually destroyed, then the effect would not be the 

 forming a strong contrast between male and female birds, but 

 rather &quot;the lessening or annihilation of the bright colours of 

 the males, owing to their continually crossing with the duller 

 females&quot; (vol. ii. p. 160). There are also many striking 

 instances in which the relation which Mr. Wallace supposes 

 to exist between a covered nest and female plumage as bright 

 as that of the male does not obtain. &quot; Thus the male house- 

 sparrow (Passer domesticus) differs much from the female, 

 the male tree-sparrow (P. montanus) differs hardly at all, 

 and yet both build well-concealed nests. TJie two sexes of 

 the common fly-catcher (Muscicapa grisolci) can hardly be 

 distinguished, whilst the sexes of the pied fly-catcher (M. luc- 

 tuosa) differ considerably, and both build in holes. The 

 female blackbird (Turdus merula) differs much, the female 



