490 The Descent of Man. Part 11. 



■■ eolonr." '" Calling to my recollection the desert-birds of South 

 America, as well as most of the ground-birds of Great Britain, 

 it appeared to me that both sexes in such cases are generally 

 coloured nearly alike. Accordingly, I applied to Mr. Tristram 

 "with respect to the birds of the Sahara, and he has kindly 

 given me the following information. There are twenty-six 

 species belonging to fifteen genera, which manifestly have their 

 plumage coloured in a protective manner; and this colouring, is 

 all the more striking, as with most of these birds it differs from 

 that of their congeners. Both sexes of thirteen out of the 

 twenty-.six species are coloured in the same manner ; but these 

 belong to genera in which this rule commonly prevails, so that 

 they tell us nothing about the protective colours being the same 

 in both sexes of desert-birds. Of the other thirteen species, three 

 belong to genera in which the sexes usually differ from each 

 other, yet here they have the sexes alike. In the remaining ten 

 species, the male differs from the female ; but the difference is 

 ccmfined chiefly to the under surface of the plumage, which is 

 cimccalcd when the bird crouches on the ground ; the head and 

 back being of the same sand-coloured hue in the two sexes. So 

 that in these ten species the upper surfaces of both sexes have 

 been acted on and rendered alike, through natural selection, for 

 the sake of protection ; whilst the lower surfaces of the males 

 alone have been diversified, through sexual selection, for the 

 sake of ornament. Here, as both sexes are equally well pro- 

 tected, we clearly see that the females have not been prevented 

 by natural selection from inheriting the colours of their male 

 parents; so that we must look to the law of sexually-limited 

 transmission. 



In all parts of the world both sexes of many soft-billed birds, 

 especially those which frequent reeds or sedges, are obscurely 

 coloured. No doubt if their colours had been brilliant, they 

 would have been much more conspicuous to their enemies ; but 

 whether their dull tints have been specially gained for the sake 

 of protection seems, as far as I can judge, rather doubtful. It 

 is still more doubtful whether such dull tints can havg been 

 gained for the .sake of ornament. We must, however, bear in mind 

 that male birds, though dull-coloured, often differ much from 

 their females (as with the common sparrow), and this leads to 

 the belief that such colours have been gaineft thnragh sexual 

 selection, from being attractive. Many of the soft-billed birds 

 are songsters ; and a discussion in a former chapter should not 



"• ' Ibis,' 1859, vol. .. p. 429, et hii! experience of the S.nliara, thit 

 t^q. Dr. Jiohlfs, however, remarks statement is too .strong, 

 to mo in a letter ttiat, accor.:line to 



