284 DARWINISM chap. 



sexual characters which may have been produced by natural 

 selection. Such are the various sounds and odours which are 

 peculiar to the male, and which serve as a call to the female 

 or as an indication of his presence. These are evidently a 

 valuable addition to the means of recognition of the two sexes, 

 and are a further indication that the pairing season has 

 arrived ; and the production, intensification, and differentiation 

 of these sounds and odours are clearly within the power of 

 natural selection. The same remark will apply to the peculiar 

 calls of birds, and even to the singing of the males. These 

 may well have originated merely as a means of recognition 

 between the two sexes of a species, and as an invitation from 

 the male to the female bird. When the individuals of a 

 species are widely scattered, such a call must be of great 

 importance in enabling pairing to take place as early as 

 possible, and thus the clearness, loudness, and individuality of 

 the song becomes a useful character, and therefore the subject 

 of natural selection. Such is especially the case with the 

 cuckoo, and with all solitary birds, and it may have been 

 equally important at some period of the development of all 

 birds. The act of -singing is evidently a pleasurable one ; and 

 it probably serves as an outlet for superabundant nervous 

 energy and excitement, just as dancing, singing, and field 

 sports do with us. It is suggestive of this view that the 

 exercise of the vocal power seems to be complementary 

 to the development of accessory plumes and ornaments, 

 all our finest singing birds being plainly coloured, and 

 with no crests, neck or tail plumes to display; while the 

 gorgeously ornamented birds of the tropics have no song, 

 and those which expend much energy in display of plumage, 

 as the turkey, peacocks, birds of paradise, and humming- 

 birds, have comparatively an insignificant development of 

 voice. Some birds have, in the wings or tail, peculiarly 

 developed feathers which produce special sounds. In some of 

 the little manakins of Brazil, two or three of the wing- 

 feathers are curiously shaped and stiffened in the male, so 

 that the bird is able to produce with them a peculiar 

 snapping or cracking sound ; and the tail-feathers of several 

 species of snipe are so narrowed as to produce distinct 

 drumming, whistling, or switching sounds when the birds 



