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 2 )resence. 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 tho 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 
