124 
SEXUAL SELECTION: BIRDS. 
Part IL 
wing-featliers, and erects his ocellated plumes in the 
right position for their full effect ; or again, how the 
male goldfinch alternately displays his gold-bespangled 
wings, we ought not to feel too sure that the female 
does not attend to each detail of beauty. We can 
judge, as already remarked, of choice being exerted^ 
only from the analogy of our own minds ; and the 
mental powers of birds, if reason be excluded, do not 
fundamentally differ from ours. From these various 
considerations we may conclude that the pairing of 
birds is not left to chance ; but that those males, which 
are best able by their various charms to please or excite 
the female, are under ordinary circumstances accepted. 
If this be admitted, there is not much difiSculty in 
understanding how male birds have gradually acquired 
their ornamental characters. All animals present indi- 
vidual differences, and as man can modify his domesti- 
cated birds by selecting the individuals which appear 
to him the most beautiful, so the habitual or even occa- 
sional preference by the female of the more attractive 
males would almost certainly lead to their modification ; 
and such modifications might in the course of time be 
augmented to almost any extent, compatible with the 
existence of the species. 
Variahility of Birds, and especially of their secondary 
Sexual Characters , — Variability and inheritance are the 
foundations for the work of selection. That domesti- 
cated^ birds have varied greatly, their variations being 
inherited, is certain. That birds in a state of nature 
present individual differences is admitted by every 
one ; and that they have sometimes been modified into 
distinct races, is generally adinitted.^^ Variations are 
According to Dr. Blasius (‘ Ibis,’ vol. ii. 1860, p. 297), there are 
425 indubitable species of birds which breed in Europe, besides 60' 
