Chap. XIV.] VARIABILITY. 119 



how the male goldfinch alternately displays his gold-be- 

 spangled 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 fundamen- 

 tally 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 difficulty in understanding how male 

 birds have gradually acquired their ornamental characters. 

 All animals present individual differences, and as man can 

 modify his domesticated birds by selecting the individuals 

 which appear to him the most beautiful, so the habitual or 

 even occasional preference by the female of the more at- 

 tractive males would almost certainly lead to their modi- 

 fication ; and such modifications might in the course of 

 time be augmented to almost any extent, compatible with 

 the existence of the species. 



Variability of Birds, and especially of their Secondary 

 Sexual Characters. — Variability and inheritance are the 

 foundations for the work of selection. That domesticated 

 birds have varied greatly, ther variations being inherited, 

 is certain. That birds in a state of nature present indi- 

 vidual differences is admitted by every one ; and that 

 they have sometimes been modified into distinct races, is 

 generally admitted. 31 Variations are of two kinds, which 



31 According to Dr. Blasius (' Ibis,' vol. ii. 1860, p. 297), there are 

 425 indubitable species of birds which breed in Europe, besides 60 

 forms, which are frequently regarded as distinct species. Of the latter, 

 Blasius thinks that only 10 are really doubtful, and that the other 50 

 ought to be united with their nearest allies ; Dut this shows that there 

 must be a considerable amount of variation with some of our European 



