SEXUAL SELECTION 545 



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 attractive males would almost certainly lead 

 to their modification; and such modifications might in the 

 course of time be augmented to almost any extent compati- 

 ble with the existence of the species. 



Variability of Birds, and especially of their Secondary Sex- 

 ual Characters. — Variability and inheritance are the founda- 

 tions for the work of selection. That domesticated birds 

 have varied greatly, their variations being inherited, is cer- 

 tain. That birds in a state of nature have been modified 

 into distinct races is now universally admitted." Variations 

 may be divided into two classes: those which appear to our 

 ignorance to arise spontaneously, and those which are di- 

 rectly related to the surrounding conditions, so that all or 

 nearly all the individuals of the same species are similarly 

 modified. Oases of the latter kind have recently been ob- 

 served with care by Mr. J. A. Allen," who shows that in 



'"" According to Dr. Blaaiua ("Ibis," vol. ii. 1860, p. 29'7), there are 425 

 indubitable species of birds which breed in Europe, besides sisty forms, which 

 are frequently regarded as distinct species. Of the latter, Blasius thinks that 

 only ten are really doubtful, and (hat the other fifty ought to be united with 

 their nearest alUes ; but this shows that there must be a considerable amount 

 of variation with some of our European birds. It is also an unsettled point 

 with naturalists, whether several North American birds ought to be ranked as 

 gpecifically distinct from the corresponding European species. So again many 

 North Amencan forms which until lately were named as distinct species are 

 BOW considered to be local races. 



*• "Mammals and Birds of East Florida," also an "Ornithological Recon- 

 ooissance of Kansas," etc. Notwithstanding the influence of climate on the 

 colors of birds, it is difficult to account for the dull or dark tints of almost all 

 the species inhabiting certain countries, for instance, the G-alapagos Islands 

 ■under the equator, the wide temperate plains of Patagonia, and, as it appears, 

 Egypt (see Mr. Hartshorne in the "American Naturalist," 18'73, p. 747). 

 These countries are open, and afford little shelter to birds; but it seems 

 doubtful whether the absence of brightly colored species can be explained on 

 the principle of protection, for on the Pampas, which are equally open, though 

 covered by green grass, and where the birds would be equally exposed to 

 danger, many brilliant and conspicuously colored species are common. I have 



