480 THE DESCENT OF 



ations may be divided into two classes; those which appeal' 

 to our ignorance to arise spontaneously, and those which are 

 directly related to the surrounding conditions, so that all or 

 nearly all the individuals of the same species are similarly 

 modified. Cases of the latter kind have recently been observed 

 with care by Mr. J. A. Allen,* who* shows that in the 

 United States many species of birds gradually become 

 more strongly colored in proceeding southward, and more 

 lightly colored in proceeding westward to the arid plains 

 of the interior. Both sexes seem generally to be affected 

 in a like manner, but sometimes one sex more than the 

 other. This result is not incompatible with the belief that 

 the colors of birds are mainly due to the accumulation of 

 successive variations through sexual selection ; for even 

 after the sexes have been greatly differentiated, climate 

 might produce an equal effect on both sexes, or a greater 

 effect on one sex than on the other, owing to some consti- 

 tutional difference. 



Individual differences between the members of the same 

 species are admitted by every one to occur under a state of 

 nature. Sudden and strongly marked variations are rare ; 



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

 latter, Blasius thinks that only ten are really doubtful and that the 

 other fifty ought to be united with their nearest allies; but this 

 shows that there must be a considerable amount of variation with 

 some of ^>ur European birds. It is also an unsettled point with 

 naturalists, whether several North American birds ought to be 

 ranked as specifically distinct from the corresponding European 

 species. So again many North American forms which until lately 

 were named as distinct species, are now considered to be local races. 



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

 Reconnaissance of Kansas, etc." Noth withstanding 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 Galapagos Islands under the equator, the wide, tem- 

 perate plains of Patagonia, and, as it appears, Egypt (see Mr, Harts- 

 home in the "American Naturalist," 1873, p. 747). These countries 

 are open and afford little shelter to birds; but it seems doubtfuV 

 whether the absence of brightly colored species can be explained on 

 the principle of protection, for on the PampaSj 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 sometimes speculated whether the pre- 

 vailing dull tints of the scenery in the above-named countries may 

 not have affected the appreciation of bright colors by the birds inhali 

 iting them, 



