Ohap. XIV. Variability. 423 



thai in the Quitod States many species of birds gradually become 

 more strongly coloured in proceeding southward, and more 

 lightly coloured 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 colours of 

 birds are mainly due to the accumulation of successive •varia- 

 tions through sexual selection; for even after the sexes have 

 been greatly differentiated, climate might produce an equal 

 effect on both f-exes, or a greater effect on one sex than on the 

 other, owing to some constitutional 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 ; it is 

 also doubtful whether if beneficial they would often be preserved 

 through selection and transmitted to succeeding generations.'^ 

 Nevertheless, it may be worth while to give the few cases which 

 I have been able to collect, relating chiefly to colour, — simple 

 albinism and melanism being excluded. Mr. Gould is well 

 known to admit the existence of few varieties, for he esteems 

 very slight differences as specific; yet he states'* that near 

 Bogota certain humming-birds belonging to the genus Cynanthus 

 aie divided into two or three races or varieties, which differ 

 from each other in the colouring of the tail — " some having the 

 " whole of the feathers blue, while others have the eight central 

 ' ones tipped with beautiful green." It does not appear that 

 intermediate gradations have been observed in this or the 

 following cases. In the males alone of one of the Australian 

 parrakeets "the thighs in some are scarlet, in others grass- 

 " green." In another parrakeet of the same country " some 

 " individuals have the band across the wing-coverts bright- 



^'^ * Oris;!!! of Species,* fifth edit, valued individuals of each hreed, 



1869, p. 104. 1 had always per- without any intention on his part 



ceived, that rare and strongly- to modify the characters of the 



marked deviations of structure, de- breed. But until I read an able 



serving to be called monstrosities, arttele in the ' North British Ee- 



could seldom be preserved through view' (March 1867, p. 289, et seq.'/, 



natural selection, and that the pre- which has been of more use to me 



serration of even highly-beneficial than any other Review, I did not 



variations would depend to a certain see how great the chances were 



extent on chance. I had also fully against the preservation of varia- 



appreciated the importance of mere tions, whether slight or strongly 



jidividual diflferences, and this led pronounced, occurring only in single 



me to insist so strongly on the im- individuals, 



portanoe of that unconscious form ^° ' tntroduct. to the Troshilid«e. 



of selection by man, which follows p. 102. 

 from the preservation of the most 



