Chap. XIV. 



Variability, 



423 



that in the United 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 sexes, 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. 35 

 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 36 that near 

 Bogota certain humming-birds belonging to the genus Cynanthus 

 are 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- 



83 'Origin f Species,' fifth edit. 

 1869, p. 104. I had always per- 

 ceived, that rare and strongly- 

 marked deviations of structure, de- 

 serving to be called monstrosities, 

 could seldom be preserved through 

 natural selection, and that the pre- 

 servation of even highly-beneficial 

 variations would depend to a certain 

 extent on chance. I had also fully 

 appreciated the importance of mere 

 Individual differences, and this led 

 me to insist so strongly on the im- 

 portance of that unconscious form 

 of selection by man, which follows 

 from the preservation of the most 



valued individuals of each breed, 

 without any intention on his part 

 to modify the characters of the 

 breed. But until I read an able 

 article in the ' North British Re- 

 view ' (March 1 867, p. 289, et seq.j, 

 which has-been of more use to me 

 than any other Review, I did not 

 see how great the chances were 

 against the preservation of varia- 

 tions, whether slight or strongly 

 pronounced, occurring only in single 

 individuals. 



36 ' Introduct, to the Trochilidae, 

 p. 102. 



