646 THE DESCENT OF MAN 



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; 

 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 color — ^simple albinism and melanism being ex- 

 cluded. 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 are divided into 

 two or three races or varieties, which differ from each other 



Bometimes speculated whether the prevailing dull tints of the scenery in tlis 

 above named countries may not have affected the appreciation of bright colota 

 by the birds inhabiting them. 



^ "Origin of Species," fifth edit., 1869, p. 104. I had always perceived 

 that rare and strongly marked deviations of structure, deserving to be called 

 monstrosities, could seldom be preserved through natural selection, and that 

 the preservation of even highly beneficial variations would depend to a certain 

 extent on chance. I had also fully appreciated the importance of mere indi- 

 vidual differences, and this led me to insist so strongly on the importance of 

 that unconscious form of selection by man which follows from the preservation 

 of the moat 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 Review" (March, 186T, p. 289, et seg.), which has been of more 

 nse to me than any other Review, I did not see how great the chances were 

 against the preservation of variations, whether slight or strongly pronounced, 

 occurring only in single individuals. 



^ "Introduct. to the Troohilidae, " p. 102. 



