VARIABILITT. 417 



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 In- 

 compatible 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 differen- 

 tiated, climate might produce an equal effect on both sexes, or a 

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

 stitutional difference. 



Individual differences between the members of the same spe- 

 cies are admitted by every one to occur under a state of nature. 

 Sudden and strongly marked variations are rare; it is also doubt- 

 ful whether if beneficial they would often be preserved through 

 selection and transmitted to succeeding generations.'" Neverthe- 

 less, 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 excluded. Mr. Gould is well known to admit 

 the existence of a few varieties, for he esteems very slight differ- 

 ences as specific; yet he states'" that near Bogota certain hum- 

 ming-birds belonging to the genus Cynanthus are divided into 



for instance, the Galapagos Islands under the equator, the wide tem- 

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

 Hartshorne in the 'American Naturalist,' 1873, p. 747). These coun- 

 tries are open, and afford little shelter to birds; hut 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 sometimes speculated whether the prevailing 

 dull tints of the scenery in the above named countries may not have 

 affected the appreciation of bright colors 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 

 bo called monstrosities, could seldom be preserved through natural 

 selection, and that the preservation of even highly-beneficial varia- 

 tions 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 importance 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 Bis 

 part to modify the characters of the breed. But until I read an able 

 article in the 'North British Review' (March, 1867, p. 289, et seq.), 

 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 variations, 

 whether slight or strongly pronounced, occurring only in single indi- 

 viduals. 

 2» 'Introduct. to the Trochilidae,' p. 102. 

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