Chap. XIV. VARIABILITY. 125 



of two kiiifk, which insensibly graduate into each other, 

 namelv, sli2:ht differences between all the members of 

 the same species, and more strongly-marked deviations 

 which occur only occasionally. These latter are rare 

 with birds in a state of nature, and it is very doubtful 

 whether they have often been preserved through selec- 

 tion, and then transmitted to succeeding generations.^^ 

 Nevertheless, it may be worth while to give the fevv 

 cases relating chiefly to colour (simple albinism and 

 melanism being excluded), which I have been able to 

 collect. 



Mr. Gould is well known rarelv to admit the existence 



ft/ 



of varieties, for he esteems very slight differences as 

 specific ; now he states ^^ that near Bogota certain hum- 

 ming-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 



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

 Bksius thinks that only ten are really dt)ubtful, and that the other fifty 

 ought to be united with their nearest allies ; but this shews that there 

 must be a considerable amount of variation with some of our 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. 



33 t Origin of Species,' fifth edit. 1869, p. 104. I had always per- 

 ceived, that rare and strongly-marked deviations of structure, deserving 

 to be called monstrosities, cuuld seldom be preserved through natural 

 selection, and that the pres( rvation of even highl5'-bent'ficial variations 

 would depend to a certain extent on chance. I had also fully appre- 

 ciated the importance of mere individual differences, and this led me 

 to insist so strongly on the importance of th->it 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 ci araeters of the breed. But until I read an able article in 

 the 'North British Eeview' (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 again^it the preservation of variations, whether 

 slight or strongly promiunoed, occurring only in single individuals. 



33 ' Introduct. to the Trochilidre,' p. 102. 



