Chap. XIY. 
VARIABILITY. 
125 
of two kinds, which insensibly graduate into each other, 
namely, slight 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 few 
cases relating chiefly to colour (simple albinism and 
melanism being excluded), which I have been able to 
collect. 
Mr. Gould is well knowm rarely to admit the existence 
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, 
Blasius thinks that only ten are really doubtful, 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 
Korth American birds ought to be ranked as specifically distinct from 
the corresponding European species. 
32 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, 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 appre- 
ciated 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 his 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 individuals. 
‘ Introduct. to the Trochilidge,’ p. 102. 
