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 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.) 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. Introduction to the Trochlidae, p. 102.) 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-yellow, while
in others the same part is tinged with red. (37. Gould, Handbook to
Birds of Australia, vol. ii. pp. 32 and 68.) In the United States some
few of the males of the scarlet tanager (Tanagra rubra) have a
beautiful transverse band of glowing red on the smaller wing-coverts
(38. Audubon, Ornithological Biography, 1838, vol. iv. p. 389.); but
this variation seems to be somewhat rare, so that its preservation
through sexual selection would follow only under usually favourable
circumstances. In Bengal the Honey buzzard (Pernis cristata) has either
a small rudimental crest on its head, or none at all: so slight a
difference, however, would not have been worth notice, had not this
same species possessed in Southern India a well-marked occipital crest
formed of several graduated feathers. (39. Jerdon, Birds of India,
vol. i. p. 108; and Mr. Blyth, in Land and Water, 1868, p. 381.)
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