Chap. IV. 
NATUBAL SELECTION. 
85 
parts of the Continent persons are warned not to keep 
white pigeons, as being the most liable to destruction. 
Hence I can see no reason to doubt that natural selec- 
tion might be most effective in giving the proper colour 
to each kind of grouse, and in keeping that colour, 
when once acquired, true and constant. Nor ought we 
to think that the occasional destruction of an animal of 
any particular colour would produce little effect: we 
should remember how essential it is in a flock of white 
sheep to destroy every lamb with the faintest trace of 
black. In plants the down on the fruit and the colour 
of the flesh are considered by botanists as characters of 
the most trifling importance: yet we hear from an 
excellent horticulturist. Downing, that in the United 
States smooth-skinned fruits suffer far more from a 
beetle, a curculio, than those with down; that purple 
plums suffer far more from a certain disease than yellow 
plums; whereas another disease attacks yellow-fleshed 
peaches far more than those with other coloured flesh. 
If, with all the aids of art, these slight differences make 
a great difference in cultivating the several varieties, 
assuredly, in a state of nature, where the trees would 
have to struggle with other trees and with a host of 
enemies, such differences would effectually settle which 
variety, whether a smooth or downy, a yellow or purple 
fleshed fruit, should succeed. 
In looking at many small points of difference be¬ 
tween species, which, as far as our ignorance permits 
us to judge, seem quite unimportant, we must not forget 
that climate, food, &c., probably produce some slight 
and direct effect. It is, however, far more necessary 
to bear in mind that there are many unknown laws of 
correlation of growth, which, when one part of the 
organisation is modified through variation, and the 
modifications are accumulated by natural selection for 
