Chap. IV. NATUKAL 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 between 

 species, which, as far as our ignorance permits us to 

 judge, seem to be 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 



