104 NATURAL SELECTION. [Chap. IV. 



of their lives would increase in countless numbers; they 

 are known to suffer largely from birds of prey; and 

 hawks are guided by eyesight to their prey — so mucb 

 so, that on parts of the Continent persons are warned 

 not to keep white pigeons, as being the most liable to 

 ('destruction. Hence natural selection might be 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 a lamb with 

 the faintest trace of black. We have seen how the 

 colour of the hogs, which feed on the " paint-root " in 

 Virginia, determines whether they shall live or die. Id 

 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 horti- 

 culturist. 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 differ- 

 ence 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 differ- 

 ences 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 



