CHARLES DARWIN 231 



and so onwards step by step. Seeing that individual differences of 

 the same kind perpetually recur, this can hardly be considered as an 

 unwarrantable assumption. But whether it is true, we can judge only 

 by seeing how far the hypothesis accords with and explains the 

 general phenomena of nature. On the other hand, the ordinary 

 belief that the amount of possible variation is a strictly limited 

 quantity is likewise a simple assumption. 



Although natural selection can act only through and for the good 

 of each being, yet characters and structures, which we are apt to con- 

 sider as of very trifling importance, may thus be acted on. When we 

 see leaf-eating insects green, and bark-feeders mottled gray ; the 

 Alpine ptarmigan white in winter, the red-grouse the colour of heather, 

 we must believe that these tints are of service to these birds and in- 

 sects in preserving them from danger. Grouse, if not destroyed at 

 some period of their lives, would increase in countless numbers ; they 

 are known to suffer largely from birds of prey; and hawks aic 

 guided by eyesight to their prey — so much 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. 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 arts, 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 



