98 ORIGIN OF SPECIES 



only by seeing how far the hypothesis accords with and ex- 

 plains 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 consider as of very trifling importance, may 

 thus be acted on. When we see leaf-eating insects green, 

 and bark- feeders mottled-grey; 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 insects in 

 preserving them from danger. Grouse, if not destroyed at 

 some period of their lives, would increase in countless num- 

 bers ; they are known to suffer largely from birds of prey; 

 and hawks are 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 par- 

 ticular colour would produce little effect: we should remem- 

 ber 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 im- 

 portance : yet we hear from an excellent horticulturist. Down- 

 ing, 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. 



