78 NATURAL SELECTION, 



so onward, step by step. Seeing that individual differences 

 of the same kind perpetually recur, this can hardly be con- 

 sidered as an unwarrantable assumption. But whether it 

 is true, we can judge only by seeing how far the hypothe- 

 sis 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 consider 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 color of heather, we 

 must believe that these tints are of service to these birds 

 and insectr 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 are guided by e3^esight to 

 their prey — so much so that on j^arts of the continent per- 

 sons are warned not to keep white pigeons, as being the 

 most liable to destruction. Hence natural selection might 

 be effective in giving the j)roper color to each kind of 

 grouse, and in keeping that color, when once acquired, 

 true and constant. Nor ought we to think that the oc- 

 casional destruction of an animal of any particular color 

 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 

 color of 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 color 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 that smooth-skinned fruits suffer 

 far more from a beetle, a Curcuho, than those with down; 

 that purple jolums suffer far more from a certain disease 

 than yellow plums; whereas another disease attacks yellow- 

 fleshed peaches far more those with other colored 

 flesh. If, with all the aids of art, these slight differ- 

 ences make a great difference in cultivating the several 

 varieties, assuredly, in a state of nature, where the trees 



