1488 THE STORY OF THE UNIVERSE 



formed must again, perhaps after a long interval of 

 time, vary or present individual differences of the 

 same favorable nature as before; and these must be 

 again preserved, and so onward step by step. Seeing 

 that individual differences of the same kind perpet- 

 ually recur, this can hardly be considered as an un- 

 warrantable assumption. But whether it is true, we 

 can judge only by seeing how far the hypothesis ac- 

 cords 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 mot- 

 tled-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 insects in 

 preserving them from danger. Grouse, if not de- 

 stroyed 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 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 destruc- 

 tion. Hence natural selection might be effective in 

 giving the proper color to each kind of grouse, and 

 in keeping that color, when once acquired, true and 

 constant. Nor ought we to think that the occasional 

 destruction of an animal of any particular color 



