66 THE ORIGIN OF SPECIES 



wherever opportunity offers, at the improvement of each organic 

 being in relation to its organic and inorganic conditions of life. We 

 see nothing of these slow changes in progress, until the hand of 

 time has marked the lapse of ages, and then so imperfect is our 

 view into long-past geological ages that we see only that the forms 

 of life are now different from what they formerly were. 



In order that any great amount of modification should be ef- 

 fected in a species, a variety, when once formed, must again, per- 

 haps after a long interval of time, vary or present individual dif- 

 ferences of the same favorable nature as before; and these must 

 again be preserved, and so onward, step by step. Seeing that indi- 

 vidual 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 ac- 

 cords with and explains the general phenomena of nature. On the 

 other hand, the ordinary belief that the amount of possible varia- 

 tion 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 insects 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 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 natu- 

 ral 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 de- 

 struction 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 Vir- 

 ginia, 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 the smooth-skinned fruits suffer far more from a beetle, a 

 Curculio, than those with down; that purple plums suffer far mere 

 from a certain disease than yellow plums; whereas another disease 



