CHAP. IV.] NATURAL SELECTION. 61 



hourly scrutinising, throughout the world, the slightest variations ; 

 rejecting those that are bad, preserving and adding up all that are 

 good ; silently and insensibly working, wlienever and wlierever 

 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 effected 

 in a species, a variety when once formed must again, perhaps after 

 a long interval of time, vary or present individual differences of 

 the same favourable nature as before ; and these must be again 

 preserved, 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 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 '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 

 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, Down- 

 ing, that in the United States smooth-skinned fruits suffer far 

 more from a beetle, a Curculio, than those with down ; that purplo 

 plums suffer far more from a certain disease than yellow plums . 



