66 Natural Selection. Chap. i\t 



crood- silently and insensibly working, whenever and 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 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 pre- 

 served, 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 con- 

 sidered by botanists as characters of the most trifling importance i 

 yet we hear from an excellent horticulturist, Downing, 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 





