Chap. IV.] NATURAL SELECTION. 103 



is daily and 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, whenever and wherever oppor- 

 tunity 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 fa- 

 vourable nature as before; and these must be again pre-l 

 served, and so onwards step by step. Seeing that individ- 

 ual differences of the same kind perpetually recur, this 

 can hardly be considered as an unwarrantable assump- 

 tion. 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 struc- 

 tures, 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-ieeders 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 

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