230 CLASSICS OF MODERN SCIENCE 

 every internal organ, on every shade of constitutional difference, on 

 the whole machinery of life. Man selects only for his own good : 

 Nature only for that of the being which she tends. Every selected 

 character is fully exercised by her, as is implied by the fact of their 

 selection. Man keeps the natives of many climates in the same coun- 

 try; he seldom exercises each selected character in some peculiar 

 and fitting manner ; he feeds a long and a short-beaked pigeon on the 

 same food; he does not exercise a long-backed or long-legged quad- 

 ruped in any peculiar manner ; he exposes sheep with long and short 

 wool to the same climate. He does not allow the most vigorous males 

 to struggle for the females. He does not rigidly destroy all inferior 

 animals, but protects during each varying season, as far as lies in 

 his power, all his productions. He often begins his selection by 

 some half-monstrous form; or at least by some modification promi- 

 nent enough to catch the eye or to be plainly useful to him. Under 

 nature, the slightest differences of structure or constitution may well 

 turn the nicely-balanced scale in the struggle for life, and so be pre- 

 served. How fleeting are the wishes and efforts of man! how short 

 his time ! and consequently how poor will be his results, compared 

 with those accumulated by Nature during whole geological periods! 

 Can we wonder, then, that Nature's productions should be far *'truer" 

 in character than man's productions; that they should be infinitely 

 better adapted to the most complex conditions of life, and should 

 plainly bear the stamp of far higher workmanship? 



It may metaphorically be said that natural selection is daily and 

 hourly scrutinizing, throughout the world, the slightest variations ; re- 

 jecting those that are bad, preserving and adding up all that are good ; 

 silently and sensibly working, whenever and wherever opportunity 

 offers, at the improvement of each organic being in relation to its or- 

 ganic 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, 



