chap, v NATURAL SELECTION 103 



a state of nature. We have now to inquire whether there is 

 any analogous process in nature, by which wild animals and 

 plants can be permanently modified and new races or new 

 species produced. 



Effect of Struggle for Existence under Unchanged Conditions. 



Let us first consider what will be the effect of the struggle 

 for existence upon the animals and plants which we see around 

 us, under conditions which do not perceptibly vary from year 

 to year or from century to century. We have seen that every 

 species is exposed to numerous and varied dangers throughout 

 its entire existence, and that it is only by means of the exact 

 adaptation of its organisation — including its instincts and habits 

 — to its surroundings that it is enabled to live till it produces 

 offspring which may take its place when it ceases to exist. 

 We have seen also that, of the whole annual increase only a 

 very small fraction survives ; and though the survival in indi- 

 vidual cases may sometimes be due rather to accident than 

 to any real superiority, yet we cannot doubt that, in the long- 

 run, those survive which are best fitted by their perfect organisa- 

 tion to escape the dangers that surround them. This " survival 

 of the fittest" is what Darwin termed "natural selection," 

 because it leads to the same results in nature as are produced 

 by man's selection among domestic animals and cultivated 

 plants. Its primary effect will, clearly, be to keep each species 

 in the most perfect health and vigour, with every part of its 

 organisation in full harmony with the conditions of its existence. 

 It prevents any possible deterioration in the organic world, and 

 produces that appearance of exuberant life and enjoyment, of 

 health and beauty, that affords us so much pleasure, and which 

 might lead a superficial observer to suppose that peace and 

 quietude reigned throughout nature. 



The Effect under changed Conditions. 



But the very same process which, so long as conditions re- 

 main substantially the same, secures the continuance of each 

 species of animal or plant in its full perfection, will usually, 

 under changed conditions, bring about whatever change of 

 structure or habits may be necessitated by them. The changed 

 conditions to which we refer are such as we know have occurred 



