18 THE THEORY OF EVOLUTION 



a matter of the commonest observation that no two 

 individuals of a species are exactly alike and, when 

 a very large number of specimens are studied, the 

 range of variation is often found to be exceedingly 

 wide. Such variations are fluctuating and repeated 

 in every generation. 



(2) The second fact is the struggle for existence. 

 Every species in a state of nature produces vastly 

 more young than can possibly survive to maturity; 

 if every egg of the herring should develop to an 

 adult fish and reproduce in its turn, it would not be 

 long before the Atlantic Ocean would fail to contain 

 them. A statistical study of many species leads 

 to the conclusion that, so long as the conditions of 

 life remain the same, the number of individuals of 

 a given species is a substantially constant quantity, 

 subject of course to fluctuations of increase and de- 

 crease. Thus, for every young animal or plant that 

 reaches maturity, an old one must die to make place 

 for it. The struggle for existence is not to be thought 

 of as a combat to slay and devour, but as a competi- 

 tion for food, light and the other necessities of life, 

 as well as resistance or escape from the attacks of 

 predatory enemies, parasites, diseases and the like. 

 Not only is there intense competition between mem- 

 bers of the same species, but also between allied 

 species with similar habits and needs. 



(3) The third fact is natural selection, or, in Herbert 

 Spencer's happier phrase, the survival of the fittest. 

 Survival to the age of reproduction is not a matter 



