60 ADAPTATION AND PROGRESS 



organic being naturally increases at so high a rate that, if not 

 destroyed, the earth would soon be covered by the progeny of a 

 single pair." Darwin gives many instances of the prodigality of 

 nature and these have been supplemented by contributions from 

 more recent exponents of " natural selection." l 



He considers various checks to the increase of members of a 

 species including enemies, lack of food supply and climate and 

 shows the complex relations of all animals and plants to each 

 other in the struggle for existence, concluding that " battle 

 within battle must be continually recurring with varying suc- 

 cess." 2 



This prodigality and struggle for existence, according to the 

 author under consideration, is just the condition most favorable 

 for progress by means of natural selection, for in this struggle 

 those individuals which by slight favorable variations are best 

 adapted to the conditions of life will survive whereas the least 

 adapted will perish. As to the working of natural selection, 

 Darwin says: " Let the endless number of slight variations and 

 individual differences occurring in our domestic productions, and, 

 in a lesser degree, in those under nature, be borne in mind; as 

 well as the strength of the hereditary tendency. . . . Can it 

 then be thought improbable, seeing that variations useful to man 

 have undoubtedly occurred, that variations useful in some way 

 to each being in the great and complex battle of life, should occur 

 in the course of many successive generations ? If such do occur, 

 can we doubt (remembering how many more individuals are born 

 than can possibly survive) that individuals having any advantage, 

 however slight, over others, would have the best chance of surviv- 

 ing and of procreating their kind ? On the other hand, we may 

 feel sure that any variation in the least degree injurious would be 

 rigidly destroyed." He goes on to say that " variations neither 

 useful nor injurious would not be affected by natural selection, 

 and would be left either a fluctuating element ... or would 

 ultimately become fixed, owing to the nature of the organism and 

 the nature of the conditions." 3 



1 Wallace, op. tit., pp. 25 f.; Conn, op. cit., pp. 52 ff.; Morgan, Evolution and 

 Adaptation, p. in. 



2 Origin of Species, p. 57. 3 Ibid., pp. 62, 63. 



