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.” 4 
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.””? 
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. cit., pp. 25 f.; Conn, op. cit., pp. 52 ff.; Morgan, Evolution and 
Adaptation, p. 111. 
% Origin of Species, p. 57. 3 [bid., pp. 62, 63. 
