48 THE THEORY OF NATURAL SELECTION 
present individual differences in almost every part of 
their structure, and this cannot be disputed ; if there 
be, owing to their geometrical rate of increase, a severe 
struggle for life at some age, season, or year, and this 
certainly cannot be disputed ; then, considering the 
infinite complexity of the relations of all organic 
beings to each other and to their conditions of life, 
causing an infinite diversity in structure, constitu- 
tion, and habits, to be advantageous to them, it would 
be a most extraordinary fact if no variations had ever 
occurred useful to each being’s own welfare, in the same 
manner as so many variations have occurred useful to 
man. But if variations useful to any organic being ever 
do occur, assuredly individuals thus characterized will 
have the best chance of being preserved in the struggle 
for life ; and from the strong principle of inheritance, 
these will tend to produce offspring similarly charac- 
terized. This principle of preservation, or the survival 
of the fittest, I have called Natural Selection. It leads 
to the improvement of each creature in relation to its 
organic and inorganic conditions of life, and, conse- 
quently, in most cases, to what must be regarded as an 
advance in organisation. Nevertheless, low and 
simple forms will long endure, if well fitted for their 
simple conditions of life.’* 
We have here a very definite and concise statement 
of the way in which Darwin believed the principle of 
natural selection to take effect in the production of new 
kinds of organisms. It will be our business in this and 
in succeeding chapters to show how far the modern 
* ‘Origin of Species,’ sixth edition, p. 96. 
