146 THEORY OF EVOLUTION 



that explanation probable. No such great 

 body of evidence had ever been brought to- 

 gether before, and it wrought, as we all know, 

 a revolution in our modes of thinking. 



Darwin also set himself the task of showing 

 how evolution might have taken place. He 

 pointed to the influence of the environment, to 

 the effects of use and disuse, and to natural 

 selection. It is to the last theory that his name 

 is especially attached. He appealed to a fact 

 familiar to everyone, that no two individuals 

 are identical and that some of the differences 

 that they show are inherited. He argued that 

 those individuals that are best suited to their 

 environment are the most probable ones to sur- 

 vive and to leave most offspring. In conse- 

 quence their descendants should in time replace 

 through competition the less well-adapted 

 individuals of the species. This is the process 

 Darwin called natural selection, and Spencer 

 the survival of the fittest. 



Stated in these general terms there is noth- 

 ing in the theory to which anyone is likely to 

 take exception. But let us examine the argu- 

 ment more critically. 



