120 EVOLUTION AND GENETICS 



evolution has taken place. He pointed to the influ- 

 ence of the environment, to the effects of use and 

 disuse, and to natural selection. It is to the last the- 

 ory that his name is especially attached. He appealed 

 to a fact familiar to everyone, that no two individ- 

 uals 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 survive and to leave 

 offspring. As a consequence, 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 Spen- 

 cer called the survival of the fittest. 



Stated in these general terms tliere is nothing in 

 the theory to which anyone is likely to take excep- 

 tion ; for, it may appear little more than a truism to 

 state that the individuals that are the best adapted to 

 survive have a better chance of surviving than those 

 not so well adapted to survive. But Darwin did much 

 more than appeal to any such generality. He pointed 

 out that variations occur in all directions; that at 

 least some of these variations are transmitted; and 

 that on an average more offspring are produced by 

 each pair than survive. He appealed directly to a 

 large amount of biological evidence in support of his 

 theory. 



Since 1859 a great deal of work has been done that 

 bears on the interpretation that Darwin placed on 



