The Evolution of Organisms 161 



Directive Factors in Evolution. We must pass now 

 to the directive factors which operate upon the raw 

 material afforded by variability. The only di- 

 rective factors we know of are included in the 

 terms Selection and Isolation. These are the 

 twin directive genii. 



Selection. The theory of Natural Selection, 

 which Darwin and Wallace first expounded, is 

 very familiar, and admits of brief statement. 

 Variability is a fact of life. The members of a 

 family or of a species are not born alike; some have 

 qualities which give them an advantage, both as 

 to " hunger" and as to " love"; others are relatively 

 handicapped. But a struggle for existence is also 

 a fact, being necessitated especially by the 

 abundance of life and by the changefulness of the 

 environment. Two parents usually produce many 

 more than two children, and the population thus 

 tends to outrun the means of subsistence; more- 

 over, living creatures are at the best only relatively 

 well adapted to the conditions of their life, which 

 are changeful. As the result of this struggle for 

 existence, there is discriminate elimination, the 

 relatively less fit being eliminated before they 

 reproduce. : 'Of fifty seeds, she often brings 

 but one to bear." The relatively fitter tend to 

 survive and to reproduce, handing on their ad- 

 vantages to their progeny. If advantageous vari- 

 ations are transmitted, if variations in the same 



