THEORIES OF EVOLUTION 113 



faith are the apparently sound principles which 

 they supply. Our point of departure is narrowed 

 down at once to the two great schools of thought, 

 the Darwinian and the Lamarckian. Whether we 

 adhere to the original views of the two men or to 

 the modifications introduced by their followers the 

 separation is not particularly well taken, but in 

 the main the Darwinian idea emphasizes the organ- 

 ism as the source of change while the Lamarckian 

 idea emphasizes the part played by the environ- 

 ment in the adaptation of the individual. 



Since Darwin made no attempt to explain the 

 origin of variations, but merely accepted them as 

 a potential source of evolution through selection, 

 it is only natural that his views should have been 

 supplemented by many others. He emphasized 

 the usefulness of variations as a potent factor in 

 evolution, and we have come to realize that much 

 evolution has been accomplished among characters 

 for which we can discover no use. Even in the 

 expression of his great theory of natural selection 

 are possibilities which it has remained for his fol- 

 lowers to bring out. 



Briefly, Darwin's theory of evolution, or natural 

 selection, rests on the several postulates of over- 

 production, the resulting struggle for existence, the 

 survival value of variations, and the cumulative 

 modification of the characters involved through 

 succeeding generations. That there are serious ob- 



