THEORIES OF EVOLUTION 161 



paleontology and other sources that species may 

 also arise by slow accumulations of small variations. 



One great value of his work is that it is based on 

 experiments and that it has given a great stimulus to 

 experimental studies. Experiment was likewise a 

 feature of Darwin's work, but that seems to have 

 been almost overlooked in the discussions aroused by 

 his conclusions. De Vries, by building upon exper- 

 imental evidence, has led naturalists to realize that 

 the method of evolution is not a subject for argu- 

 mentative discussion, but for experimental investi- 

 gation. 



Other Theories. In addition to the four theories 

 briefly outlined other theories have been advanced, 

 which, in their relation to the Darwinian hypothesis 

 of natural selection, fall into two categories. There 

 are competing theories designed to replace that of 

 natural selection, and there are auxiliary, or support- 

 ing theories, that are designed to throw new light on 

 the conditions of species-forming, and to strengthen 

 the natural selection theory by its more complete 

 elucidation. Such an extensive literature has grown 

 up in the discussion of these matters, that even 

 summaries would unduly prolong the subject of this 

 chapter. The entire case has been presented with 



