EXPLANATIONS OF EVOLUTION 337 



perish. Natural selection is thus said to result in the 

 "survival of the fittest," or it may be even more accurately 

 described as the "destruction of the unfit." The in- 

 dividuals thus preserved hand down to their progeny the 

 favorable variation; and, as this selection is repeated 

 generation after generation, the favorable variation is in- 

 creased, and finally a new species is the result. 



This explanation is not now regarded as entirely satis- 

 factory, for the multiplication of facts has introduced 

 serious difficulties. But whether natural selection pro- 

 duces new species or not, it must play an important role 

 in the preservation of certain forms and in the destruction 

 of others. 



4. The fourth epoch in the history of evolution was 

 introduced during the present decade when DeVries of 

 Amsterdam announced his "mutation theory." He had 

 observed that a certain species of evening primrose oc- 

 casionally produced forms so different from the parent that 

 he regarded them as distinct species. They were not the 

 small variations that Darwin used in his theory of natural 

 selection, but the large ones that had been called "sports," 

 and had been disregarded as of any significance in the 

 origin of species. DeVries cultivated these plants for 

 many years, and observed them generation after generation. 

 The sports, which he called "mutations" or "mutants," 

 continued to appear, but in very small numbers in propor- 

 tion to the total number of progeny. In this way he ob- 

 served this single species of evening primrose produce 

 several new species, all of which bred true and showed no 

 tendency to run back to the parent. He concluded that 

 new species may be thus produced "at a single bound," 



