EVOLUTION 323 



whether or not the organism is relatively quiescent or active in 

 producing mutations. 



Present Status of Darwin's Theory. — Darwin explained 

 evolution by the gradual accumulation of small variations under 

 the direction of natural selection. De Vries emphasized muta- 

 tions because they were hereditary and also because they repre- 

 sented larger steps in evolution. For a while many geneticists 

 favored the mutation idea and made use of mutations in the study 

 of heredity. Now as a result of intensive study it has been 

 learned that small mutations are more numerous than large ones, 

 which in fact wipes out the distinction between continuous and 

 discontinuous variation. If mutations occur in small steps as 

 well as in large, we return once more to Darwin's view that the 

 origin of species involves the accumulation of small heritable 

 variations. It may be fairly said that on the whole the work of 

 three-quarters of a century since the publication of Darwin's 

 "Origin of Species" has tended to support and verify his theory 

 of evolution by the natural selection of heritable characters. 

 Natural selection offers the only scientific explanation of the 

 coordinated adaptation detectable in living things. 



