188 GENERAL ZOOLOGY 



unsuccessful experiments have been performed in the at- 

 tempt to furnish this direct evidence. 



The Mutation Theory. It will be remembered that Dar- 

 win laid stress upon indefinite, or fluctuating, variations as 

 furnishing the material for selection. The mutation theory 

 stands in sharp contrast with the selection theory in em- 

 phasizing the hereditary transmission of definite variations. 

 With the mutation theory is associated the name of Hugo de 

 Vries, a Dutch botanist of Amsterdam, Holland. He was 

 led to express the principle from his studies of the variations 

 in a species of evening primrose introduced from America 

 and found growing in waste places near Amsterdam. 



According to this principle new species have been pro- 

 duced by sudden and perfectly definite changes (mutations) 

 in the organism, though it is not necessary to assume that 

 these changes are always great. The theory makes no 

 attempt to account for the presence of mutations, but when 

 they occur it is a striking fact that the characters tend to be 

 handed on to the descendants. Many of de Vries's conclu- 

 sions regarding the mutations which he observed in prim- 

 roses are today explained as due to impure stock giving rise 

 to new combinations according to Mendel's principles. How- 

 ever, he formulated a principle which has become the basis of 

 one of the most active fields in the investigation of heredity. 



Experimental Study of Evolution. The study of heredity 

 that is being carried on so actively in the research labora- 

 tories of all civilized countries is really the study of some 

 aspects of evolution. New kinds of plants and of animals 

 are being continually produced by artificial selection which 

 essentially is very similar to the principle of natural selec- 

 tion expounded by Darwin. New mutations are being ob- 

 served regularly in the laboratory experiments in genetics. 

 It is becoming more and more obvious that Nature is not 

 restricted to a single method of producing new forms of life. 



