THEORIES OF EVOLUTION 127 



modification of existing things beyond existing 

 limits, the establishment of new trends in develop- 

 ment. Variation may be taken for granted, but 

 this acceptance does not explain the origin of 

 variations, which is the critical point of evolution. 

 It has been shown that selection may be carried 

 to a certain point and that beyond that point it 

 does not result in change. Johanssen's work on 

 beans, which established the principle of pure lines, 

 is the classical example of this limitation. 



Only two of the older theories have soundly at- 

 tacked the problem of the origin of variations, 

 namely, the Lamarckian theory and the mutation 

 theory. The former must be considered more fully 

 in later pages; the latter is of interest here because 

 it has so often been advanced as an explanation of 

 the origin of variations within the organism. 



The reality of mutations cannot logically be 

 doubted. De Vries' work, which bears the same re- 

 lation to the mutation theory that Darwin's bears 

 to evolution generally, was based on the sudden ap- 

 pearance among evening primroses of individual 

 plants with heritages demonstrably different from 

 the parent heritage. These plants bred true, save 

 that they might, in turn, produce mutations in a 

 small percentage of their progeny. Mutations now 

 play a prominent part in genetics and have been 

 recorded in domestic animals and plants and in 

 various organisms picked at random in nature. The 



