EXPERIMENTAL EVOLUTION 537 



found that they bred true (except for additional muta- 

 tions) when propagated by seed for over 25 years — that 

 is, they were true mutations. 



457. Relation of Mutation Theory to Darwinism. — The 

 mutation theory is not intended by de Vries to supplant 

 the theory of natural selection, but to demonstrate that 

 the materials upon which selection acts in the formation 

 of new species are mutations, and mutations only — never 

 fluctuating or individual variations. In the second place 

 the mutation theory explains away numerous objections 

 to natural selection. It shows how characters that are 

 never of vital importance' — i.e., matters of actual Hfe or 

 death — to a species may arise and be perpetuated. With- 

 out mutation this is diflB.cult to explain,^ and yet many, if 

 not most, of the characteristics by which different species 

 are distinguished from each other are of this kind — not, 

 so far as we can see, absolutely essential to the life of the 

 species. Mutation also offers a method by which evolu- 

 tionary changes may take place within a much shorter 

 time-period than was demanded by the natural selection 

 of fluctuations. 



Incidentally, the mutation theory clearly shows that the 

 absence of "connecting links" between species is no argu- 

 ment against evolution, but is, on the contrary, just what 

 we might expect to find. 



458. Value of the Mutation Theory. — As stated above, 

 the elaboration of the mutation theory has furnished the 



' As required by Darwin's theory. See quotation on p. 516. 



^ Other explanations have been offered. For example, sometimes two 

 characters appear to be always associated, so that the presence of one 

 involves the presence of the other; as a mane and maleness in the lion, 

 dicotyledony and exogeny in Angiosperms. 



