OTHER THEORIES OF SPECIES-FORMING. 339 



all or nearly all that try to go in other directions. By this 

 means natural selection is the one directing cause of the 

 broad lines of evolution." 



While de Vries admits that recorded mutations are few ; 

 "mutations under observation are as yet very rare; enough 

 to indicate the possible and most probable ways but no 

 more;" 1T yet he strongly maintains that there is no scientific 

 proof of the origin of species in any other way than by 

 mutation and that there is such proof of their actual muta- 

 tional origin. He says : "I intend to give a review of the 

 facts obtained from plants which go to prove the assertion 

 that species and varieties have originated by mutation and 

 are, at present, not known to originate in another way." 



But in any consideration of de Vries's work and theories, 

 one must have clearly in mind the distinctive meaning 

 which de Vries attaches to the word species. However 

 little biologists agree on any absolute definition of species, 

 the term nevertheless is consistently used to refer to differ- 

 entiated organic types between any two of which there is 

 considerable obvious describable difference, either quali- 

 tative or quantitative. If two types of such obvious differ- 

 ence in one or several characteristics (usually external or 

 at least externally noticeable differences are the ones used) 

 are connected by a series of connecting gradatory forms 

 existing either in the same territory or in other regions, the 

 two forms are not referred to as distinct species but as 

 varieties ; at least the form at one end of the series is called 

 a variety of the form at the other end. But de Vries's 

 species and varieties are of different stuff. Specific dis- 

 tinctions with him are based on differences in aggregation 

 of the elementary units, the Einheiten, that go to compose 

 the specific types. "Species is a word," says de Vries, 

 "which always has had a double meaning. One of them is 

 the systematic species, which is the unit of our system. 

 But these units are not at all indivisible. Long ago Lin- 



