SPECIES AND THEIR ORIGIN 303 



the illustration shows that the concept of species 

 is an abstract one. The species is real enough, but 

 the criterion of the species exists in the student's 

 mind, just as does that of the genus or the higher 

 groups. Such species, frequently called Linncean 

 species, are but convenient categories for classifying 

 aggregates of plants and animals which closely 

 resemble one another and are of (assumed) common 

 descent. 



Elementary Species. It will be recalled that 

 there is considerable evidence for the belief that 

 discontinuous variations or mutations are of a very 

 different sort from fortuitous variations. Their 

 constancy, their abrupt origin, and their behavior 

 in heredity, sharply set them off from the latter. 

 De Vries called his mutants " elementary species:" 

 He believes that they are not in any sense the product 

 of environmental influence, as geographical " vari- 

 eties " may be, but are distinct entities. There have 

 been found over two hundred such species of the 

 " whitlow grass," Draba verna, and more than that 

 of the hawthorn (Cratcegus). It is obvious that to 

 separate and name all of these would defeat the 

 purpose of classification, which is to reduce chaos 

 to order, and complexity to simplicity, so the Lin- 

 naean species will probably continue to be used as 

 the practical units of the classifier. 



The " pure lines " or genes which have been dis- 

 covered in both plants and animals are probably the 

 real units of organic nature, the centers of stability 



