152 A TEXT-BOOK OF GRASSES 



more may his judgment be trusted when defining the 

 limits of species with which he is famihar, and the more 

 nearly should his taxonomic ideas approach the truth. 

 The truth for which the taxonomist seeks is a knowledge 

 of genetic relationships; the grouping of organisms into 

 species, genera and other divisions is a convenience which 

 is intended as nearly as may be to express this truth. 

 In the ever-diverging lines of descent, certain groups of 

 individuals have been cut off, as it were, from their allies, 

 so that in these cases the species of the taxonomist prob- 

 ably does express the truth. In other cases the groups 

 are in process of formation and separation, and are not 

 actually distinct. It is here that the taxonomist meets his 

 greatest difficulties. Even with complete knowledge, his 

 taxonomic ideas can be no more distinct than are the 

 groups as they exist in nature. In proportion to his lack 

 of knowledge is the probability that his taxonomic ideas 

 fail to represent the truth. It follows, then, that a 

 classification submitted by a botanist is accepted by his 

 co-workers in proportion to their faith in his judgment and 

 their knowledge of his experience. The members of a 

 complex group of allied species may have been defined 

 and their limits placed with approximate accuracy and 

 yet it may be impossible definitely to refer every individual 

 to its proper species. According to the degree of divergence 

 of allied species in their descent from a common origin, 

 there are a greater or less number of intermediate indi- 

 viduals. The existence of individuals intermediate between 

 two species should not invalidate those species; rather 

 they emphasize the fact that species do not exist in nature, 

 that they are ideas according to which most of the indi- 

 viduals may be classified. 



193. Genera. — A genus is a group of species that are 



