152 A TEXT-BOOK OF GRASSES 
more may his judgment be trusted when defining the 
limits of species with which he is familiar, 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 
