158 THE CLASSIFICATION OF PLANTS 
bly all of the earliest kinds have disappeared. If we 
had before us all of the plants that ever existed the task 
of arranging them so as to show their relationship would 
still be a difficult one, but with many forms irretrievably 
lost the difficulty of the task is very greatly increased. 
Some lower plants are probably still much like their 
primitive ancestors, while others have been greatly 
modified. We may think of the plants that we now see 
as having developed through shorter or longer distances; 
some perhaps have stood still in their original: places, 
others have moved forward short distances to where we 
now find them, while still others have gone much farther 
along their evolutionary pathway to their present 
positions. 
Or SPECIES AND GENERA 
206. In studying plants we notice that they exist as 
kinds, and there has been a general agreement to speak of 
each recognizable kind as a “‘species.”” Thus we speak 
of the species of Oaks, Elms, Ashes, Magnolias, etc., 
meaning the kinds of Oaks (White Oak, Red Oak, Black 
Oak, ete.), or Elms (White Elm, Slippery Elm, Cork Elm, 
etc.), or Ash (White Ash, Green Ash, Black Ash, ete.), 
etc., etc., and in all these cases we recognize that we refer 
to a quite definite kind—a species. While in many cases 
the distinctions are less definite, it is still true that in any 
particular locality plants are recognizable as kinds 
(species). Now these species are sufficiently stable so 
that under constant conditions, in any particular locality 
they change slowly, if at all, while they are sufficiently 
plastic so that under changed conditions, as when they 
are carried to other habitats, they change more or less, 
and this may be great enough so that we regard them as 
different species. 
