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 dif&cult 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. 



Of 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, etc.), or Elms (White Elm, Slippery Elm, Cork Elm, 

 etc.), or Ash (White Ash, Green Ash, Black Ash, etc.), 

 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. 



