THE CLASSIFICATION OF PLANTS 219 



relationship. In grouping species together into genera, 

 Lmnseus recognizes this bond of union between species ; 

 but in his wider groupings he abandoned the natural 

 system, and separated the genera into divisions based 

 upon the number of stamens in the flower. This gave 

 him a convenient key for the identification of plants, 

 but it was extremely artificial, and afforded no ghmpse 

 of their true affinities. A more natural system was out- 

 lined afterwards by Linnaeus himself, but the working out 

 of a natural system of classification has been the work of 

 his successors. In British floras the system of Bentham 

 and Hooker is generally adopted; on the Continent and 

 in modern treatises that of Engler is followed. No 

 classification can be said to be quite satisfactory, and 

 none ever will be until the last word on plant-relation- 

 ships has been said, and we are very far from that at 

 present. 



The Unit of Classification. — Every exact science is based 

 on units. In physics and mathematics these are piu:ely 

 arbitrary. In botany, as in zoology, the species is re- 

 garded as the unit of classification. Each species is sup- 

 posed to represent a definite race of plants. Before the 

 idea of evolution became prevalent, most people thought 

 that the species originated by an act of special creation, 

 and remained unchanged and unchangeable throughout 

 all time ; others thought that the species was mutable, 

 but that the genus was not. Linnaeus, for example, 

 looked upon the genus as a special creation, but not the 

 species derived from it. We know now that the species 

 is, at most, only a relatively constant thing. No two 

 plants even of the same parent are exactly alike, and 

 observation will show that considerable variation occurs 

 among the members of the same species. When these 

 variations are pronounced and apparently constant, we 

 caU the variation a variety. What, therefore, constitutes 

 a variety and what a species ? The question is a very 

 difficult one to answer. What one person will call a 

 variety another will call a true species. The Floras are 

 full of instances of this kind. In fact, there are two 

 schools of botanical systematists — those who lump 

 together under the same specific name as many varietal 

 forms as possible, and those who separate them out, 

 wherever possible, as true and distinct species. Bentham 



