160 THE CLASSIFICATION OF PLANTS 



So the Vegetable Kingdom contains 

 Phyla 



Classes (also Sub-classes) 



Orders (also Super-orders, and Sub-orders) 

 Families (also Sub-families) 

 Genera 

 Species. 



The foregoing may be called the framework of the 

 classification of plants used in this book. 



209. It must be borne in mind that in this classification 

 we are dealing with individuals as the only actually ex- 

 istent things. For our own convenience we form a 

 mental concept of an aggregation, of similar individuals, 

 and this we hold as "kind" ("species"). So also we 

 form a mental picture of an aggregation of similar species, 

 and this is what we call the genus. Quite similarly we 

 form a concept of aggregated genera, and call it a family, 

 and so on for orders, classes and phyla. 



EVOLUTION 



210. For the present purpose the more important 

 points included in the general doctrine of evolution may 

 be summarily stated as follows: 



1. The first species were lower plants, and these gave 

 rise to higher plants. 



2. Evolution while generally upward (progressive) is 

 often downward (retrogressive). 



3. Evolution does not necessarily involve all organs of 

 the plant equally in any particular period, and one organ 

 may be progressing at the same time that another is 

 retrograding. 



4. Hysterophytic retrogression of plants is persistent, 

 and the hysterophytic phylum does not afterward be- 

 come holophytic. 



