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. 
