462 The Living Plant 



Such are the five primary groups, sometimes called Classes, of 

 Plants, Each is divided into sub-groups called Orders, and those 

 again into others called Families, and those again into others 

 called Genera, and those into Species. It is theoretically possible 

 to follow out the branches of our genealogical tree through smaller 

 and smaller ramifications to the ultimate tips, representing the 

 species, of which there would be some two hundred and fifty 

 thousand ; and the construction of such a tree is the aim of every 

 student of classification. It is, however, no part of our present 

 business to follow this matter any farther, for, while the primary 

 groups are distinguished very largely by differences of habit, this 

 becomes less and less true with the groups that are smaller, and 

 hardly at all with the species, which are mostly marked off from 

 one another by characters having little connection with adapta- 

 tion. 



The Flowering Plants are the highest yet developed within the 

 Plant Kingdom. Are there then no higher possibilities in plant 

 evolution? So far as concerns any new field for them to expand 

 in, there seems to be none, unless they follow the example of man, 

 and take to free flight in the air. But the world is not yet finished, 

 nor are all the possibilities of variational experimentation ex- 

 hausted ; and until such times come, evolution is not likely to 

 cease. 



There remains one other aspect of classification to be men- 

 tioned before this chapter can be finished. Although the large 

 genealogical groups we have been considering happen to be dis- 

 tinguished pretty well from one another in habit, and thus con- 

 stitute also ecological groups, the correspondence between gen- 

 ealogy and ecology is by no means exact. Examples, indeed, of 

 the ecological intrusion of the genealogical groups into one an- 

 other have been given in the preceding pages; and further study 

 only serves to increase the number of such cases. Every group 

 is striving to expand to its utmost, and whenever it can find an 

 unoccupied crevice in the territory of another, it is not deterred 



