PRINCIPLES OF CLASSIFICATION. 



Before writing about plants at all it is necessary to give them 

 names; and as it is obviously impossible to have a different name 

 for every individual plant (as we have for human beings), it long 

 ago became customary to use the same name for all individuals 

 which appear essentially alike. Although no two trees are exactly 

 alike (as if cast in the same mold), there is not an infinite variety, 

 as there appears to be among clouds, pebbles on the seashore, etc. 

 Generally speaking, all trees (and other organisms) are grouped 

 into categories which we call species, and all the individuals of the 

 same species resemble each other more than they do those in any 

 other species (after making allow^ance for different stages of 

 growth, abnormalities, etc.) Within a species there are sometimes 

 minor groups called varieties or forms ; and the species are assem- 

 bled for convenience into larger groups which we call genera, the 

 genera into families, the families into orders, etc. For example, 

 the red oaks and white oaks belong to the same genus, the oaks 

 and chestnuts to the same family, and so on. 



Before the principles of taxonomy were well understood, it 

 was a common belief that all individuals of the same species were 

 descended from similar ancestors, and that all species were 

 created simultaneously at the beginning of time. According to this 

 view there could be no relationship between different species, and 

 genera, families, etc., were merely arbitrary groups. It is much 

 more logical, however, to assume that the degree of resemblance 

 indicates the degree of relationship, though direct proof is difficult 

 if not impossible. Whether this is true or not, it is no longer pos- 

 sible to maintain that species are fixed and definite. They are 

 simply categories, or pigeonholes, established for convenience, and 

 no two authorities agree exactly on the classification of a large 

 number of organisms, either as to species or genera. 



As the sum of scientific knowledge about the vegetable king- 

 dom increases from day to day, and is recorded in print, it is in- 

 evitable that more and more differences between plants previously 

 thought alike should be discovered, and the number of recognized 

 genera, species, varieties, etc. increased. Not only are unsuspected 

 differences of long standing continually brought to light, but the 

 plants themselves may change from one generation to another 



