374 BIOLOGY 



between the family and the genera there are sometimes recog- 

 nized what we call sub-families, between the classes and the 

 orders we find sub-classes, etc. 



THE SIGNIFICANCE OF CLASSIFICATION 



Why should there be a classification? As soon as we 

 recognize the principle of divergence from type it becomes evi- 

 dent that the classification of animals has a meaning. < 'l:i>sifi- 

 cation means history, and if we could get a perfect classification 

 we should have pictured the history of organisms. The first 

 step in the development of the organisms of the world was the 

 divergence of animals and plants from one another, thus form- 

 ing the two kingdoms of plants and animals. Then the process 

 was repeated in each kingdom, where there appeared a still 

 further divergence, a number of different lines of descent start- 

 ing from common centers, giving rise to the various sub-king- 

 doms. Again each of these broke up into other lines of descent, 

 and the smaller groups thus made their appearance. Thus 

 types continued breaking up and branching out in various 

 directions, giving rise to a classification which may be compared 

 to a tree, the trunk being the original type of organisms, the vari- 

 ous large branches representing the first lines of divergence from 

 the original stock, while the numerous subordinate branches 

 represent the successive types that appeared, by the same gen- 

 eral law. The minute twigs at the end of the branches are the 

 species of to-day, and they are aH connected by this line of 

 descent with the original trunk. 



The classification of animals is the attempt to reconstruct 

 this treelike arrangement of organisms according to their histori- 

 cal relationship. The members of the same species are supposed 

 to have had a common ancestor in a fairly recent period; the 

 different species of the same genus had a common ancestor a 

 little farther back in history; the different genera of the same 

 family had a still earlier common ancestor; the families of the 

 same order had their connecting point farther back still, and so 



