94 EVOLUTION OF LIVING GKGANISMS. 



each distinguished by some small constant variation in 

 character. Indeed, it is the universal experience of 

 naturalists engaged in the classification of quite modern 

 closely allied "species," that the great difficulty of the 

 work is due to the fact that it is usually scarcely possible 

 to find any character at all sufficiently conspicuous and 

 constant to distinguish them from each other. Sudden 



Speciess 



Species 



Order 

 FIG. 3. Diagram to illustrate the principles of classification. 



mutations and sharp distinctions would be welcomed by 

 all systematists. 



The only "fixed points" in a phylogenetic system of 

 classification are the points of bifurcation, where one 

 branch diverges from another (Fig. 3). It is here that 

 our divisions should be made ; and our generic, family, 

 and ordinal distinctions naturally come at each fork 

 farther and farther down the stem of the phylogenetic 

 tree. The phylogeny of organisms, however, is but 

 incompletely known ; and often the actual point of diver- 



