PRINCIPLES OF CLASSIFICATION 15 



having sprung in natural sequence, by slow processes of 

 change, from creatures of earlier geological periods. It is 

 said that in classification we have " three ancestral docu- 

 ments " : Morphology, Embryology, and Paleontology. In 

 Morphology we compare one form with another, thus 



a. Elephant. b. Coney. c. Rhinoceros. d. Horse. 



FIG. 3. Homology of digits of four odd-toed mammals, showing gradual reduction 

 in number and consolidation of bones above. After ROMANES. 



tracing resemblances and differences. In Embryology we 

 trace the development of individuals from the egg, thus 

 finding clues in heredity that will enable us to trace the 

 development of the race. In Paleontology we study the 

 extinct forms directly, thus often finding evidence \as to 

 the origin of forms now existing. 



17. Scientific names. Each of the different kinds of 

 animal or plant is called a species. There is no better 

 definition of species. Thus the red squirrel is a kind or 

 species of squirrel, the gray squirrel is another, the fox 

 squirrel a third. The black squirrel of the East is not a 

 species, because black squirrels and gray squirrels are some- 

 times found in the same nest, born from the same parents. 



A genus is a group of closely related species one or 

 more separated from other genera by tangible structural 

 characters. Thus all the squirrels named above constitute 



