CLASSIFICATION 29 



different species resemble each other much closer than do 

 animals of two different families, and still fewer characters 

 are common to animals belonging to different orders. 

 Thus, the horse, Equus caba'lus, and the ass, Equus asi- 

 nuSj differ chiefly as to the tail and ears only. Both belong 

 to the order Ungulata and family Equidce. The cow, Bos 

 taurus, of the same order but of the family Bovidce, differs 

 much from the horse and ass in possessing shorter legs with 

 two toes, a pair of horns, and no upper incisor, or canine 

 teeth. The black bear, Ursus americanus, presents great 

 contrast in structure to the above animals and is therefore 

 placed in another order, Carnivora. It feeds on flesh, has 

 teeth adapted to cutting instead of grinding, and claws in 

 place of hoofs. 



Since a fish resembles the above animals in having a 

 back bone, a brain, and nerve cord on the dorsal side it is 

 placed in the same subkingdom, Vertebrata, but must be 

 assigned to another class, Pisces, on account of the absence 

 of hair, limbs, lungs, and other features common to the 

 mammals. 



