6 MANUAL OF ZOOLOGY 



are placed together in the family Felida. Similarly the 

 civets and mongooses form the family Viverridce ; the dogs, 

 wolves, jackals, foxes, etc., the family Canidce; bears, the 

 family Ursidce; and so on. 



All the foregoing animals have sharp teeth adapted to a 

 flesh diet, and their toes are armed with claws. They there- 

 fore differ fundamentally from such animals as sheep, deer, 

 pigs, and horses, which have flat teeth adapted for grinding 

 vegetable food, and hoofed feet. The differences here are 

 obviously far greater than those between any two of the 

 families mentioned above, and are emphasised by placing 

 the flesh-eater in the order Carnivora, the hoofed animals 

 in the order Ungulata. In the same way gnawing animals, 

 such as rats, mice, and beavers, form the order Rodentia , 

 pouched animals, such as kangaroos and opossums, the 

 order Marsupialia ; and so on. 



Carnivora, Ungulata, Rodentia, Marsupialia, etc., although 

 differing from one another in many important respects, agree 

 in the possession of a hairy skin and in the fact that they all 

 suckle their young. They thus differ from birds, which have 

 a covering of feathers, and hatch their young from eggs. 

 The differences here are considerably more important than 

 those between the orders of quadrupeds referred to, and are 

 expressed by placing the latter in the class Mammalia, while 

 birds constitute the class Aves. In the same way the scaly, 

 cold-blooded lizards, snakes, tortoises, etc., form the class 

 Reptilia; the slimy-skinned, scaleless frogs, toads, and sala- 

 manders the class Amphibia ; and the finned, water-breathing 

 fishes the class Pisces. 



Mammals, birds, reptiles, amphibians, and fishes all agree 

 with one another in the possession of red blood and an inter- 

 nal skeleton — an important part of which is the backbone 

 or vertebral column — and in never having more than two 



