4 ZOOLOGY 



is one member of the cat-tribe, the Cheetah, or Hunting Leopard, 

 which differs from all its allies in having imperfectly retractile 

 claws and certain peculiarities in its teeth. It is therefore placed 

 in a distinct genus, Gyncdurus, to mark the fact that the differences 

 separating it from any species of Felis are of a more fundamental 

 character than those separating the species of Felis from one 

 another. 



The nearest allies of the Cats are the Hyaenas, but the presence 

 of additional teeth and of non-retractile claws — to mention only 

 two points — makes the interval between Hyfenas and the two 

 genera of Cats far greater than that between Felis and Cynalurus. 

 The varying degree of difference is expressed in classification by 

 placing the Hyanas in a separate family, the Hymnidm, while 

 Felis and Cyntelurus are placed together in the family FcKdm. 

 Similarly, the Civets and Mongooses form the family Viverridce ; 

 the Dogs, Wolves, Jackals, Foxes, &c., the family Canidm ; Bears, 

 the family Ursidce ; and so on. 



All the foregoing animals have sharp teeth adapted to a flesh 

 diet, and theii' toes are armed with claws. They therefore 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-eaters 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 

 Bodcntia ; pouched animals, such as Kangaroos and Opossums, the 

 order Marsvpialia ; and so on. 



Carnivora, Ungulata, Rodentia, Marsupialia, &c., 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 Avcs. In 

 the same way the scaly, cold-blooded Lizards, Snakes, Tortoises, &c., 

 form the class BejJtilia; the slimy-skinned, scaleless Frogs, Toads, and 

 Salamanders the class Am])]iibia ; 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 internal skeleton — 

 an important part of which is an axial rod or vertebral column — 

 and in never having more than two pairs of limbs. They thus 

 differ in some of the most fundamental features of their organisation 

 from such animals as Crabs, Insects, Scorpions, and Centipedes, 

 which have colourless blood, a jointed external skeleton, and 



