104 PRACTICAL ZOOLOGY 



number of large groups called phyla. The animals in each phy- 

 lum were subdivided into groups called classes ; the classes into 

 subgroups, the orders ; the orders into families ; the families into 

 genera; and the genera into species. Each species consists of 

 a group of closely similar individuals. By referring to animals 

 familiar to every one it will be possible to make the system of 

 classification used by scientists perfectly clear. 



HORSE AS AN EXAMPLE OF SPECIES. - - The common horse, 

 although represented by over sixty domesticated races, belongs 

 to a single species which is known to scientists as Equus caballus. 

 The term " caballus " is used only for the common horse, but 

 the term Equus is also employed when writing of certain near 

 relatives of the horse, such as the zebra, Equus zebra. 



Equus is known as the generic name, and the horse and zebra 

 are said to belong to the same genus. The horse and zebra, to- 

 gether with a number of other horselike animals, make up the 

 genus Equus. A genus may be defined as a group of similar 

 species. 



As in the case of the horse, every species is referred to in 

 scientific writings by two terms : the generic name comes first 

 and is written with a capital; the specific name second with a 

 small letter. These two terms are often followed by the name 

 of the scientist who first applied the name to any particular 

 animal. 



The genus Equus, the extinct genus Protohippus, and several 

 other genera are grouped together into one family, the horse 

 family EquidcE. 



This family and several others, including the family Tapirida, 

 which contains the tapirs, and the family Rhinocerotida, which 

 contains the rhinoceroses, belong to the order Perissodactyla. 

 All of the animals in this order have an odd number of toes, and 

 each toe bears a hoof. 



The order Perissodactyla belongs with the order Rodentia 

 (gnawing animals, like the rabbit and mouse), the order Carniv- 

 ora (flesh-eating animals, such as the cat and dog), the order 



