INTRODUCTION. 9 



Classification. There are estimated to be upward of 

 250,000 species of animals now living on the surface of the 

 earth. How all these forms are related and how they dif- 

 fer comprises what is called the classification of animals, or 

 SYSTEMATIC ZOOLOGY. 



When Linnaeus, the father of natural history, undertook 

 to classify animals, he divided the animal kingdom into 

 classes, orders, genera, and species. Thus at present all ani- 

 mals, such as fishes, birds, or mammals, which have a back- 

 bone are placed together in the branch or sub-kingdom of 

 Vertebrates; those vertebrates, such as the cat, horse, or 

 cow, which suckle their young are placed in the class of 

 Mammals; those mammals which have claws and teeth 

 adapted for seizing and chewing flesh are carnivorous, i.e., 

 belong to the order of Carnivora. The order of Carnivora 

 is composed of a number of families, such as the cat family, 

 the dog family, etc. A family is composed of one or more 

 genera, and a genus is made up of species and varieties, the 

 latter being composed of individuals. 



Thus the principle of zoological classification consists in 

 placing animals which are alike by themselves into distinct 

 groups. The following table expresses the zoological posi- 

 tion of the cat: 



Kingdom of Animals; 



Sub-kingdom, or branch, Vertebrates; 

 Class, Mammalia; 

 Order, Carnivora; 

 Family, Felidae; 

 Genus, Felis; 



Species, Felis domesticus Linnaeus; 

 Variety, Felis domesticus Angorensis. 



The animal kingdom is divided into two series of 

 branches: those for the most part composed of a single cell 

 are represented by a single branch, the Protozoa (animal- 

 cules). Those animals whose bodies are formed of many 



