INTRODUCTION 



dioxide expelled. Secretions for digestive and 

 other purposes must be elaborated, and poi- 

 sonous excretions discarded. Only the fittest 

 among each species sur\ave in the desperate 

 struggle for existence. 



Maintenance of the race 



The ability of an animal to maintain itself 

 in its habitat is not enough; it would soon 

 die out if others of its kind were not repro- 

 duced. As a matter of fact, the powers of 

 reproduction of animals are enormous; any 

 species would soon overrun the world if all 

 offspring were to grow to maturity and repro- 

 duce their kind. The struggle for existence, 

 due largely to limits in space and food sup- 

 ply, is responsible for the destruction of 

 most of the young that are brought into the 

 world each year. The number of each species 

 of animal is thus kept more or less constant 

 from year to year. Occasionally a species be- 

 comes extinct, such as the passenger pigeon 

 (Fig. 334), or unusually abundant, as the 

 lemming, but ordinarily a state approaching 

 equilibrium exists in nature with respect to 

 the number and character of the animals 

 present in any locality. 



SCIENTIFIC CLASSIFICATION 

 OF ANIMALS 



When a large number of dissimilar objects 

 are collected, it is natural to place them in 

 groups according to the presence or ab- 

 sence of certain characteristics. This is called 

 classification. The science of classification is 

 known as taxonomy. Animals may be classi- 

 fied in several ways. 



Artificial classification 



This groups animals according to some 

 superficial resemblance in structure, color, 

 habitat, etc. For example, certain animals 

 are called aquatic because they live in the 

 water; others are called terrestrial, because 

 they live on land; some are called carnivo- 



rous because they eat flesh; others are called 

 herbivorous because they live on vegetable 

 food; and still others are called omnivorous 

 because they devour both animal and vege- 

 table matter. This is called artificial classifi- 

 cation, and it is often convenient to use. 



Natural classification 



For all scientific work, natural classifica- 

 tion is employed. This is based on similarity 

 in structure, physiology, embr}'ology, and 

 other factors. Natural classification is based 

 on the principle of evolution and is an effort 

 to show true genetic relationships of ani- 

 mals. A number of large divisions of the 

 animal kingdom known as phyla are recog- 

 nized by zoologists. Each phylum is made up 

 of one or more classes, each class of one or 

 more orders, each order of families, each 

 family of genera, and each genus of species. 



A phylum is a wide group of animals hav- 

 ing some characteristics in common. A class 

 is a somewhat narrower group, composed of 

 individuals which have not only the struc- 

 tures peculiar to the phylum, but additional 

 common structural characteristics. An order 

 is a still smaller group in which the individ- 

 uals have the same phylum and class char- 

 acteristics, and, in addition, some common 

 characteristics peculiar to the order. Like- 

 wise, the family, genus, and species repre- 

 sent smaller and smaller groups of individ- 

 uals which possess the characteristics of the 

 larger groups, but, in addition, each has its 

 own identifying characteristics. 



The timber wolf, for example, belongs to 

 the species lupus of the genus Canis. This 

 genus and others, such as the genus Vulpes, 

 which contains the red fox, constitute the 

 family Canidae. The Canidae are included 

 with the bears (family Ursidae), the seals 

 (family Phocidae), and a number of other 

 groups of flesh-eating animals in the order 

 Carnivora. Nineteen related orders, of which 

 the Carnivora form one, are placed in the 

 class Mammalia. Mammals possess hair and 

 mammary glands; these characteristics dis- 

 tinguish them from the six other classes 



