INTRODUCTION ix 



(5) FISHES : Cold-blooded aquatic animals, which breathe 

 air through gills during their whole lives. 



The above five large classes, together with three others 

 consisting of small marine creatures, make up the Verte- 

 brate sub-kingdom of the great Animal kingdom. 



It might seem superfluous to go further and define 

 what an " animal " really is, were it not that there 

 occurs not infrequent misconception on the point. An 

 animal, then, is that which can be classed neither as a 

 vegetable nor as a mineral, and so the term includes 

 all birds, reptiles, fishes, and insects. 



Other descriptive terms used are explained in the 

 text. A few of the most common, however, are, for 

 convenience of reference, given below. 



ARBOREAL : Used to describe animals which live in, or 

 habitually frequent trees. 



NOCTURNAL : Used to describe animals which lie up 

 during the daytime and issue forth to hunt their 

 food, &c., at night. 



MIGRATORY : Used to describe animals which wander 

 regularly or occasionally from one region to another. 



PACHYDERMS : Thick-skinned animals, such as the 

 elephant, rhinoceros, and hippopotamus. 



UNGULATES : Hoofed animals. 



SPECIALIZATION : The development of an organ of the body 

 in some special direction. In whatever degree groups 

 or species of animals have developed peculiarities 

 of structure distinguishing them each from older 

 and more simply formed groups or species, so are they 

 described as being more or less highly specialized. 



ENVIRONMENT : The surroundings of the animal, and 

 the external conditions with which it constantly 

 comes into contact. 



BOOK III b 



