=Zoology and its divisions.=--What things we do know about the dog, however, and about its relatives, and what things others know, can be classified into several groups, namely, things or facts about what the dog does, or its behavior, things about the make-up of its body, things about its growth and development, things about the kind of dog it is and the kinds of relatives it has, and things about its relations to the outer world, and its special fitness for life. All that is known of these different kinds of facts about the dog constitutes our knowledge of the dog and its life. All that is known by scientific men and others of these different kinds of facts about all the 500,000 or more kinds of living animals, constitutes our knowledge of animals and is the science _zoology_.[3] Names have been given to these different groups of facts about animals. The facts about the bodily make-up or structure of animals constitute that part of zoology called animal _anatomy_ or _morphology_; the facts about the things animals do, or the functions of animals, compose animal _physiology_; the facts about the development of animals from young to adult condition are the facts of animal _development_; the knowledge of the different kinds of animals and their relationships to each other is called _systematic_ zoology or animal _classification_; and finally the knowledge of the relations of animals to their external surroundings, including the inorganic world, plants and other animals, is called animal _ecology_. Any study of animals and their life, that is, of zoology, may include all or any of these parts of zoology. Most zoologists do, indeed, devote their principal attention to some one group of facts about animals and are accordingly spoken of as anatomists, or physiologists, systematists, and so on. But such a specialization of study should be made only after the zoologist has acquired a knowledge of the principal or fundamental facts in all the other branches of zoology.