THE STUDY OF ANIMALS 13 



the placoid scales gradually become more and more tooth-like 

 as regards shape, all the intermediate stages being found. To 

 this it may be added that the mouth -cavity develops as a pit 

 or depression in the surface of the body, so that its lining is 

 really specialized skin, and might be expected to produce similar 

 structures to those characteristic of skin. Piecing together these 

 various facts we arrive at the conclusion — "Teeth have resulted 

 from the modification of scales belonging to the mouth -cavity", 

 a conclusion which is rendered still more probable when we 

 find that teeth are by no means limited to the edges of the 

 jaws, but may also be found (in some fishes, reptiles, &c.) on 

 the roof and other parts of the mouth -cavity. That teeth 

 should persist in animals like mammals, which have long since 

 lost the original scaly covering of remote ancestral forms, is ex- 

 plained by the principle of " change of function ", often exemplified 

 by organs which are out of work from having lost their original 

 job. This is a case of primarily defensive structures which have 

 been pressed into the service of the digestive system, though 

 they may also, in certain instances, again be modified for defence 

 and offence on new lines. 



4. The Physiologist's Stand-point. — Students of physiology are, 

 like naturalists, interested in the life -manifestations of animals, 

 but their enquiries are more subtle, and aim at determining the 

 uses or functions of the various parts revealed by anatomy and 

 histology. To determine the nature of life is the final aim of the 

 physiologist. The several departments of zoology are so intimately 

 related that they are briefly described here under separate headings 

 merely as a matter of convenience. This is particularly true of 

 physiology and morphology, which are so closely interwined that 

 it is impossible to separate them. The study of form would be 

 extremely dull and largely profitless without some knowledge of 

 function, and the study of function presupposes a preliminary 

 knowledge of form. For example, a far more vivid interest is 

 imparted to a description of the structure of the eye if the uses 

 of its various parts are kept in view, while any explanation of 

 vision would obviously be impossible were the related anatomical 

 facts ignored. A very good example is afforded by the tooth 

 problem, dealt with in the last section, where questions both of 

 morphology and physiology are involved. 



5. The Embryologist s Stand-point. — The study of form and 



