236 PRACTICAL ZOOLOGY 



zoological gardens with living wild animals, or museums with 

 exhibits of stuffed animals. Many of us have had a chance to 

 see a few different kinds of wild creatures in circuses and have 

 therefore some idea of their appearance. 



According to some authorities we should not study any ani- 

 mals that we cannot see in the laboratory, but while it is cer- 

 tainly true that we remember what we see better than what we 

 read about, still we can obtain by means of descriptions and pic- 

 tures rather accurate ideas of animals we have never seen on the 

 basis of what we know of familiar creatures. In the same way 

 does our knowledge of the lakes, rivers, and creeks, the hills and 

 the plains enable us to study intelligently similar forms in 

 Africa and other foreign lands which we have never seen. It is, 

 therefore, the plan of our discussion of the classes of verte- 

 brates to point out first the peculiarities in the structure of the 

 animals which adapt them to their surroundings, and then with 

 the aid of pictures to describe a few of the more important mem- 

 bers of each class. 



The activities of these different groups of vertebrates are often 

 very diverse, and the structures that adapt them to their differ- 

 ent habitats are quite varied ; nevertheless the plan of structure 

 is similar in all. The best example of the vertebrates that is 

 small enough to be used conveniently in the laboratory, and that 

 can be obtained for large classes without prohibitive labor and 

 expense, is undoubtedly the frog. Fish answer these require- 

 ments also, but their construction differs more widely from that 

 of man than does the anatomy of the frog. It is desirable that 

 we learn as much as possible about man from our study of the 

 lower animals, and the best method of beginning is to study the 

 most available vertebrate, comparing its structures and physio- 

 logical processes with those of human beings. The frog will 

 therefore be discussed quite fully in the succeeding chapter. 



The Body as a Machine. — The body of a vertebrate, such as 

 the frog or man, may be considered a sort of machine consisting 

 of many complex parts. There are, of course, many differences 



