2 ELEMENTARY ZOOLOGY 



our neighbor's dog is of another kind, a greyhound, it 

 may be. We know accordingly that there are different 

 kinds of tame dogs, and we may know that wolves 

 are so much like dogs that they might indeed be 

 called wild dogs, or dogs called a kind of tame wolf. 

 But how little we really know about the dog's body and 

 its life is apparent at a moment's thought. We see only 

 the outside of the dog, but what an intricate complex of 

 parts really composes this animal! We see it eat and 

 breathe and run ; of what is done with the food and air 

 inside its body, and of the series of muscle contractions 

 and mechanical processes which cause its running, we have 

 but the slightest conception. We see that the pup gets 

 larger, that is, grows ; that it changes gradually in appear- 

 ance, that is, develops ; but of the real processes and 

 changes that take place in growth and development how 

 little we know ! We know that there are other kinds of 

 dogs; that wolves and foxes are relatives of the dog; and 

 we have heard that cats and tigers are relatives also, 

 although more distant ones. We know, too, that all the 

 backboned animals, some of them very unlike dogs, are 

 believed to be related to each other, but of the thousands of 

 these animals and of their relationships our knowledge is 

 scanty. Finally, of the relations of the dog, and of other 

 animals, to the outside world, and of the wonderful man- 

 ner in which the dog's make-up and behavior fit it to live 

 in its place in the world under the conditions that surround 

 it, we have probably least knowledge of all. 



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 thejdnds of relatives it has, and 



