43^ ELEMENTARY ZOOLOGY 



lions only in Asia and Africa. On the other hand there 

 are no opossums in Europe or grizzly bears or rattlesnakes 

 anywhere else in the world than in this country. That 

 is, certain kinds of animals have a certain limited range of 

 occurrence or distribution. It is obvious, too, that certain 

 animals live only on land, while others live only in water, 

 and of these latter some are restricted to the ocean, while 

 others live only in fresh water. All of the facts regard- 

 ing the dispersion or diffusion of animals on land and in 

 water make up the science of the geographical distribution 

 of animals, or, as it is sometimes called, zoogeography. 

 Under this subject are included not only the facts of the 

 present actual distribution or occurrence of animals over 

 the world, but the facts concerning the reasons for this 

 distribution, the modes of travel and dispersion, the char- 

 acter and influence of barriers to the spread, and the 

 results, in the adaptation of old forms and the production 

 of new forms, of the phenomena of distribution. 



Just as maps are made to show graphically the facts of 

 political geography, which concerns the position and 

 extent of the various powers and States which claim 

 the allegiance of the people, and the facts of physical 

 geography, which concerns the physical character of the 

 earth's surface, so maps are made to show the geograph- 

 ical distribution of animals. Because of the great numbers 

 of animal species no one map can show the distribution 

 of all species, but a series of maps of the world or of a 

 continent or of a State or county or more limited region 

 could be made (and many such have been made) showing 

 the distribution of selected species. In a map of a limited 

 locality, say of a few square miles, the occurrence and 

 distribution of most of the commoner and more familiar 

 animals can be shown, and each high school should 

 possess such a map (see technical note at beginning of 

 this chapter). 



