=Geographical distribution.=--It is a matter of common knowledge with all of us that there are no wild lions or camels or kangaroos or monkeys or ostriches or nightingales in North America. Ostriches are found only in Africa and South America, kangaroos only in Australia, 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 regarding 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 character 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 geographical 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). =Laws of distribution.=--The laws governing the distribution of animals over the earth's surface have been recently[20] expressed in a simple statement as follows: Every species of animal is found in every part of the earth unless (_a_) its individuals have been unable to reach this region on account of barriers of some sort; or (_b_) having reached it, the species is unable to maintain itself, through lack of capacity for adaptation, through severity of competition with other forms, or through destructive conditions of environment; or (_c_) having entered and maintained itself it has become so altered in the process of adaptation as to become a species distinct from the original type.