CHAPTER V
ANIMAL LIFE
A common ground to zoologists and geographers in the exploration of which they derive mutual pleasure from assisting each other, is the geographical distribution of animals. In this connection the fauna of North America presents perhaps even more interesting problems than does its flora.
GENERAL PRINCIPLES OF GEOGRAPHICAL DISTRIBUTION
In the study of the distribution of animals over a continent, the discovery of the laws determining the intangible boundaries which the members of a species may not pass is even more difficult than the similar task in the case of plants. Plant species for the most part advance and retreat slowly as conditions change, and, with minor exceptions, there is no freedom of movement for the individual; but animals, and especially the higher forms, are sensitive to even slight changes in their environment, and there is more or less individual freedom to travel over the land, to fly through the air, or to swim through the water. Why the members of a given species which have apparently unlimited power to travel should be confined to a certain and frequently a narrowly circumscribed area has excited the curiosity of man for many centuries.
A point is gained in reference to the distribution of animal species when it is remembered that animals are either directly or indirectly dependent on plants for food, and it follows that if plants, as we have seen, are so largely controlled in their distribution by climate, the secret of the distribution of animals is to be sought in the same direction. When a thoroughly satisfactory classification of
climatic provinces is arrived at, it will no doubt be found to agree with the larger features of plant distribution, and should coincide, although perhaps less definitely, with the major divisions into which the zoologist partitions the earth's surface. This principle has been recognised by C. Hart Merriam in subdividing the United States into "life-zones and crop-zones," and in the following pages his view will be discernible, although losing much of their clearness by reflection.
The Place of North America in the Life Realms of the Earth.—The geological distribution of animals has been critically studied by P. L. Sclater, A. R. Wallace, T. H. Huxley, and others, and the entire land area of the earth subdivided into realms, regions, etc., in such a manner as to indicate the present grouping of animals. One of the latest of these broad views of the life of the earth is presented by Richard Lydekker, who, from the evidence furnished by both living and extinct mammals, has divided the world into three great "realms," two of which are again subdivided into "regions," as follows: