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: { 1. Australian region. Notogaeic realm { 2. Polynesian region. { 3. Hawaiian region. { 4. Austro-Malayan region. Neogaeic realm Neotropical region. { 1. Malagasy region. { 2. Ethiopian region. Arctogaeic realm { 3. Oriental region. { 4. Holarctic region. { 5. Sonoran region.