=23. Zoogeography.= Since animals are free for the most part, and hence not confined so strictly to one spot as plants, their dependence upon the habitat is not so evident. The relation is further obscured by the fact that no physical factor has the direct effect upon them which water or light exerts upon the plant. Vegetation, indeed, as the source of food and protection, plays a more obvious, if not a more important part. This is especially true of anthophilous insects, but it also holds for all herbivorous animals, and, through them, for carnivorous ones. The animal ecology of a particular region can only be properly investigated after the habitats and plant formations have been carefully studied. Here, as in floristics, a great deal can be done in the way of listing the fauna, or studying the life habits of its species, without any knowledge of plant ecology; but an adequate study must be based upon a knowledge of the vegetation. Although animal formations are often poorly defined, there can be no doubt of their existence. Frequently they coincide with plant formations, and then have very definite limits. They exhibit both development and structure, and are subject to the laws of invasion, succession, zonation, and alternation, though these are not altogether similar to those known for plants, a fact readily explained by the motility of animals. Considered from the above point of view, zoogeography is a virgin field, and it promises great things to the student who approaches it with the proper training. =24. Sociology.= In its fundamental aspects, sociology is the ecology of a particular species of animal, and has in consequence, a similar close connection with plant ecology. The widespread migration of man and his social nature have resulted in the production of groups or communities which have much more in common with plant formations than do formations of other animals. The laws of association apply with especial force to the family, tribe, community, etc., while the laws of succession are essentially the same for both plants and man. At first thought it might seem that man’s ability to change his dwelling-place and to modify his environment exempts him in large measure from the influence of the habitat. The exemption, however, is only apparent, as the control exerted by climate, soil, and physiography is all but absolute, particularly when man’s dependence upon vegetation, both natural and cultural, is called to mind. THE ESSENTIALS OF A SYSTEM