The essential difference between the plant and animal organism is that the animal has the power of locomotion which enables it to gather nutriment from a wider environment, but, in addition to the power to move in space, the human animal has the ability to contrive and adapt the environment to his needs. In a word, the human community differs from the plant community in the two dominant characteristics of mobility and purpose, that is, in the power to select a habitat and in the ability to control or modify the conditions of the habitat. On first consideration this might seem to indicate that human ecology could have nothing in common with plant ecology where the processes of association and adjustment result from natural unmodifiable reactions, but closer examination and investigation make it obvious that human communities are not so much the products of artifact or design as many hero-worshipers suppose.[35] The human community has its inception in the traits of human nature and the needs of human beings. Man is a gregarious animal: he cannot live alone; he is relatively weak and needs not only the company of other human associates but shelter and protection from the elements as well. Brunhes says there are three essentials to the inception of the human community: the house, the road, and water.[36] Food may be transported more easily than shelter or water; the latter two therefore constitute, even under the most nomadic conditions, the essential elements in giving a location and a spatial fixity to human relations.[37] This is exemplified under our present régime of automobile tourist life, where water and shelter become the determining factors in the location of the camp. The size and stability of the human community is however a function of the food supply and of the rôle played in the wider ecological process of production and distribution of commodities. When man makes his living from hunting or fishing, the community is small and of but temporary duration; when agriculture becomes the chief source of sustenance, the community is still small but assumes a more permanent character; when trade and commerce develop, larger communities arise at points of break in conveyance, that is, at the mouths of rivers, junctions of streams, at waterfalls, and shallows where streams are forded. As new forms of transportation arise, new points of concentration occur and old points become accentuated or reduced. Again, as goods for trade are made in communities, still other points of concentration come into existence, determined largely by sources of power and raw material.[38] II. ECOLOGICAL CLASSIFICATION OF COMMUNITIES