The advent of the trolley line and more recently of the automobile has produced still further disturbing elements in the growth of human communities. Their effect has been chiefly to modify the life of the small town or village, causing the decline of some and the sudden growth of others. The introduction of these two forms of transportation, more particularly of the automobile, has been the most potent force in our recent American history in affecting redistribution of our population and in the disorganization of our rural and small-town institutions which grew up on the basis of a horse-and-vehicle type of mobility.[45] The evolution of new types of industry is another feature that becomes a determining factor in the redistribution of the country’s population. As we review our census reports we see the emergence each decade of one or more important industries; first, the textile industry causing concentrations of population in the eastern states, then the development of the iron and steel industry with its center of operations gradually shifting farther and farther west, and more recently the advent of the automobile and oil industries making for enormous concentration of population in certain states of the Union, also the motion picture industry with its concentrated center in southern California. The emergence of a new industry has a far-reaching effect in disturbing the _status quo_ of communal life. Competition soon forces the new industry to concentrate its productive enterprises in one or two communities; these communities then serve as great magnets drawing to themselves the appropriate population elements from communities far and near. IV. THE EFFECT OF ECOLOGICAL CHANGES ON THE SOCIAL ORGANIZATION OF COMMUNITY Population migrations resulting from such sudden pulls as are the outcomes of unusual forms of release in community growth may cause an expansion in the community’s development far beyond the natural culmination point of its cyclic development, resulting in a crisis situation, a sudden relapse, disorganization, or even panic. So-called “boom towns” are towns that have experienced herd movements of population beyond the natural point of culmination.