540 THE AMERICAN JOURNAL OF SOCIOLOGY
i. e., determined by the action of internal social factors in correla- tion with all the external social factors. In a word, the theory of frontiers is everywhere and always sociological. Rivers, creeks, lakes, interior seas, oceans, mountains, deserts, and the ethnologi- cal characteristics of groups are only the elements. The frontier is a social phenomenon resulting from the combination of these elements, a social phenomenon representative at once of an equili- brium and a movement.
When the Code of Manu recommends to the king " to estab- lish himself in a city whose access is defended either by a desert, or by land, or by water, or by forest, or by soldiers, or by moun- tains, and to strive as much as possible to occupy a city protected by mountains and having a fortress," it is not a sociological theory that he advances. These words are practical counsels which he gives in regard to the existing social state. The state was then represented by a military theocracy in which the military caste was subordinated to the sacerdotal power within the principalities and distinct kingdoms. Buddhism coincided, on the contrary, with the period of the political unification of India, and with a corresponding tendency to a greater social equality. It followed the lowering and leveling of the political frontiers as well as of those existing between classes.
The Aryans, setting out from regions whose locality is still a matter of doubt, constituted nomadic hordes at a time when Egypt, Chaldea, Assyria, and China already represented great societies. The Aryan nomadic tribes of hunters and pastoral people slowly extended their sway by successive invasions in two directions. First, at the east in the basins of the Indus and Ganges, whence they dominated the aboriginal tribes of India by penetrating more and more to the interior of the peninsula. Second, at the west they scattered conformably to the natural roads which connected the Asiatico-European continent. These lines in general corresponded to a uniform distribution of cli- mates, natural products, and the chief conditions of life. They were the easiest and most advantageous routes for emigrants, and required the least effort for adapting the people to the new environments. The Aryans established themselves in this man-
