These great political groups, which, perhaps, are represented by family rituals, were essentially alike in language, custom and religion (although minor ritualistic differences probably obtained, as well as tribal preference for particular cults); while in all these respects, as well as in color and other racial peculiarities, the Aryans were distinguished from the dark-skinned aborigines, with whom, until the end of the Rig Vedic period, they were perpetually at war. At the close of this period the immigrant Aryans had reduced to slavery many of their unbelieving and barbarian enemies, and formally incorporated them into the state organization, where, as captives, slaves, or sons of slaves, the latter formed the "fourth caste." But while admitting these slaves into the body politic, the priestly Aryans debarred them from the religious congregation. Between the Aryans themselves there is in this period a loosely defined distinction of classes, but no system of caste is known before the close of the first Vedic Collection. Nevertheless, the emphasis in this statement lies strongly upon system, and it may not be quite idle to say at the outset that the general caste-distinctions not only are as old as the Indo-Iranian unity (among the Persians the same division of priest, warrior and husbandman obtains), but, in all probability, they are much older. For so long as there is a cult, even if it be of spirits and devils, there are priests; and if there are chieftains there is a nobility, such as one finds among the Teutons, nay, even among the American Indians, where also is known the inevitable division into priests, chiefs and commons, sometimes hereditary, sometimes not. There must have been, then, from the beginning of kingship and religious service, a division among the Aryans into royalty, priests, and people, i.e., whoever were not acting as priests or chieftains. When the people becomes agricultural, the difference tends to become permanent, and a caste system begins. Now, the Vedic Aryans appear in history at just the period when they are on the move southwards into India; but they are no irrupting host. The battles led the warriors on, but the folk, as a folk, moved slowly, not all abandoning the country which they had gained, but settling there, and sending onwards only a part of the people. There was no fixed line of demarcation between the classes. The king or another might act as his own priest--yet were there priestly families. The cow-boys might fight--yet were there those of the people that were especially 'kingsmen,' _r[=a]janyas_, and these were, already, practically a class, if not a caste[6]. These natural and necessary social divisions, which in early times were anything but rigid, soon formed inviolable groups, and then the caste system was complete. In the perfected legal scheme what was usage becomes duty. The warrior may not be a public priest; the priest may not serve as warrior or husbandman.