The assimilation of culture within a boundary zone is in some respects the result of race amalgamation, as, for instance, in costume, religion, manners and language; but in economic points it is often the result of identical geographic influences to which both races are alike subjected. For example, scarcity of food on the arid plateau of Central Asia makes the Chinese of western Kansu eat butter and curds as freely as do the pastoral Mongols, though such a diet is obnoxious to the purely agricultural Chinese of the lowlands.[386] The English pioneer in the Trans-Allegheny wilderness shared with the Indians an environment of trackless forests and savage neighbors; he was forced to discard for a time many essentials of civilization, both material and moral. Despite a minimum of race intermixture, the men of the Cumberland and Kentucky settlements became assimilated to the life of the red man; they borrowed his scalping knife and tomahawk, adopted his method of ambush and extermination in war; like him they lived in great part by the chase, dressed in furs and buckskin, and wore the noiseless moccasin. Here the mere fact of geographical location on a remote frontier, and of almost complete isolation from the centers of English life on the Atlantic slope, and the further fact of persistent contact with a lower status of civilization, resulted in a temporary return to primitive methods of existence, till the settlements secured an increase of population adequate for higher industrial development and for defence. A race boundary involves almost inevitably a cultural boundary, often, too, a linguistic and religionary, occasionally a political boundary. The last three are subject to wide fluctuation, frequently overstepping all barriers of race and contrasted civilizations. Though one often accompanies another, it is necessary to distinguish the different kinds of boundaries and to estimate their relative importance in the history of a people or state. We may lay down the rule that the greater, more permanent, and deep-seated the contrasts on the two sides of a border, the greater is its significance; and that, on this basis, boundaries rank in importance, with few exceptions, in the following order: racial, cultural, linguistic, and political. The less marked the contrasts, in general, the more rapid and complete the process of assimilation in the belt of borderland. [Sidenote: The boundary zone in political expansion.]