But these diversities in natural conditions, not merely thus directly produce diversities in social development, but, by producing diversities in social development, bring out in man himself an obstacle, or rather an active counterforce, to improvement. As families and tribes are separated from each other, the social feeling ceases to operate between them, and differences arise in language, custom, tradition, religion--in short, in the whole social web which each community, however small or large, constantly spins. With these differences, prejudices grow, animosities spring up, contact easily produces quarrels, aggression begets aggression, and wrong kindles revenge.[45] And so between these separate social aggregates arises the feeling of Ishmael and the spirit of Cain, warfare becomes the chronic and seemingly natural relation of societies to each other, and the powers of men are expended in attack or defense, in mutual slaughter and mutual destruction of wealth, or in warlike preparations. How long this hostility persists, the protective tariffs and the standing armies of the civilized world to-day bear witness; how difficult it is to get over the idea that it is not theft to steal from a foreigner, the difficulty in procuring an international copyright act will show. Can we wonder at the perpetual hostilities of tribes and clans? Can we wonder that when each community was isolated from the others--when each, uninfluenced by the others, was spinning its separate web of social environment, which no individual can escape, that war should have been the rule and peace the exception? "They were even as we are." Now, warfare is the negation of association. The separation of men into diverse tribes, by increasing warfare, thus checks improvement; while in the localities where a large increase in numbers is possible without much separation; civilization gains the advantage of exemption from tribal war, even when the community as a whole is carrying on warfare beyond its borders. Thus, where the resistance of nature to the close association of men is slightest, the counterforce of warfare is likely at first to be least felt; and in the rich plains where civilization first begins, it may rise to a great height while scattered tribes are yet barbarous. And thus, when small, separated communities exist in a state of chronic warfare which forbids advance, the first step to their civilization is the advent of some conquering tribe or nation that unites these smaller communities into a larger one, in which internal peace is preserved. Where this power of peaceable association is broken up, either by external assaults or internal dissensions, the advance ceases and retrogression begins.