But presently man's superior intelligence came into play in such wise that other and better methods of getting food were devised. When in intervals of peace men learned to rear flocks and herds, and to till the ground, and when they had further learned to exchange with one another the products of their labour, a new step, of most profound significance, was taken. Tribes which had once learned how to do these things were not long in overcoming their neighbours, and flourishing at their expense, for agriculture allows a vastly greater population to live upon a given area, and in many ways it favours social compactness. An immense series of social changes was now begun. Whereas the only conceivable bond of political combination had heretofore been blood-relationship, a new basis was now furnished by territorial contiguity and by community of occupation. The supply of food was no longer strictly limited, for it could be indefinitely increased by peaceful industry; and moreover, in the free exchange of the products of labour, it ceased to be true that one man's interest was opposed to another's. Men did not at once recognize this fact, and indeed it has not yet become universally recognized, so long have men persisted in interpreting the conditions of industrial life in accordance with the immemorial traditions of the time when the means of subsistence were strictly limited, so that one man's success meant another's starvation.