Passing from this simplest case of co-operation to a case not quite so simple--a case in which the homogeneity is incomplete--let us ask how a member of the group may be led without dissatisfaction to expend effort in achieving a benefit which, when achieved, is enjoyed exclusively by another? Clearly he may do this on condition that the other shall afterward expend a like effort, the beneficial result of which shall be similarly rendered up by him in return. This exchange of equivalents of effort is the form which social co-operation takes while yet there is little or no division of labor, save that between the sexes. For example, the Bodo and Dhimals "mutually assist each other for the nonce, as well in constructing their houses as in clearing their plots for cultivation." And this principle--I will help you if you will help me--common in simple communities where the occupations are alike in kind, and occasionally acted upon in more advanced communities, is one under which the relation between effort and benefit, no longer directly maintained, is maintained indirectly. For whereas when men's activities are carried on separately, or are joined in the way exemplified above, effort is immediately paid for by benefit, in this form of co-operation the benefit achieved by effort is exchanged for a like benefit to be afterward received when asked for. And in this case as in the preceding case, co-operation can be maintained only by fulfillment of the tacit agreements. For if they are habitually not fulfilled, there will commonly be refusal to give aid when asked; and each man will be left to do the best he can by himself. All those advantages to be gained by union of efforts in doing things that are beyond the powers of the single individual, will be unachievable. At the outset, then, fulfillment of contracts that are implied if not expressed, becomes a condition to social co-operation, and therefore to social development.