From these simple forms of co-operation in which the labors men carry on are of like kinds, let us turn to the more complex forms in which they carry on labors of unlike kinds. Where men mutually aid in building huts or felling trees, the number of days' work now given by one to another is readily balanced by an equal number of days' work afterward given by the other to him. And no estimation of the relative values of the labors being required, a definite understanding is little needed. But when division of labor arises--when there come transactions between one who makes weapons and another who dresses skins for clothing, or between a grower of roots and a catcher of fish--neither the relative amounts nor the relative qualities of their labors admit of easy measure; and with the multiplication of businesses, implying numerous kinds of skill and power, there ceases to be anything like manifest equivalence between either the bodily and mental efforts set against one another, or between their products. Hence the arrangement cannot now be taken for granted, as while the things exchanged are like in kind: it has to be stated. If A allows B to appropriate a product of his special skill, on condition that he is allowed to appropriate a different product of B's special skill, it results that as equivalence of the two products cannot be determined by direct comparison of their quantities and qualities, there must be a distinct understanding as to how much of the one may be taken in consideration of so much of the other. Only under voluntary agreement, then, no longer tacit and vague, but overt and definite, can co-operation be harmoniously carried on when division of labor becomes established. And as in the simplest co-operation, where like efforts are joined to secure a common good, the dissatisfaction caused in those who, having expended their labors do not get their shares of the good, prompts them to cease co-operating; as in the more advanced co-operation, achieved by exchanging equal labors of like kind expended at different times, aversion to co-operate is generated if the expected equivalent of labor is not rendered; so in this developed co-operation, the failure of either to surrender to the other that which was avowedly recognized as of like value with the labor or product given, tends to prevent co-operation by exciting discontent with its results. And evidently, while antagonisms thus caused impede the lives of the units, the life of the aggregate is endangered by diminished cohesion. § 53. Beyond these comparatively direct mischiefs, special and general, there have to be noted indirect mischiefs. As already implied by the reasoning in the last paragraph, not only social integration but also social differentiation, is hindered by breach of contract.