Now, since each of the organs has thus to be paid in nutriment for its services by the rest, it follows that the due balancing of their respective claims and payments is requisite, directly for the welfare of each organ, and indirectly for the welfare of the organism. For in a whole formed of mutually dependent parts, anything which prevents due performance of its duty by one part reacts injuriously on all the parts. With change of terms these statements and inferences hold of a society. That social division of labor which parallels in so many other respects the physiological division of labor, parallels it in this respect also. As was shown at large in the _Principles of Sociology_, Part II, each order of functionaries and each group of producers, severally performing some action or making some article not for direct satisfaction of their own needs but for satisfaction of the needs of fellow-citizens in general, otherwise occupied, can continue to do this only so long as the expenditures of effort and returns of profit are approximately equivalent. Social organs, like individual organs, remain stationary if there come to them normal proportions of the commodities produced by the society as a whole. If because the demands made on an industry or profession are unusually great, those engaged in it make excessive profits, more citizens flock to it and the social structure constituted by its members grows; while decrease of the demands and therefore of the profits, either leads its members to choose other careers or stops the accessions needful to replace those who die, and the structure dwindles. Thus is maintained that proportion among the powers of the component parts which is most conducive to the welfare of the whole. And now mark that the primary condition to achievement of this result is fulfillment of contract. If from the members of any part payment is frequently withheld, or falls short of the promised amount, then, through ruin of some and abandonment of the occupation by others, the part diminishes; and if it was before not more than competent to its duty, it now becomes incompetent, and the society suffers. Or if social needs throw on some part great increase of function, and the members of it are enabled to get for their services unusually high prices; fulfillment of the agreements to give them these high prices, is the only way of drawing to the part such additional number of members as will make it equal to the augmented demands. For citizens will not come to it if they find the high prices agreed upon are not paid.