In Part II of the _Principles of Sociology_, it was shown that the fundamental principles of organization are the same for an individual organism and for a social organism; because both consist of mutually dependent parts. In the one case as in the other, the assumption of unlike activities by the component members, is possible only on condition that they severally benefit in due degrees by one another's activities. That we may the better see what are the implications in respect of social structures, let us first note the implications in respect of individual structures. The welfare of a living body implies an approximate equilibrium between waste and repair. If the activities involve an expenditure not made good by nutrition, dwindling follows. If the tissues are enabled to take up from the blood enriched by food, fit substances enough to replace those used up in efforts made, the weight may be maintained. And if the gain exceeds the loss, growth results. That which is true of the whole in its relations to the external world, is no less true of the parts in their relations to one another. Each organ, like the entire organism, is wasted by performing its function, and has to restore itself from the materials brought to it. If the quantity of materials furnished by the joint agency of the other organs is deficient, the particular organ dwindles. If they are sufficient, it can maintain its integrity. If they are in excess, it is enabled to increase. To say that this arrangement constitutes the physiological contract, is to use a metaphor which, though not true in aspect is true in essence. For the relations of structures are actually such that, by the help of a central regulative system, each organ is supplied with blood in proportion to the work it does. As was pointed out (_Principles of Sociology_, § 254) well-developed animals are so constituted that each muscle or viscus, when called into action, sends to the vaso-motor centers through certain nerve-fibers, an impulse caused by its action; whereupon, through other nerve-fibers, there comes an impulse causing dilatation of its blood-vessels. That is to say, all other parts of the organism, when they jointly require it to labor, forthwith begin to pay it in blood. During the ordinary state of physiological equilibrium, the loss and the gain balance, and the organ does not sensibly change. If the amount of its function is increased within such moderate limits that the local blood-vessels can bring adequately-increased supplies, the organ grows: beyond replacing its losses by its gains, it makes a profit on its extra transactions; so being enabled by extra structures to meet extra demands. But if the demands made on it become so great that the supply of materials cannot keep pace with the expenditure, either because the local blood-vessels are not large enough, or for any other reason, then the organ begins to decrease from excess of waste over repair: there sets in what is known as atrophy.