Now the interchangeability of wealth necessarily involves an average between all the species of wealth of any special advantage which accrues from the possession of any particular species, for no one would keep capital in one form when it could be changed into a more advantageous form. No one, for instance, would grind wheat into flour and keep it on hand for the convenience of those who desire from time to time to exchange wheat or its equivalent for flour, unless he could by such exchange secure an increase equal to that which, all things considered, he could secure by planting his wheat. No one, if he could keep them, would exchange a flock of sheep now for their net weight in mutton to be returned next year; for by keeping the sheep he would not only have the same amount of mutton next year, but also the lambs and the fleeces. No one would dig an irrigating ditch, unless those who by its aid are enabled to utilize the reproductive forces of nature would give him such a portion of the increase they receive as to make his capital yield him as much as theirs. And so, in any circle of exchange, the power of increase which the reproductive or vital force of nature gives to some species of capital must average with all; and he who lends, or uses in exchange, money, or planes, or bricks, or clothing, is not deprived of the power to obtain an increase, any more than if he had lent or put to a reproductive use so much capital in a form capable of increase. There is also in the utilization of the variations in the powers of nature and of man which is effected by exchange, an increase which somewhat resembles that produced by the vital forces of nature. In one place, for instance, a given amount of labor will secure 200 in vegetable food or 100 in animal food. In another place, these conditions are reversed, and the same amount of labor will produce 100 in vegetable food or 200 in animal. In the one place, the relative value of vegetable to animal food will be as two to one, and in the other as one to two; and, supposing equal amounts of each to be required, the same amount of labor will in either place secure 150 of both. But by devoting labor in the one place to the procurement of vegetable food, and in the other, to the procurement of animal food, and exchanging to the quantity required, the people of each place will be enabled by the given amount of labor to procure 200 of both, less the losses and expenses of exchange; so that in each place the produce which is taken from use and devoted to exchange brings back an increase. Thus Whittington’s cat, sent to a far country where cats are scarce and rats are plenty, returns in bales of goods and bags of gold.