Now, if the vital forces of nature which give an advantage to the element of time be the cause of interest, it would seem to follow that this maximum rate of interest would be determined by the strength of these forces and the extent to which they are engaged in production. But while the reproductive force of nature seems to vary enormously, as, for instance, between the salmon, which spawns thousands of eggs, and the whale, which brings forth a single calf at intervals of years; between the rabbit and the elephant, the thistle and the gigantic redwood, it appears from the way the natural balance is maintained that there is an equation between the reproductive and destructive forces of nature, which in effect brings the principle of increase to a uniform point. This natural balance man has within narrow limits the power to disturb, and by the modification of natural conditions may avail himself at will of the varying strength of the reproductive force in nature. But when he does so, there arises from the wide scope of his desires another principle which brings about in the increase of wealth a similar equation and balance to that which is effected in nature between the different forms of life. This equation exhibits itself through values. If, in a country adapted to both, I go to raising rabbits and you to raising horses, my rabbits may, until the natural limit is reached, increase faster than your horses. But my capital will not increase faster, for the effect of the varying rates of increase will be to lower the value of rabbits as compared with horses, and to increase the value of horses as compared with rabbits.