the excessive preference of present over future would lead to the using up and wearing out of wealth, to the present enjoyment of every possible resource, on the penalty of future misery. Evidently somewhere between these two extremes there must be, in each economy, a ratio of exchange between present and future, which in fact is the interest rate. This rate applied to utilities traces through each good a line analagous to the isothermal line on the map, marking off a zone of utilities for the present and other zones for each period of the future. There is thus a close relation between saving and the rate of time-discount. [Sidenote: The less necessary goods are the ones saved] [Illustration: Present VALUE line] Let us illustrate by the case of fruit stored in the cellar for future use. In the fall after the appetite for apples has been gratified up to a certain point, there still remains a large stock which affords less gratification if consumed at once than if kept for a time. Thus wood, food, and clothing are stored in the summer for the winter's need. Even the animals act on this principle. Squirrels, bees, and ants store up in the season of superfluity for the season of scarcity. The animals recognize with their feeble intelligence or by instinct, that a time will come when these consumption goods will represent greater importance to their welfare than they do at the moment. It results from the nature of wants and the principle of diminishing utility that in many cases some portion of a large supply of present goods must be worth less now than at a future time. This part, the marginal, less necessary part, will be left for a future time, and it is to this part that our opening proposition refers. This is roughly illustrated by the diagram. Things that cannot be kept, perishable goods, do not permit of this comparison. But if goods that can be kept continue to be used after utility has fallen down the scale, their high value for the future is cast away. Man lives not alone in the present but, in a far greater measure than do any animals, he lives in the future also. His economic life and his economic judgment comprehend a great number of periods at once. With the aid of memory and imagination he forecasts the future, and compares it with the present. The diminishing utility of goods, therefore, is modified by this fact that a thing has want-gratifying power at different periods. Before man uses goods for an inferior purpose he will ask whether, if they are kept for the future, they will not gratify a greater want. [Sidenote: The less valuable rise in value with the lapse of time]