So much for the backward look. Let us next turn our gaze forward. Will the earth again become unsuitable for human habitation? Must man's occupation of it have an end as well as a beginning? The astronomers and physicists give an affirmative answer. Man's existence depends on a nice adjustment of temperature, water, and atmospheric elements. The earth may in time lose its atmosphere and moisture, just as the moon did long ago. The sun may cool off and withhold its life-giving warmth. The earth may lose its internal heat. How far off these conditions are is even more uncertain than the length of geological time; it may be several millions of years. Nor is it possible at present to foresee which of the necessary factors for man's existence on the earth will fail first. Meanwhile, glacial epochs will probably come and go; the physical world may be expected to change as much in the next million years as it has in the past. It might seem as if the most important question that can engage man's attention is this one of how long the earth will be suitable for his habitation.

But this question loses most of its practicality and becomes quite academic when we note what a short distance into the future even the most far-reaching plans extend. Tables for computing bond-values run up to one hundred years. Dealers say that they know of no bonds running longer than that, except the perpetual ones. A few leases and franchises have been granted for nine hundred and ninety-nine years. That seems to be about the limit of human foresight.

The rate of interest is a mathematical method of discounting the future, and one which can be carried to any length and to any desired degree of minuteness. High rate reflects short-sightedness and a low valuation of future goods; and conversely, low rate reflects far-sightedness and an estimate of future goods nearer to the value of the corresponding goods in the present. This method is regularly used by financiers to determine the present worth of annuities, bonds, mines, or investments U of any kind which yield definite returns for limited periods; used in a different way it determines the value of investments giving promise of perpetual income, such as waterpowers and other sites for location, agricultural lands, perpetual annuities, and shares of stock in stable companies. How the valuation of distant goods varies inversely as the rate of interest may be shown in tabular form thus:

The Value of an Investment Yielding $100 Annually

The first three lines of this table were taken from Skinner's The Mathematical Theory of Investment.

The distance into the future to which man's calculations extend is thus seen to be brief, indeed, compared even with historical time, and shrinks into insignificance in comparison with archaeological time. There would doubtless be longer foresight if there were more fore-knowledge. If it were known, for instance, that the northern ice cap would in a thousand years advance southward to its old limits, say to the present isotherm of 50 degrees F., the statesmen of Europe would be even more eager than they are now to control tropical lands.

As for the social world, it will not only change in response to changes in the physical, but it will also change from its own internal growth, that is, with the development of the social mind. It seems to be changing to-day more rapidly than ever before. Some features of the near future can be quite clearly seen.