96 ECONOMIC PRINCIPLES

mankind which promise so low a return that they are beyond
the range of present possibilities. Saving will not have
exceeded its possibilities of usefulness till capital is available
 for all purposes of industrial expansion from which
may be anticipated a net yield more than sufficient to compensate
 for the risks of the investment, so long, that is to
say, as the rate of net interest is in excess of zero. The
attractiveness of expenditure on present satisfactions is
likely to grow as increased application of capital affords
cheaper means of present satisfaction, while decrease of
interest will diminish the inducement to accumulate for
future needs. Thus a check to saving is likely to be felt, as
interest rates fall, though that is not the same thing as
stating that the rate of addition to capital will decrease.
The net productiveness of industry may be so great that the
saveable fund increases fast, and a smaller proportion of it,
reserved for future needs, may mean a steadily increasing
total so reserved. Yet the extent of the field of profitable
investment of capital borrowed at low rates may be very
great, and, in spite of rapid additions to the mass of capital,
the fall in its earning power may be slow, especially having
regard to the opportunities afforded by extension of scientific
knowledge in new fields. The indications of the beginning
of the twentieth century point to a pause in the diminution
of the marginal earning power of capital, if not to a rise.
How long this check, or reversal, of previous tendencies may
endure, we make no attempt to guess.