sources of food, but no statistics are available to show adequately
their yield. Few of them are in private possession and they do not
appear at all in a total of "capitals," yet they are more important to
the nation than a large part of the land area. They are only beginning
to be developed artificially by the propagation of oysters, clams, and
fish. The development of a proper policy in this matter is one of our
economic problems.

There were in 1910 (mostly on farms) about 64,000,000 beef and dairy
cattle, 60,000,000 swine, 56,000,000 sheep and goats, and there were
raised in the one year nearly 500,000,000 fowls of all kinds.

§ 9. #The sources of heat, light, and power#. The law of the
conservation of energy expresses the fundamental likeness of heat,
light, and power. The principal sources from which man derives these
agencies are coal and falling waters, tho wood is of importance as
fuel in some localities. About 500,000 square miles of land (about 13
per cent of the area of the country) are underlaid with coal. These
deposits are widely distributed, so that nearly every part of the
country is within 500 miles of a mine. The enormous deposits if used
at the present amounts per year would last probably 2,000 to 4,000
years, but if used at the present increasing rate (doubling the
product every ten years) they would, it has been estimated, last but
150 years. What shall be the actual rate as between these extremes
is a question whose answer depends on our economic legislation as
to ownership, exploitation, prices, use, and substitution. This is
another of our important socio-economic problems.

The one great available substitute for coal as a source of heat and
light and power is water power. It is estimated that in 1908 but