teenth-century dogma that everything must be owned.

It is not hard to see how the Romans came to the distinction that has
obtained in the books ever since. Some things were part of the
Roman's _familia_, were used by him upon the public domain which he
occupied or were traded by him to those with whom he had legal power
of commercial intercourse. He acquired them by discovery, by capture
in war, by labor in agriculture or as an artisan, by commercial
transactions or by inheritance. For these things private actions lay.
Other things were no part of his or of anyone's household. They were
used for political or military or religious purposes or, like rivers,
were put to use by everyone without being consumed thereby. As to
these, the magisterial rather than the judicial power had to be
invoked. They were protected or use of them was regulated and secured
by interdicts. One could not acquire them so as to maintain a private
action for them. Thus some things could be acquired and conveyed and
some could not. In order to be valid, however, according to juristic
theory the distinction must lie in the nature of things, and it was
generalized accordingly.

In a time when large unoccupied areas were open to settlement and
abundant natural resources were waiting to be discovered and
developed, a theory of acquisition by discovery and appropriation of
_res nullius_, reserving a few things as _res extra commercium_, did
not involve serious difficulty. On the other hand, in a crowded world,
the theory of _res extra commercium_ comes to seem inconsistent with
private property and the theory of discovery and occupation to involve
waste of social resources. As to the latter, we may compare the law of
mining and of water rights on the public domain, which developed along
lines of discovery and reduction to possession under the conditions of
1849 and the federal legislation of 1866 and 1872, with recent
legislation proceeding on ideas of conservation of natural resources.
The former requires more consideration. For the argument that excludes
some things from private ownership may seem to apply more and more to
land and even to movables. Thus Herbert Spencer says, in explaining
_res communes_:

"If one individual interferes with the relations of another to the
natural media upon which the latter's life depends, he infringes the
like liberties of others by which his own are measured."

But if this is true of air and of light and of run