It has been remarked that the best evidence of the truth of a theory, is
its ability to refer to some general principle, the greatest number of
relevant phenomena, that, like the component masses of the chiselled
arch, they may mutually bind and strengthen each other. This we claim
to be the characteristic of this theory. At the outset it was not
intended to allude to more than was actually necessary to give an
outline of the theory, and to introduce the main question, yet
untouched. We have exhibited the stones of which the arch is composed;
but they may be pasteboard,--for the reader has not handled them. We
will now produce the keystone, and put it in its place. This he shall
handle and weigh. He will find it hard,--a block of granite, cut from
the quarry of observed facts, and far too heavy to be held in its place
by a mere pasteboard structure.