ich order is what men of
science term a natural law. Whether such a law is to be regarded as an
expression of the mode of operation of natural forces, or whether it
is simply a statement of the manner in which a supernatural power has
thought fit to act, is a secondary question, so long as the existence of
the law and the possibility of its discovery by the human intellect are
granted. But he must be a half-hearted philosopher who, believing
in that possibility, and having watched the gigantic strides of the
biological sciences during the last twenty years, doubts that science
will sooner or later make this further step, so as to become possessed
of the law of evolution of organic forms--of the unvarying order of that
great chain of causes and effects of which all organic forms, ancient
and modern, are the links. And then, if ever, we shall be able to begin
to discuss, with profit, the questions respecting the commencement of
life, and the nature of the successive populations of the globe, which
so many seem to think are already answered.

The preceding arguments make no particular claim to novelty; indeed
they have been floating more or less distinctly before the minds of
geologists for the last thirty years; and if, at the present time,
it has seemed desirable to give them more definite and systematic
expression, it is because paleontology is every day assuming a greater
importance, and now requires to rest on a basis the firmness of which is
thoroughly well assured. Among its fundamental conceptions, there
must be no confusion between what is certain and what is more or less
probable. [2] But, pending the construction of a surer foundation than
paleontology now possesses, it may be instructive, assuming for the
nonce the general correctness of the ordinary hypothesis of geological
contemporaneity, to consider whether the deductions which are ordinarily
drawn from the whole body of paleontologic facts are justifiable.

The evidence on which such conclusions are based is of two kinds,
negative and positive. The value of negative evidence, in connection
with this inquiry, has been so fully and clearly discussed in an address
from the chair of this Society [3], which none of us have forgotten,
that nothing need at present be said about it; the more, as the
considerations which have been laid before you have certainly not tended
to increase your estimation of such evidence. It will be preferable to
turn to the positive f