overy 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 palaeontology 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.[1] But,
pending the construction of a surer foundation than palaeontology 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 palaeontological facts are justifiable.

[Footnote 1: "Le plus grand service qu'on puisse rendre a la science est
d'y faire place nette avant d'y rien construire."--CUVIER.]

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,[2] 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 facts of palaeontology, and to inquire what they tell us.

[Footnote 2: Anniversary Address for 1851, _Quart. Journ. Geol. Soc._
vol. vii.]

We are all accustomed to speak of the number and the extent of the