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.* 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.

[footnote] *"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*, which none of us have
forgotten, that nothing need at present be said about it; the more, as