It is not now proposed to inquire whether these doctrines are true
or false; but to direct your attention to a much simpler though very
essential preliminary question--What is their logical basis? what are
the fundamental assumptions upon which they all logically depend? and
what is the evidence on which those fundamental propositions demand our
assent?

These assumptions are two: the first, that the commencement of the
geological record is coeval with the commencement of life on the
globe; the second, that geological contemporaneity is the same thing as
chronological synchrony. Without the first of these assumptions
there would of course be no ground for any statement respecting the
commencement of life; without the second, all the other statements
cited, every one of which implies a knowledge of the state of different
parts of the earth at one and the same time, will be no less devoid of
demonstration.

The first assumption obviously rests entirely on negative evidence. This
is, of course, the only evidence that ever can be available to prove the
commencement of any series of phenomena; but, at the same time, it must
be recollected that the value of negative evidence depends entirely on
the amount of positive corroboration it receives. If A B wishes to prove
an 'alibi', it is of no use for him to get a thousand witnesses simply
to swear that they did not see him in such and such a place, unless the
witnesses are prepared to prove that they must have seen him had he
been there. But the evidence that animal life commenced with the
Lingula-flags, 'e.g.', would seem to be exactly of this unsatisfactory
uncorroborated sort. The Cambrian witnesses simply swear they "haven't
seen anybody their way"; upon which the counsel for the other side
immediately puts in ten or twelve thousand feet of Devonian sandstones
to make oath they never saw a fish or a mollusk, though all the world
knows there were plenty in their time.

But then it is urged that, though the Devonian rocks in one part of the
world exhibit no fossils, in another they do, while the lower Cambrian
rocks nowhere exhibit fossils, and hence no living being could have
existed in their epoch.

To this there are two replies: the first, that the observational basis
of the assertion that the lowest rocks are nowhere fossiliferous is an
amazingly small one, seeing how very small an area, in comparison to
that of the whole world, has yet been fully searched; the second