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.