al 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, that the
argument is good for nothing unless the unfossiliferous rocks in question
were not only _contemporaneous_ in the geological sense, but
_synchronous_ in the chronological sense. To use the _alibi_ illustration
again. If a man wishes to prove he was in neither of two places, A and B,
on a given day, his witness