encement of the deposition of the sedimentary rocks; and then
succeeded one another, in such a manner, that totally distinct faunas
and florae occupied the whole surface of the earth, one after the other,
and during distinct epochs of time.

A geological formation is the sum of all the strata deposited over the
whole surface of the earth during one of these epochs: a geological
fauna or flora is the sum of all the species of animals or plants which
occupied the whole surface of the globe, during one of these epochs.

The population of the earth's surface was at first very similar in all
parts, and only from the middle of the Tertiary epoch onwards, began to
show a distinct distribution in zones.

The constitution of the original population, as well as the numerical
proportions of its members, indicates a warmer and, on the whole,
somewhat tropical climate, which remained tolerably equable throughout
the year. The subsequent distribution of living beings in zones is the
result of a gradual lowering of the general temperature, which first
began to be felt at the poles.

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 phaenomena; 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 t