ermediate spot, and the
chances against exact coincidence of migration and of imbedding are
infinite.

In point of fact, however, whether the hypothesis of single or of
multiple specific centres be adopted, similarity of organic contents
cannot possibly afford any proof of the synchrony of the deposits which
contain them; on the contrary, it is demonstrably compatible with the
lapse of the most prodigious intervals of time, and with interposition
of vast changes in the organic and inorganic worlds, between the epochs
in which such deposits were formed.

On what amount of similarity of their faunae is the doctrine of the
contemporaneity of the European and of the North American Silurians
based? In the last edition of Sir Charles Lyell's "Elementary Geology"
it is stated, on the authority of a former President of this Society,
the late Daniel Sharpe, that between 30 and 40 per cent. of the species
of Silurian Mollusca are common to both sides of the Atlantic. By way of
due allowance for further discovery, let us double the lesser number and
suppose that 60 per cent. of the species are common to the North
American and the British Silurians. Sixty per cent. of species in common
is, then, proof of contemporaneity.

Now suppose that, a million or two of years hence, when Britain has made
another dip beneath the sea and has come up again, some geologist
applies this doctrine, in comparing the strata laid bare by the upheaval
of the bottom, say, of St. George's Channel with what may then remain of
the Suffolk Crag. Reasoning in the same way, he will at once decide the
Suffolk Crag and the St. George's Channel beds to be contemporaneous;
although we happen to know that a vast period (even in the geological
sense) of time, and physical changes of almost unprecedented extent,
separate the two.

But if it be a demonstrable fact that strata containing more than 60 or
70 per cent. of species of Mollusca in common, and comparatively close
together, may yet be separated by an amount of geological time
sufficient to allow of some of the greatest physical changes the world
has seen, what becomes of that sort of contemporaneity the sole evidence
of which is a similarity of facies, or the identity of half a dozen
species, or of a good many genera?

And yet there is no better evidence for the contemporaneity assumed by
all who adopt the hypotheses of universal faunae and florae, of a
universally uniform climate, and of a sensible cool