heoretical Geology," published now nearly thirty years
ago, and will carry out the arguments there most luminously stated, to
their logical consequences, may very easily convince themselves that even
absolute identity of organic contents is no proof of the synchrony of
deposits, while absolute diversity is no proof of difference of date. Sir
Henry De La Beche goes even further, and adduces conclusive evidence to
show that the different parts of one and the same stratum, having a
similar composition throughout, containing the same organic remains, and
having similar beds above and below it, may yet differ to any conceivable
extent in age.

Edward Forbes was in the habit of asserting that the similarity of the
organic contents of distant formations was _prima facie_ evidence, not of
their similarity, but of their difference of age; and holding as he did
the doctrine of single specific centres, the conclusion was as legitimate
as any other; for the two districts must have been occupied by migration
from one of the two, or from an intermediate 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 the
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 s