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.