cceed where physical geology fails? Standard
writers on paleontology, as has been seen, assume that she can. They
take it for granted, that deposits containing similar organic remains
are synchronous--at any rate in a broad sense; and yet, those who will
study the eleventh and twelfth chapters of Sir Henry De La Beche's
remarkable 'Researches in Theoretical 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 suppos