518 ANNIVERSARY ADDRESS TO THE GEOLOGICAL SOCIETY 
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 con- 
ceivable extent in age. 
Edward Forbes was in the habit of asserting that the similarity 
of the organic contents of distant formations was primd 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 
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 faunze is the doctrine of the 
contemporaneity of the European and of the North American Silu- 
tians 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 4o 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 contempo- 
raneity. 
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 
