ANNIYEES,VRY ADDRESS. xlv 



De la Beche's remarkable * Kesearches in Theoretical Geology,' pub- 

 lished 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 sjTichrony of deposits, while absolute 

 diversity is no proof of difference of date. Sir Henry De la Beche 

 goes even fui'ther, 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 \Yns> 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 Jiypothesis 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 similaiity of their fauna? is the doctrine of the 

 contemporaneity of the European and of the Forth American Silu- 

 i-ians 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 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 tlie bottom, say, of St. George's Channel 

 with what may then remain of the Suffolk Crag, llcasoning 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 compara- 

 tively cloac together, may yet be separated by an amount of geolo- 



