x.] GEOLOGICAL CONTEMPORANEITY. 183 



or are they a million of years younger or a million of years 

 older ? 



Is palaeontology able to succeed 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 &quot; Researches in 

 Theoretical Geology,&quot; 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 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 coinci 

 dence 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 faune is the doctrine 



