IX GEOLOGICAL CONTEMPORANEITY 283 



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 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 hypo- 

 thesis 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 con- 

 trary, 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. 



