CONTEMPORANEITY OF STRATA. 55 



munication would allow of a fresh immigration and the for- 

 mation of a fresh " colony," and the process might be indefi- 

 nitely repeated. . -Finally, however, we must suppose that the 

 Bohemian area was permanently thrown open to immigration 

 from the general European area, when the Upper Silurian 

 fauna of the latter would succeed in permanently and com- 

 pletely displacing the old Lower Silurian fauna of the former 

 region. The phenomenon, therefore, of " colonies " may be 

 denned as " the coexistence of two general faunas, which, con- 

 sidered in their entirety, are nevertheless distinct ; " and it is 

 to be regarded as merely a case of migration under certain 

 peculiar and exceptional circumstances. 



It must be borne in mind that the phenomenon of 

 " colonies," even if we regard it as established beyond con- 

 tradiction in the Bohemian area, has never been recognised 

 with any certainty elsewhere, and that it must, under any 

 circumstances, be of very rare and exceptional occurrence. 

 In fact, if " colonies " were at all common, it is quite clear 

 that we should before now have met with plenty of instances 

 of their existence, seeing that we have innumerable examples 

 in which we can show that the migration of marine organisms 

 from one area to another has taken place ; and it is further 

 clear that the occurrence of " colonies " as a common thing 

 would greatly lower the general value of paleeontological 

 evidence as bearing upon the age of fossiliferous strata. 





