62 INTRODUCTION. 



munication would allow of a fresh immigration and the formation of 

 a fresh " colony," and the process might be indefinitely repeated. 

 Finally, however, we must suppose that the Bohemian area was per- 

 manently thrown open to immigration from the general European 

 area, when the Silurian fauna of the latter would succeed in perma- 

 nently and completely displacing the old Ordovician fauna of the 

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

 denned as " the coexistence of two general faunas, which, considered 

 in their entirety, are nevertheless distinct ; " and it is to be regarded 

 as merely a case of migration under certain peculiar and exceptional 

 circumstances. 



Not only have the phenomena described by M. Barrande in con- 

 nection with his " colonies " never been recognised with any certainty 

 in any region outside Bohemia ; but there are strong grounds for 

 believing that the actual facts in this area will not bear the interpre- 

 tation which has been placed upon them by this distinguished palae- 

 ontologist. Thus, Mr J. E. Marr, after a careful examination of 

 the facts on the spot, was led to the conclusion that the so-called 

 " colonies " have no real existence, but are the result of disturbances 

 of the strata, being due, in reality, to the repeated " faulting down " 

 of a band of the Silurian rocks among the underlying Ordovician 

 deposits. This explanation seems to account adequately for the 

 observed phenomena, and relieves us of the necessity of accepting a 

 theory which can with difficulty be reconciled with the ascertained 

 laws of the distribution of animals in past time. 



