CH. Ill] SYNCHRONOUS FAUNAS. 163 



In this case it would necessarily follow, on account of 

 the disparity of age, that there need be no similarity in 

 conditions between the different parts of the world during 

 the same geological period. Prof. Nicholson 1 , accepting 

 this criticism, observes, " Most of the facts bearing upon 

 this question may be elicited by a consideration of such a 

 widely extended and well-known formation as the Moun- 

 tain Limestone or Sub-Carboniferous Limestone. This 

 formation occurs in localities as remote from one another 

 as Europe, Central Asia, North America, South America 

 and Australia; and it is characterised by an assemblage 

 of well-marked fossils, amongst which Brachiopods be- 

 longing to the genus Productus may be specially singled 

 out. Now if we believe that the Carboniferous Limestone 

 in all the widely distant localities was strictly contempo- 

 raneous, we should be compelled to admit the existence 

 of an ocean embracing all these points, and in spite of its 

 enormous extent so uniform in temperature and depth 

 and the other conditions of marine life, that beings either 

 the same or very nearly the same inhabited it from end 

 to end." These two statements are ingeniously met by 

 Prof. Heilprin. They imply, he says, that every animal 

 has migrated in exactly the same way and direction. 

 And it is hardly conceivable, in fact impossible, that this 

 can have been everywhere the case. " Given the possible 

 equivalence in age, as is argued, of the Silurian fauna of 

 North America with the Devonian of the British Isles, 

 and the Carboniferous of Africa or any similar arrange- 

 ment, why has it never happened that when migration, 



1 Manual of Paleontology. 



112 



