350 ORTMANN— THE ALLEGHENIAN DIVIDE. [April i8, 



Chapter 6. 



Explanation of Distributional Facts. 



We are now ready to study the faunistic facts with regard to 

 their genesis, and shall take them up according to the classification 

 given above (Chapter 4, pp. 338-341). 



Fact I., i. 



The fact that the eastern and western faunas are sharply distinct, 

 and that the Allegheny system actually forms a sharp faunistic bar- 

 rier of the freshwater faunas, does not need any comment, for moun- 

 tain ranges generally are most apt to act as divides between rivers 

 and their faunas unless the elements of these faunas have excep- 

 tional means of dispersal (by transport). The very fact that the 

 western forms generally have not crossed the divide, nor have the 

 eastern forms, indicates that among three of the groups discussed 

 here (Najades, Pleuroceridse, Crayfishes) no such exceptional means 

 of dispersal have acted to any considerable degree. However, as 

 we shall see farther on, there are some exceptions. 



One point, however, deserves special mention. There have been 

 periods of general base-leveling, the last important one belonging to 

 the Cretaceous time. It is very likely that at this time the barrier 

 was not so well marked, and that a more general interchange of the 

 faunas was possible. If any cases in the present distribution are to 

 be traced back to this time, there are very few of them, and the 

 majority of the cases, chiefly of the Najadcs, does not show any 

 evidence of this. This means to say that probably the bulk of the 

 Najad-iauna of the Appalachian River systems is not older than the 

 Cretaceous time, probably largely Postcretaceous. 



This is an important conclusion in view of the fact that we know 

 from fossil remains that Na jades existed in North America in 

 Jurassic time and possibly even earlier. But it should be noted that 

 these fossils are known practically exclusively from the western 

 parts of the continent. This, however, cannot be followed up any 

 farther, since it would lead us too far away from our present purpose. 



While thus the western fauna could not cross the Alleghenian 



