I9I3.] ORTMANN— THE ALLEGHENIAN DIVIDE. 359 



There is no question that this is one of tlie factors, which has 

 largely brought about the more or less universal distribution of the 

 species of the Atlantic slope, and has permitted their spreading from 

 one river system into others, notwithstanding the contrary opinion 

 of Johnson (1905), who does not believe that "river captures" are 

 to be assumed in this region, but that passive transportation accounts 

 for the universal distribution of certain Najades over the Atlantic 

 slope. Indeed, it is not river capture in the strict sense, which 

 caused the present conditions, but what Adams (/. c.) calls " removal 

 of barriers " in a country approaching base-level. This is also prac- 

 tically the opinion of Simpson (1893, p. 354, footnote 2), when he 

 says, that shells may migrate from river to river " across overflowed 

 regions near the sea, in times of floods." (We always must bear 

 in mind that the migration was by the help of fish, which carried the 

 larvse.) 



This lowland zone reaches all the way up the coast to New York 

 state. But we know, that at certain times it extended even farther 

 north, when the continent stood at a higher elevation, and when the 

 coastal plain was wider than at present. We must also consider, 

 that at other times the coast was more submerged than now, and 

 that then also the Piedmont Plateau was more or less at base-level, 

 offering the same conditions favorable to a migration of the fauna. 



Moreover, we have seen, that there was stream-capture in the 

 region of the mountains, and that the northern rivers had a tend- 

 ency to encroach upon the southern. This should have caused a 

 migration of southern forms northward in the mountain region, but 

 not of northern forms southward. There is indeed evidence of it 

 in the fact, that forms with a northern center of dispersal (those 

 falling under II., 2, b) availed themselves, in their southern disper- 

 sal, of the coastal route, for instance, Lampsilis radiata, cariosa, 

 ochracea and Cambarus limosus, for they become more and more re-- 

 stricted to the lowlands in the southern parts of their range. On the 

 other hand, those forms, which have a more general distribution, 

 also in the mountain region, are chiefly southern in their origin, as 

 for instance : Elliptio complanatns, Alasmidonta undnlata, Gonio- 

 basis vivginica, and these may have availed themselves, in their 



