19I3-] ORTMAXX— THE ALLEGHEXIAX DIVIDE. 361 



sion, of rejuvenation, indicated by falls or rapids beginning some- 

 where in the lower parts, have had time to work back to the head- 

 waters (the cycle being completed), while on the western side these 

 cycles, at least some of them, are not quite finished, and are indi- 

 cated by falls and rapids lying at various distances below the head- 

 waters (see profiles, PL XIII., fig. 2, PL XIV., fig. 1). 



It does not require any further discussion to see that this dif- 

 ference of the eastern and western streams is finally to be referred 

 to the different general slope of the rivers, the former being short 

 and more direct in their course to the sea, and thus working faster. 



The consequence is, that the aquatic life of the lower sections of 

 the Atlantic streams finds congenial conditions up to near the head- 

 waters, since the conditions are more nearly uniform all along the 

 stream. Only close to the headwaters, there is a rather sudden 

 change, and here the fauna deteriorates also quite suddenly. 



Fact IL, 2, (a). 



We have seen that a differentiation of elements within the At- 

 lantic fauna is indicated, and that first of all, a southern element 

 is clearly distinguishable. A number of Najades belong here, the 

 snail Goniobasis virginica, and two crayfishes, Cambaras blandingi 

 and acuminatus (see p. 340). 



In all these forms it is evident that they have their center of 

 radiation somewhere in the southern section of the Atlantic slope 

 (Carolinas, Georgia), whence they migrated northward (see Simp- 

 son, i8g6b, p. 337). But we notice that the different forms have 

 advanced northward to different points. Some of them spread all 

 over the Atlantic slope, northward even beyond the section dis- 

 cussed here; so, for instance, Elliptio complanatus, Alasmidonta 

 undulata (possibly also Alasmidonta heterodon) , which go to New 

 England; Goniobasis virginica has reached the state of New York, 

 and Cambarus blandingi (restricted to the lowlands) has reached 

 middle New Jersey. 



Others do not go so far. Elliptio fisherianus, a lowland form, 

 goes northward to the lower Delaware; Elliptio productus to the 

 Potomac; Elliptio lanceolatus and Cambaras acuminatus to the 



