president's address. 51 



extensively the earth's surface features must have been changed 

 during the successive geological periods, it would at first sight e:eem 

 impossible that continuous lines of descent of such animals as these 

 could have been preserved through any considerable portion of them. 

 That is, those gill-bearing animals which may have occupied any 

 given river system could not effect their distribution, or even their 

 preservation, by migrating beyond its limits, in case their habitat 

 should be destroyed by movements of the earth's crust. They could 

 not pass over the land to any portions of other river systems, nor 

 could they pass through the sea to reach the moutlis of other rivers. 

 We have, however, very satisfactory evidence that a large part of 

 the living gill-bearing animals of North American fresh waters have 

 come down by unbroken genetic lines from some period at least as 

 remote as the close of the Cretaceous. 



Now, a continuity of these lines of descent necessarily implies a 

 continuity of their fresh-water habitat from the time of the origin 

 of those lines to the present time. This continuity again implies 

 the integrity of those river systems in which the mollusks originated, 

 from those early times to the present. This last proposition, as a 

 geological one, is comparatively new ; but the labors of Powell, 

 Gilbert, and Button have shown that rivers, in many instances at 

 least, have been among the most permanent of geological, as well 

 as geographical, features; that even the elevation of mountain 

 ranges across their course has not swerved them from their ground ; 

 but that they have cut their way through the ranges as fast as they 

 arose. 



A vast number of rivers, which have drained the land in past 

 geological times, have undoubtedly been destroyed by the submer- 

 gence of the land and other causes ; but I think we are justified in 

 the assumption that many of the streams which were established, 

 even as far back as the close of the Cretaceous period, are still 

 flowing as parts of existing river systems. In this way, a large part 

 of the gill-bearing faunas of the rivers of to-day have, by direct 

 lineal descent, and in unbroken habitats, been transmitted from 

 long past geological periods. 



