July 27, 1900.] 



SCIENCE. 



135 



lasmus group of Unios, found abundantly in 

 the Chattalioocliee River, can hardly be 

 separated from forms of TJnio ohesus Lea, 

 found in the streams of the Atlantic drain- 

 age from North Carolina to Florida. Two 

 or three members of the BueMeyi group of 

 Unios seem to inhabit both the Chattahoo- 

 che Eiver and its branches, and the Savan- 

 nah River and nearby streams of the Atlan- 

 tic drainage. One member of this group, 

 Unio tortivus Lea, is common to certain 

 streams flowing into the Atlantic, a consid- 

 erable part of Florida, the Chattahoochee 

 River system and the Black Warrior River, 

 in Alabama. 



These remarkable facts of Unione distri- 

 bution led me long ago to believe that at a 

 former period, during the lifetime of some 

 of the present species of Unionidse, some- 

 time in the middle or later Tertiary, per- 

 haps, the Tennessee River must have flowed 

 southward into some one of the streams of 

 the Alabama drainage, and through this dis- 

 charged its waters into the Gulf of Mexico. 

 It seemed most likely that this connection 

 was by way of the Coosa on account of its 

 nearness to Tennessee, and because the 

 genus Pleurohema is more abundantly repre- 

 sented in that river than in the Cahawba or 

 Black Warrior. It seemed likely, too, that 

 during this or some nearby time there had 

 been for a limited period connection be- 

 tween the waters of the Tennessee and the 

 Chattahoochee system, either directly across 

 to the upper part of the latter, or in some 

 way by the Alabama system. I could ac- 

 count for the disti-ibution of these forms of 

 life in no other way, because they cannot 

 travel overland from river to river, but must 

 have water communication in order to pass 

 from one stream to another. 



I concluded that the connection of the 

 Tennessee with the Alabama drainage had 

 been severed permanently, certainly as far 

 back as the later Tertiary. That the Pleu- 

 robemas being somewhat susceptible to the 



influence of environment had changed un- 

 til new though closely allied groups had 

 been developed in the Alabama region since 

 the Tennessee began to flow into the Ohio ; 

 that other species of southern drainage 

 had developed from closely allied ances- 

 tors of northern origin. Others less sus- 

 ceptible to environmental influence had 

 only changed to new varieties in their new 

 location, while still others in which the char- 

 acters were firmly fixed only changed 

 slightly in appearance. 



Although it is possible that forms of the 

 Complanatiis and other groups of Unios 

 might have migrated from the Atlantic 

 along the low shores, of a former strait in 

 upper Florida connecting that ocean with 

 the Gulf of Mexico, and from thence up the 

 Chattahoochee River system, yet it would 

 seem more likely that these had passed fi'om 

 the Savannah to the Chattahoochee River 

 by water connection at or near the head of 

 these two streams which have their sources 

 very near together. 



In this brief sketch I have not gone ex- 

 haustively into the evidence presented by 

 the Unionidse. There are many other species 

 found in the Alabama River system which 

 are evidently identical or nearly related to 

 Tennessee River forms, but which have no 

 very close relationships with the species of 

 any other region and which are, most likely, 

 descendants of Tennessee forms. In fact it 

 is probable that nearly all the Unionidse of 

 the Alabama River system have been de- 

 rived from the Tennessee. 



This subject will be discussed to some ex- 

 tent in my forthcoming synopsis of the 

 Naiades. 



These conclusions almost exactly coincide 

 with those arrived at by Messrs. Hayes and 

 Campbell, who have made a very careful 

 and exhaustive study of the geomorphology 

 of the Southern Appalachians. And it is 

 indeed interesting that the geologist and 

 biologist, though working along entirely 



