TERTIARY HISTORY OF THE TENNESSEE RIVER 217 



into Central America. The embryos, in some cases, may be carried by aquatic 

 birds in the manner elsewhere mentioned in this paper; in others, they probably 

 migrate across overflowed regions near the sea, in time of floods. 



Farther on (p. 358), in accounting for the presence of Unio luteolus 

 in both the Missouri and Columbia Rivers, Mr. Simpson says: 



I have traced it up the Missouri River to near its source, and when it is taken 

 into consideration that the Marias, a tributary of this stream, heads within a few 

 miles of Flathead Lake on Clarke's River, a branch of the Columbia, and that the 

 embryos of Unios are provided with hooks by which they can attach themselves 

 to the feet or feathers of aquatic birds, it is very easy to see how this species might 

 have been carried from the waters of one drainage system to those of another. 



Mr. Simpson does not consider the possibility of such means of dis- 

 persal when discussing the Tennessee problem, the whole force of 

 his argument in that case lying in the statement: "These forms 

 [the Unios] cannot travel overland from river to river, but must have 

 water communication in order to pass from one stream to another." 

 It is believed, then, that the well-authenticated means of dispersal 

 of Mollusca, and more especially the facts of Unionidae distribution, 

 are such as to render the argument in favor of the theory of capture, 

 based on the evidence of the Unionidae, invalid; for while it is believed 

 that such a capture as has been proposed would result in the dis- 

 persal of the Pleurobema and other forms throughout the two basins, 

 it is equally believed that other means of dispersal will account for 

 all of the phenomena noted, some of which are inexplicable on the 

 theory of capture alone. 



SUMMARY OF EVIDENCE IN FAVOR OF THE THEORY OF CAPTURE 



To review our consideration of the four lines of evidence in favor 

 of the theory of a recent diversion of the Tennessee to its westward 

 course through the gorge in Walden Ridge: 



It has been shown that divides similar to that between the Tennes- 

 see and Coosa-Alabama basins exist in other regions at points where 

 no large stream is ever supposed to have held its course. A considera- 

 tion of the behavior of river-brought sediments under marine control, 

 and of the character of the Tertiary sediments in Alabama, is believed 

 to show the impossibility of properly basing any argument in favor 

 of the capture on the comparison of sediments now exposed with 



