April 14, 1905.] 



SCIENCE. 



591 



Pleurohema. If we find it impossible to hold 

 river-capture responsible for the distribution 

 of identical species in all these cases, then 

 mere similarity of forms in the Tennessee and 

 Coosa-Alabama basins can not be regarded as 

 proof of river-capture near Chattanooga. 



If we carry this line of argument to its 

 logical conclusion, the objections to it become 

 even more apparent. One group of Unios is 

 recorded as occurring everywhere in the 

 streams draining into the Atlantic from Lab- 

 rador to Georgia. If the occurrence of the 

 Pleurohema in the Tennessee and Coosa-Ala- 

 bama rivers proves river-capture in that case, 

 then the distribution just referred to must 

 prove a great succession of river-captui'es from 

 Labrador to Georgia. The same species is, in 

 one instance, found in Europe, northern Asia, 

 Japan, northern North America and Iceland. 

 According to the argument advanced this 

 means world-wide land connections and an 

 inconceivable series of river-captures. Such 

 violent hypotheses compel the conclusion that 

 some other means than river-capture is most 

 commonly operative in effecting the distribu- 

 tion of fresh-water shells. 



It is believed that all the phenomena noted 

 are easily explicable independently of the 

 theory of capture, and, in this connection, it 

 is well to note that the presence of a longi- 

 tudinal, open valley across the low divide be- 

 tween the two basins is peculiarly favorable 

 for the operation of some of the means of 

 dispersal referred to above. The northward 

 and southward migrations of birds along cer- 

 tain valleys is known, and where a low divide 

 in a prominent valley alone separates the 

 waters of two river systems it is to be ex- 

 pected that more or less mingling of forms 

 will very likely take place. 



In our consideration of this line of evidence 

 it is of interest to recall Mr. Simpson's state- 

 ments regarding the dispersal of these shells, 

 which appear in his paper on the ' Distribu- 

 tion of JSTorth American Unionidse,'* published 



* ' On the Relationships and Distributicn of the 

 North American Unionidse, with Notes on the 

 West Coast Species,' American Naturalist, XXVII., 

 353-358, 1893. 



seven years prior to his Tennessee paper. In 

 a footnote (p. 354) he observes : " In many 

 cases the UnionidEe seem to have had no diflfi- 

 culty in migrating across the country from 

 river to river; an example of this being the 

 Mississippi Valley species which inhabit all 

 the rivers of Texas, and some of those of 

 eastern Mexico; while on the other hand, 

 species of South America extend up 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 over- 

 flowed regions near the sea, in time of floods." 

 Further on (p. 358), in accounting for the 

 presence of Unio luteolus in both the Mis- 

 souri and Columbia rivers, Mr. Simpson says: 

 " I have traced it up the Missouri River to 

 near its source, and when it is taken into con- 

 sideration that the Marias, a tributary of this 

 stream, heads within a few miles of Flathead 

 Lake on Clarke's River, a branch of the Co- 

 lumbia, 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." 



It is believed, then, that the well-authen- 

 ticated means of dispersal of moUusca, and 

 more especially the facts of molluscan dis- 

 tribution, are such as to render it impossible 

 to regard the distribution of fresh-water 

 faunas as a conclusive evidence of drainage 

 modifications. We must place this line of 

 evidence in the second of the two classes out- 

 lined above — it being the result of river-cap- 

 ture in some cases, but being also produced as 

 a result of other agencies. Whether the Ten- 

 nessee River has suffered capture or not is a 

 question which can not be settled on the basis 

 of this class of evidence. Other sources must 

 be appealed to. And while the Tennessee 

 problem is a question in itself, it may not be 

 amiss, especially in view of the wide attention 

 attracted by the application of this line of 

 evidence to that problem, to say that I have 

 elsewhere pointed out certain objections to 

 the theory of recent capture, and called atten- 

 tion to a variety of evidence now available 



