TERTIARY HISTORY OF THE TENNESSEE RIVER 213 



in the Tennessee River. It is not found throughout the other por- 

 tions of the Mississippi basin. But it is found abundantly in the 

 Coosa and Alabama Rivers. Also certain other forms of Unio common 

 to the Mississippi-Tennessee basin are found in the Coosa-Alabama 

 basin. From these facts Mr. Simpson concludes that at some time 

 the upper Tennessee River must have flowed southward into the Coosa- 

 Alabama River, since, to quote his words, "these forms cannot travel 

 overland from river to river, but must have water communication in 

 order to pass from one stream to another." 



Mr. Simpson's argument in favor of the theory of capture is based 

 on the assumption that the fresh-water mussels must have direct 

 water communication (evidently meaning direct fresh-water com- 

 munication) in order to pass from one stream to another. It appears 

 that the recorded observations of many naturalists, and the facts of 

 Unionidas distribution, are both contrary to this assumption. In 

 the first place, there are so many authentic cases where birds, insects, 

 etc., have been taken with fresh- water shells attached to them, that 

 students of the subject are compelled to believe in this method of 

 dispersion of these forms from place to place. Darwin proved that 

 young mollusks just hatching will attach themselves to the feet of a 

 duck, and remain alive in this position out of water from twelve to 

 twenty hours (p. 174). Mr. Arthur F. Gray, of Danversport, Mass., 

 had in his possession the foot of a waterfowl to which was attached 

 a bivalve shell. Canon Tristran shot a bird in the Sahara which 

 had attached to it the eggs of some mollusk. Some shells attach 

 themselves to plants which are carried away by birds (Darwin, p. 

 174). Insects are frequently taken with shells attached. There are 

 at least five recorded cases of the capture of the water scorpion, 

 Nepa, a large flying-bug, with small shells attached. The great 

 water beetle, Dytiscus, is known similarly to aid in the dispersion of 

 fresh-water Mollusca. The same is true of Dineutes. Mr. Albert 

 P. Morse, of Wellesley, has kindly shown me specimens of these last 

 two forms having attached shells. Notonecta has likewise been 

 proved to carry these forms from place to place. Some of these 

 insects are powerful flyers. Darwin records the capture of one of 

 them out at sea, 45 miles from the nearest land (p. 174). Beddard, 

 Kew, and other students of zoogeography regard birds and insects 



