NO. 2.] SNAILS OF THE GENUS 10— ADAMS. 71 



3. AGENCIES AND MEANS OF DISPERSAL. 



The present geographic relations of lo, as have been indicated in the chapter on the history 

 of the Upper Tennessee drainage, can not be understood without considering the present condition 

 as a product of changes which have taken place in the past. In addition to the historical influ- 

 ences there are those of the present time which make possible the presence of the animal under 

 existing conditions, such as rapidly flowing water, certain algae, a firm substratum and a supply 

 of hme. The persistence of this kind of animal in this region, for perhaps millions of years, 

 impUes that in aU probability very similar conditions are of equal dm-ation. It therefore seems 

 very evident that where the time element is concerned there has been an abundance of it. 

 It is frequently stated that the antiquity of the fresh-water fauna is an important element in its 

 extensive range, but in this case we have an ancient group which is of relatively limited range, 

 and most of the different forms are also of very hmited range, in spite of the fact that they 

 frequent a habitat which would be expected to give wading and water birds many chances to 

 disperse them to other drainage areas. Personally I have felt that undue confidence in the 

 influence of the "accidental means" of dispersal, particularly of macroscopic aquatic animals 

 by birds, has favored the negligence of other more normal or usual methods of dispersal. 

 Thus a whole volume has been devoted to the dispersal of moUusks (as Kew, The Dispersal 

 of Shells. 1893) and with such httle consideration, that it amounts to neglect, of the 

 influence of the movements of the animals themselves or to the migration of their environ- 

 ment. But before turning to what might be considered the normal methods of dispersal of 

 lo mention should be made of the fact that these sheUs were transported by the aboriginal 

 Indians. I have found hundreds of these shells upon the sites of ancient Indian camps, 

 particularly upon the Chnch, Nolichucky, and along the Tennessee proper. Also the two shells 

 of turrita which form lot 200, were taken from an Indian mound near Chattanooga. There can 

 therefore be no question that the shells were transported, at least short distances. These shells 

 found about the camps are of the same kind as those hving in the near-by rivers, or when the 

 living snails were not found the dead ones possessed enough uidividuaHty to make it certain 

 that they had not been carried from some other stream, or were contradictory to the natural 

 probabilities of their occurrence. That the snails could survive such transportation seems likely 

 because lo wiH hve for several hours, or even a few days out of water, as I have received live 

 specimens at Chicago sent through the mail from Chnchport, Va. 



The locahties where transportation was most Hkely to occur are at the important portages, 

 and perhaps one of the most important of these was at Big Moccasia Gap, through Chnch Moun- 

 tain, at Gate City, Va., plate 59. This might make possible a transference between the upper 

 Clinch and Holston. There were probably other portages, but if their influence was large it 

 seems that these shells should also have been estabHshed in some streams outside of the Ten- 

 nessee. In view of these observations and inferences I conclude that the Indians were not an 

 important factor in the inter-stream dispersal of these shells. 



There are very few observations made or recorded which bear directly upon the means 

 of dispersal of lo. Its habit of hving on shoals isolates it and retards or prevents extensive 

 excursions. Near the headwaters of rivers, where lo flourishes, the rifiles are relatively close 

 together but vary much, depending upon local conditions; in places these are probably two or 

 three to a mile of the river, while farther downstream they are often a mile apart, and in the 

 lower reaches, where the stream has more nearly cut its bed to grade, the shoals or bars are 

 then often several miles apart, and the water on the shoals is relatively deep. 



There is a constant tendency for the rapidly flowing stream to wash loose the animals 

 and carry them into the pools below the shoals. This in the case of headwater streams is 

 probably of no serious disadvantage because of the shallow waters along whose margins and 

 bottoms the snail may be able to crawl back to the rapid water and upstream. But farther 

 downstream where the pools are deeper, muddier, and the distances to other shoals are greater, 

 dislodgment is much more serious and tends to isolate more completely the different colonies 

 of snails. The greater velocity of headwater streams must also increase the chances of carrying 

 shells from farther upstream downward, much more so than in the lower courses of the streams. 



