144 THE DISPERSAL OF SHELLS. 



of that Species are sometimes carried into the streams by 

 rains and floods, and are borne away on the waters to- 

 zvards the Pacific Coast. Occasionally some of the 

 specimens must find or make a lodgment along the 

 banks of the streams, and if the conditions are favourable 

 a colony will spring up and perhaps spread over the 

 neighbourhood. The banks of the Columbia between 

 the Dalles and the mouth of Snake River, a distance of 

 one hundred and fifty miles, are destitute of timber, and 

 are covered for several miles back with loose drifting 

 sand, quite unfavourable to the existence and spread of 

 land -shells. The locality where I found the variety cas- 

 taneus zvas on the bank of the Columbia near Celilo^ about 

 fifteen miles above the Dalles, on the cast side of the Cas- 

 cades, but on the ivest side of the Blue Mountains. This 

 colony must have sprung from specimens brougJit down the 

 stream by floods. At a subseqicent visit it had dis- 

 appeared. . . . Very likely the original strigosa [the 

 original specimen, so named, was found on or near the 

 Pacific CoastJ may have come from some colony planted 

 in this way." 



Obviously, as was remarked by the Rev. H. H. 

 Higgins, animals can be carried by means of rivers to 

 lower localities only ; it is clear, of course, that lands 

 bordering on the higher waters of a river can never be 

 thus reached : ^ it is almost equally clear, however, as 

 the elder Binney observed, that ** a species having by 

 its own powers attained the summit of a range of moun- 



' H. H. Higgins, " Proc. Lit. and Philos. Soc, Liverpool," xxxvi. 

 (1882), pp. xliv.-xlv. 



