54 GEOGRAPHICAL DISTEIBUTIOX. 



would be liable to be concealed in and transported away by floating 

 timbers. In this manner they could be drifted away for several hun- 

 dreds of miles, and, under exceptionally favourable circumstances, 

 to possibly one or two thousand, the more readily since some of 

 these animals possess an enormous amount of vital tenacity, even 

 under the most adverse conditions of existence. Thus, a Helix from 

 North Africa (H. desertorum), contained in the British Museum 

 collection, and glued on to a tablet, was found by the conservators 

 to be alive after a period of more than four years. A similar in- 

 stance of resuscitation, although after a less protracted period, has 

 been noted in the case of one of the tabulated snails of the Acade- 

 my of Natural Sciences of Philadelphia. Again, it has been con- 

 clusively shown by Darwin and others that the eggs of pond and 

 other fresh-water bivalve-mollusks are occasionally found attached to 

 the feet of wading-birds — ducks, and the like — visiting such waters, 

 and are by them liable to be carried to very considerable distances 

 from their true homes, and thereby to have their range almost 

 inimitably widened. Such a method of transport, although exer- 

 cised to a much more limited extent, has been observed to be effect- 

 ed even by species of water-beetle, whose legs may have become 

 entrapped between the valves of the shell, as well as by newts and 

 other amphibians. The broad distribution of allied or identical 

 generic and specific forms of fluviatile moUusks over the most ex- 

 tended or widely remote geographical areas receives a partial ex- 

 planation in the circumstance that the physical forces operating 

 upon the earth's crust, causing movements in it of a differential 

 character — i. e., elevation at one point and subsidence at another — 

 tend to destroy the permanency of river courses, turning them now 

 to one side, then to another, and ultimately, possibly, uniting the 

 basins of streams whose waters were at one time quite remote from 

 each other. With this union or coalescence of the waters there 

 will necessarily also be a union of the contained moUuscan faunas, 

 and, by a repetition of the process, a general transferrence may 

 in course of time be effected of the same or but barely modified 

 forms over the most distant portions of the earth's surface. Ex- 

 istence, under the new conditions of habitation, will be rendered 

 possible or materially facilitated by the comparatively slight al- 

 teration in its physical properties to which the watery medium 

 vrill in many or most cases be subjected. 



