WILD LIFE OF ORCHARD AND FIELD 



been elevated from an altitude at which any mol- 

 lusk could probably have lived upon their sum- 

 mits to one that makes them a barrier to many 

 species. Such changes may have happened any- 

 where, again and again, and thus the two halves 

 of a community been divided. In succeeding cen- 

 turies the members of the parted sections may have 

 diverged in their development, until on this side of 

 a mountain range, or desert, or sea, we now find 

 one set of species, and on that side another set, which 

 belong to the same genera, and may in some cases 

 be proved, as well as surmised, to have had an iden- 

 tical origin. 



But the main explanation of their dispersion is 

 undoubtedly to be found in a much more recent 

 land connection between various islands of present 

 archipelagoes, and between these and the neigh- 

 boring mainlands. It has been pretty satisfacto- 

 rily demonstrated that during the glacial period 

 the oceans must have been drained of water repre- 

 senting a universal depth of one thousand feet, in 

 order to construct the enormously thick ice-caps 

 which covered the polar hemispheres. This would 

 expose a vast area of shallows, before and since 

 deeply submerged, across which snails might easily 

 migrate to other latitudes ; when, at the end of the 



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