IN A SNAILERY. 29 
But the main explanation of their dispersion is undoubt- 
edly to be found in a land connection once existing be- 
tween the different islands of present archipelagoes, and 
between these and the neighboring main-lands. It has been 
pretty satisfactorily demonstrated that during the glacial 
period the oceans must have been drained of water repre- 
senting a universal depth of 1000 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 glacial period, the melted ice reclaimed the shallows, 
the snails would be left colonized upon the high points now 
widely separated by water. 
More casual circumstances have always contributed to 
this world-wide distribution. Snails frequently conceal 
themselves in crevices of bark, or firmly attach themselves 
to branches and foliage, and thus might be drifted long dis- 
tances, since they are able to resist starvation for an im- 
mense period, and protect themselves against injury from 
salt-water or excessive heat by means of opercula and epi- 
phragms; violent storms might frequently transport living 
shells a considerable distance, aquatic birds carry them or 
their eggs from pond to pond attached to feet or plumage. 
