THE ARGTJMENTS }?ROM DISTRIBUTION. 393 



many cases, of the kinds most liable to be thus deserted by 

 their medium. Let us consider what the sea- shore shows 

 us. Twice a-day the rise and the fall of the tide., 



covers and uncovers countless plants and animals, fixed and 

 moving ; and through the alternation of spring and neap 

 tides, it results that the exposure of the organisms living low 

 down on the beach, varies both in frequency and duration : 

 while some of them are left dry only once a fortnight for a 

 very short time, others a Kttle higher up, are left dry during 

 two or three hours at several ebb tides every fortnight. 

 Then by small gradations we come to such as, living at the 

 top of the beach;; are bathed by salt-water only at long in- 

 tervals ; and still higher to some which are but occasionally 

 splashed in stormy weather. What, now, do we find among 

 the organisms thus subject to various regular and irregular 

 alternations of media ? Besides many plants and many fixed 

 animals, we find numerous moving animals ; some of which 

 are confined to the lower zones of this littoral region, but others 

 of which wander over the whole of it. Omitting the humbler 

 animal forms, it will suffice to observe that each of the two 

 great sub-kingdoms, Mollusca and Articulata, supplies ex- 

 amples of creatures having a \\dde excursiveness within this 

 region. We have gasteropods which, when the tide is down, 

 habitually creep snail-like over sand and sea- weed, even up as 

 far as high- water mark. We have several kinds of crustaceans, 

 of which the crab is the most conspicuous, running about on 

 the wet beach, antl sometimes rambling beyond the reach of 

 the water. And then note the striking fact, that each of these 

 forms thus habituated to changes of media, is allied to forms 

 that are mainly or wholly terrestxial. On the West Coast of 

 Ireland, marine gasteropods are found on the rocks three hun- 

 dred feet above the sea, where they are only at long intervals 

 wetted by the spray ; and though between gasteropods of this 

 class and land- gasteropods the differences are considerable, yet 

 the land-gasteropods are more closely allied to them than to 

 any other Mollusca, Similarly, the two highest orders oi 

 26 



