THE ARGUMENTS FROM DISTRIBUTION. 481 



out water; and those which show the power to change 

 their media temporarily or permanently, are in very 

 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 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 little 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 intervals; 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 alterations of 

 media? Besides many plants and many fixed animals, we 

 find moving animals of numerous kinds; 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 

 types, it will suffice to observe that each of the two great 

 sub-kingdoms, Mollusca and Arthropoda, supplies examples of 

 creatures having a wide excursiveness within this region. 

 We have gasteropods which, when the tide is down, habitu- 

 ally 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, and sometimes rambling beyond the reach of 

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

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

 which are mainly or wholly terrestrial. On the West Coast 

 of Ireland marine gasteropods are found on the rocks thre? 

 hundred feet above the sea, where they are only at long in- 

 tervals wetted by the spray; and though between gasteropod- 

 of this class and land-gasteropods the differences are con- 



