The Moon Snail 47 



have learned to breathe In the open air. The garden 

 snail and the slug are outstanding examples. Once 

 upon a time these quaint creatures were possessed of 

 gills and lived in the water, like their relatives in the 

 pond and sea. But now they breathe with lungs, and 

 they are no more able to live under water, contin- 

 uously, than any other air-breathing animal. Ihe 

 question, therefore, at once arises as to how this 

 transformation was brought about. How, in the evo- 

 lutionary progress of these creatures, came they so 

 completely to change in their habits? 



The truth is, no one know^s. But men of learning 

 have given this question considerable thought. And 

 they have formed some very definite opinions. They 

 have, moreover, given us evidence that greatly supports 

 the apparent probability of their conclusions. One 

 aspect of this evidence is well worth pausing here to 

 ponder. At any rate, in so doing, it will enable us to 

 get an interesting viewpoint of this problem. 



This brings us again to the tides. As we have seen, 

 the periodic rise and fall of the waters leave many ani- 

 mals exposed for a greater or lesser duration of time. 

 Now there Is much variation between the capacities of 

 different shore animals to resist drying when exposed 

 by the retreating tides, but these capacities are invar- 

 iably in direct relationship to the positions the animals 

 occupy on the shore. Thus, certain of the periwinkles 

 {Littor'mid^) are found only high above the low-tide 

 line. Some (L. neritoides) live, in shady nooks just 

 above high-tide mark. But there are some tropical 

 members of this family (L. varia, L. fasciata, L. pul- 

 chra) that actually live in trees entirely out of reach 



