48 Dwellers of the Sea and Shore 



of the salt water. It will be seen, therefore, that the 

 Littorinas are well on the road toward a land life. 

 They are, in fact, gradually substituting lungs for gills. 

 That the Littorinas first acquired the habit of living 

 out of the water through the influence of the tides is not 

 unreasonable to assume. And succeeding centuries 

 gave rise to individuals which were increasingly able to 

 withstand long exposure to the air. But, while this 

 may hold good in the case of this particular group 

 which is essentially marine, there would seem to be 

 some difficulty in accounting for those air-breathing 



littorina; a periwinkles shell. 



types which undoubtedly had their origin in fresh 

 water. And those types, by the way, have so far pro- 

 gressed that they must have been a long time on the 

 road; in fact, the first to start in the evolutionary pro- 

 cession. 



But the answer to this is forthcoming. It is pre- 

 sumed that the age of the ocean since the earth 

 assumed its present form is somewhat less than 100,- 

 000,000 years. Now, as is well known, the sea re- 

 ceives from the land in addition to other elements more 

 than 60,000,000 tons of sodium every year. This sub- 

 stance, dissolved out of the rocks by rains and carried 



