2 VARIETIES OF STATION chap. 



narrow our point of view to the Mollusca, the development of 

 hard shells, or exoskeletons, hence the sand-burrowing, rock- 

 boring, rock-clinging instincts of various genera and species.^ 



What was the primitive form of molluscan life is little likely 

 to be ever positively known, although, on grounds of comparative 

 anatomy, something approaching to the archi-mollusc is often 

 constructed, with more or less probability, by careful observers. 

 From one of the oldest known geological strata, the Cambrian, 

 nearly four hundred species of Mollusca are known, which include 

 representatives of nearly all the great Orders as they exist at the 

 present day, and without the slightest sign of approximation to 

 one another. With regard to the origin of the land and fresh- 

 water Mollusca some definite conclusions can be arrived at, 

 which will be given in their proper place. 



Scarcely any portion of the coast-line of the world is desti- 

 tute of molluscan life, except in regions where extreme cold 

 forbids its existence. Thus along the shores of Northern Asia 

 there is no proper littoral fauna, the constant influence of travel- 

 ling ice sweeping it all away; animal life begins at about 

 three fathoms. But in every coast region not positively hostile 

 to existence Mollusca make their home. Each description of 

 habitat has its own peculiar species, which there flourish best, 

 and exist precariously, if at all, elsewhere. Thus the sandy 

 waste of estuaries, the loose and shingly beaches, the slimy mud- 

 flats beset with mangroves, the low stretches of jagged rock, 

 and even the precipitous cliffs, from whose base the sea never 

 recedes, have all their own special inhabitants. The same is 

 true of the deep sea, and of the ocean surface. And when we 

 come to examine the land and fresh-water Mollusca, it is found 

 not merely that some Mollusca are terrestrial and others fluvia- 

 tile, but that certain species haunt the hills and others the 

 valleys, some the recesses of woods and others the open meadow 

 sides, some prefer the limestone rocks, others the sandy or clayey 

 districts, some live only in still or gently moving waters, while 

 others are never found except where the current is rapid and 

 powerful. 



It is within the tropics that the Mollusca become most nu- 

 merous, and assume their finest and quaintest forms. A tropical 

 beach, especially where there is a good tide-fall and considerable 



1 See especially Moseley, Nature, 1885, p. 417. 



