v.] INTRODUCTION. IXXXI 



of marine currents will effect for tlieir countless race 

 and generations that which is denied to animals of 

 greater locomotive powers but of less number. A small 

 tribe of gigantic animals would be far more easily ex- 

 terminated than a host of puny shell-fish. When the 

 Mollusca have, in the course of ages, become thus spread 

 over a certain space, their further progress is arrested 

 by some geological convulsion or change. The land or 

 sea-bed, which they inhabited or roamed over, is either 

 suddenly or gradually covered with water or dried up ; 

 plains are raised and converted into mountains; trees 

 and succulent vegetation disappear; deserts become 

 swamps, and rivers estuaries ; the sea-shore sinks many 

 fathoms deep ; the climate of the land and the tempera- 

 ture of the sea are altered ; and conditions unfavourable 

 to molluscan life succeed. By some of these means 

 many species are entirely destroyed within the area 

 which is the scene of such a convulsion or change ; 

 others are reduced in number and dwindle away ; while 

 a few of a more hardy nature survive and continue to 

 flourish. Frequent alterations in the relative level of 

 sea and land, accompanying the alternate elevation or 

 depression of more or less extensive districts, will doubt- 

 less account in a great measure for the irregular distri- 

 bution of some species and groups of Mollusca. But 

 shell-fish do not " retire " or " retreat," as has been 

 conjectured by some naturalists. Their instinctive im- 

 pulse is to advance only. When aquatic mollusks sud- 

 denly and unwillingly find themselves on dry land, or 

 snails are immersed in a sea-bath for a long time, they 

 have no alternative but to die at their posts like brave 

 soldiers ; while their comrades are starved to death, owing 

 to the failure of the commissariat. 

 With respect to the distribution of the marine Mol- 



c?5 



