LAND SHELLS : THEIR TENACITY OF LIFE. 99 



variation has been constantly checked by the destruc- 

 tion, first in one part^ then in another, of all the fresh- 

 water species ; and on these places being again occupied 

 by fresh-water they would be colonized by forms from 

 other parts of the world. Thus species of restricted 

 range were always exposed to destruction because their 

 habitat was temporary and their retreat impossible, 

 and only families of wide distribution could be preserved. 

 Hence I believe it is that the types of fresh-water 

 productions are few and world-wide, whilst the sea has 

 molluscs innumerable, and the land great variety and 

 wealth of species. This variety is in the ratio of the 

 continuity of their habitats in time and space.^' ^ 



Before proceeding, in Chapter VI., to a consideration 

 of possible means for the dispersal of land-shells, I will 

 venture to append here a small collection of facts on 

 their tenacity of life. As is well known, some of these 

 creatures possess in a remarkable degree the power of 

 retaining vitality under conditions of the most adverse 

 kind, being able to live for long periods, even for years, 

 without either food or moisture ; and this faculty must 

 doubtless be of the greatest use to them in their 

 involuntary migrations, during which they must often 

 suffer great hardships ; and thus it seems likely that 

 they are fitted for transportal by means which can never 

 operate for frailer creatures. Even if they have had to 

 depend almost solely on voluntary migration, as some 

 writers seem to suppose, the ability to remain dormant 



^ T. Belt, "Naturalist in Nicaragua," ed. 2, 1888, pp. 334-5- 

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