X ATTEMPTS AT EXPLANATION 283 



probability that the whole river system of the earth has been 

 mixed up and reconstructed again and again, with a very 

 thorough blending of adjacent fauna. 



It is possible that the very uniform conditions under which 

 fresh-water Mollusca live may have sotnething to do with the 

 uniformity of their distribution and the comparative sameness 

 in their development. There can scarcely be any question that 

 the environments of fresh-water species are in themselves less 

 varied and less liable to fluctuation than those of species whose 

 home is the land. Water is very like water, all the world over ; 

 it may be running or motionless, warm or cold, clear or muddy, 

 but the general tendency is for it to be free from extremes of 

 any kind. Even if the surface water of a lake or river freezes, 

 or becomes unusually hot, there is generally plenty of water at a 

 lower stratum which maintains a less extreme temperature, and 

 to vrhich creatures can retire on the first symptoms of a change. 

 From this two results will follow. Not only will the inhabitants 

 of a piece of water not be inclined to vary much from the type, 

 since their whole surroundings, food, etc., continue very much 

 the same, but, if transported by any accident or cataclysm else- 

 where, they will be exceedingly likely to arrive at a place which 

 closely resembles their former home in all essentials. Thus 

 the tendency for new types to be formed would be constantly 

 checked, or rather would very seldom arise. 



Mr. Belt, while recognising the importance of changes of 

 level as affecting the distribution of fresh-water species, appears 

 to regard the operations of such changes from a rather different 

 point of view to that described above. " I think it probable," 

 he writes,^ " that the variation of fresh-water species of animals 

 and plants has been constantly checked by the want of continuity 

 of lakes and rivers in time and space. In the great oscillation 

 of the surface of the earth, of which geologists find so many 

 proofs, every fresh-water area has again and again been de- 

 stroyed. . . . 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." 



The terrestrial surface of the globe has been divided, as indi- 

 cating the facts of geographical distribution, into six regions — 

 1 The Naturalist in Nicaragua^ p. 334 f. 



