CHAPTER II. 



MEANS OF DISPERSAL. 



The manner in which fresh-water shells are distributed 

 over the globe, the wide ranges enjoyed by many, and 

 their local diffusion in isolated waters, as we have seen, 

 clearly imply the existence of means of dispersal. The 

 creatures must certainly have been carried from pool to 

 pool, from river-basin to river-basin, and from mainland 

 to island, even, indeed, to some of the most isolated 

 archipelagoes of the open ocean. 



Objections to suggested means have sometimes been 

 raised on the ground that the creatures would probably 

 be unable to withstand the exposure to which they 

 would be subjected, but, as is pretty generally known, 

 many kinds, at least, are able to live for a time out of 

 water, and under the most adverse conditions. Of a 

 number of pond-snails [Limncea truncatuld), for instance, 

 placed by Professor A. P. Thomas in an open vessel, in 

 a dry laboratory, where the sunshine fell upon them for 

 an hour or so daily, rather more than fifty per cent, 

 survived for twenty-six days, and some few were alive 

 after more than six weeks.' A specimen oi Paludina 



^ A. P. Thomas, "Quart. Journ. Micro. Sci.," (n.s.),xxiii. (1883), 

 131. 



