6 THE DISPERSAL OF SHELLS. 



stocked by special acts of creation ; curiously enough, 

 when a boy, I believe before reading any books, I was 

 much puzzled at seeing some living thing in a newly- 

 formed pool, and remember concluding that it must 

 have been created there ! Mr. Clement Reid^ who has 

 attended to this subject, has been in the habit of noting 

 from time to time the species occurring in artificial 

 ponds and other isolated waters ; by so doing, as he 

 puts it, " one can see the accumulated results of many 

 years' dispersal, and can get some idea as to the extent 

 to which it must be going on." As a rule, he tells me, 

 such waters only contain Limnccoi (generally L. peregrd) 

 but the species miricularia^ truncahda^ stagnalis, and 

 palustris also occur, associated, sometimes, with the 

 smaller Planorbes (generally spirorbis and vortex)^ and 

 Physa fontinalis. The presence of bivalves, he adds, 

 " is quite exceptional ; the smaller Pisidia, especially 

 P, pusillmn^ occasionally occur, and I have once found 

 SpkcBrziim corneuuL^ but never Unio or Anodon ;" oper- 

 culate pond-snails are almost invariably absent. More 

 recently, this observer has published an able paper on 

 the " Natural History of Isolated Ponds," based, for the 

 most part, upon observations made on the South 

 Downs. ^ These undulating chalk hills, it is stated, 

 constitute a pre-eminently dry district in which, in order 

 to provide water for the cattle, ponds have to be dug at 

 comparatively short distances from each other. When 

 rendered impervious by puddling with clay or chalk- 



^ Clement Reid, Trans. Norfolk and Norwich Nat. Soc, v. 

 (1892), 272. 



