4 THE DISPERSAL OF SHELLS. 



in connection with it seems completely cut off from 

 every other river-system of the same country, and still 

 more complete, of course, is the separation between the 

 fresh-waters of distinct islands and continents/ Some 

 of the univalves live much out of water, but, even with 

 these, powers of voluntary migration over land must be 

 very limited. I find, however, in Mr. Musson's manu- 

 script, a statement that a pond-snail (Linincea trimcatiild) 

 has been seen travelling across dry fields. Bivalves 

 seem practically incapable of migrating voluntarily 

 beyond the limits of their own element and its imme- 

 diate surroundings ; some kinds, as is well known, are 

 occasionally encountered in a living state, buried in 

 sand, or amongst moss, &c., at distances from water,^ 

 but it is unlikely that they intentionally proceed to 

 such spots.^ Notwithstanding these disabilities, how- 

 ever, we find a wide distribution characterizing many of 

 the creatures, both univalves and bivalves, together 

 also with most other freshwater productions. Some 

 species range very widely, and, as Mr. Darwin observes, 



1 Darwin, "Nature," xviii. (1878), p. 120, and see "Origin," 



p. 343- 



3 See, for instance, W. Thompson, "Ann. and Mag. Nat. Hist.," 



vi. (1841), p. 195; J. G. Jeffreys, "British Conchology," i. 



(1862), p. 17 ; L. Reeve, " Land and Fresh-water Mollusks," 1863, 



p. 236; G. Roberts, "Zoologist," (3), ix. (1885), p. 471; J. G. 



Milne, "Journ. of Conch.," vi. (1891), pp. 4I3-I4j 4i8 ; C. T. 



Simpson, "Nautilus," v. (1891), p. 16. 



^ One of the pea-shells {Pisidium pusillum)^ it is true, is said to 



have been seen creepmg in damp moss 6 — 20 paces from water ; 



D. Weinland, quoted in "Zoological Record," xiii. (1876), Moll, 



P- 57- 



