92 THE DISPERSAL OF SHELLS. 



range very widely, some having species scattered at 

 random in various parts of the world/ and almost all 

 oceanic-islands (even those in which fresh-water shells 

 are very scarce or altogether absent) are inhabited by 

 land-snails, often in great plenty and variety. Many 

 of the genera and higher groups have limited ranges, 

 but it is notorious that some of the families and genera 

 are almost universally distributed. The family Helicidcc, 

 a group of immense extent, is described as absolutely 

 cosmopolitan in its range, being found in the most 

 barren deserts, and on the smallest islands, all over the 

 globe, and reaching to near the line of perpetual snow 

 on mountains, and to the limit of trees, or even con- 

 siderably beyond it, in the arctic regions : the genus 

 Helix also is said to be universal, having found its way 

 to every country and to almost every island in the 

 world ; several other genera with very wide ranges might 

 be named ; e.g., Pupa^ like Helix, inhabits each of the 

 six zoological regions.^ Mr. Wallace has maintained 

 that all the animals now inhabiting truly " oceanic 

 islands " must have reached them by crossing the ocean 

 or be the descendants of ancestors which did so, for 

 such islands have been produced in mid-ocean and 

 have nev^er formed part of a continent ; but in some 

 cases, of course, the creatures may possibly have 

 migrated from former and unknown lands lying nearer 



* See Wallace, "Geographical Distribution," ii. (1876), pp. 

 512-13, 522. 



5 "Geographical Distribution," ii. (1876), pp. 512-13, 522; " Island 

 Life," p. 76 ; ed. 2, p. 78. 



