CHAPTER V. 



LAND SHELLS; THEIR TENACITY OF LIFE. 



Among land-shells some of the species are spread over 

 large areas : of this our own little white snail [Helix 

 pulchelld)^ which independently of dissemination by 

 human agency seems to have had a very extensive range, 

 is a familiar example, and many others might be men- 

 tioned ; Professor Semper has remarked upon the fact 

 that the species of Trochomorpha (a sub-genus of Helix) 

 are extremely similar in appearance whether they come 

 from India, the Moluccas, the Philippines, or the islands 

 of the Pacific, and several, he says, are distributed 

 throughout this vast region almost without any varia- 

 tion in their shells.^ But as a general rule terrestrial 

 species are restricted in distribution, and often wonder- 

 fully so ; most islands, even very small ones, have some 

 peculiar species, and it is even found that single moun- 

 tains or valleys sometimes possess species or varieties 

 found nowhere else in the world.'"^ It is notorious, for 



^ K. Semper, " Animal Life," ed. 4, 1890, p. 288. 

 2 See Wallace, "Geographical Distribution," ii. (1876), 524; 

 " Island Life," 1880, p. 76 ; ed. 2, (1892), p. 78. 



