LAND SHELLS : THEIR TENACITY OF LIFE. QI 



instance, that numbers of species are confined to 

 the Madeiras/ and the islands Madeira and Porto 

 Santo, each in sight of the other, possess assem- 

 blages of land-shells which, though representative, are 

 for the most part different or proper to each.^ From 

 the Sandwich Islands between three and four hundred 

 species of land-shells have been described, and all, it is 

 said, are endemic ! In the sub-family Achatinellincs 

 (entirely confined to the islands) the average range 

 of a species, according to the observations of the Rev. 

 J. T. Gulick, is " five or six miles, while some are re- 

 stricted to but one or two square miles, and only very 

 few have the range of a whole island." It is even said 

 that each valley, often each side of a valley, and 

 sometimes each ridge or peak possesses its own peculiar 

 species.^ 



Fresh-water species, in some groups at least, generally 

 range much more widely than terrestrial species, and 

 it might at first sight seem probable that the former 

 possess far greater facilities for overcoming natural 

 barriers — indeed the latter seem to have been regarded 

 by some writers as practically destitute of all means of 

 dispersal* — but it seems improbable that such is the 

 case, for certain terrestrial genera and higher groups 



1 See R. B. Watson, " Journ. of Conch.," vii. (1892), 4-5. 



^ Lyell, '' Antiquity of Man," ed. 4, 1873, P- 49^- 



3 Wallace, "Island Life," pp. 303-4; ed. 2, pp. 316-18; J. T. Gulick, 

 "Journ. Lin. Soc," 1873, P- 49^, and " Proc. Zool. Soc," 1873, 

 p. 80, as quoted by Mr. Wallace. 



* See, for instance, H. H. Higgins, " Proc. Lit. and Philos. 

 Soc. Liverpool," xxxvi. (1882), pp. xliv-xlv. 



