LAND SHELLS : THEIR TENACITY OF LIFE. 95 



Woodward, Cydostoma elegans is found in the greatest 

 profusion on the bosses of chalk that appear arnong the 

 overlying Tertiary gravels and clays, and is not met 

 with in the intervening areas. ^ 



Mr. Reeve, it will be remembered, took it for granted 

 that these creatures possess even " greater facilities of 

 migration " than the fresh-water kinds,^ but this conclu- 

 sion was not based upon definite data and has but little 

 value ; one is certainly inclined to agree with Mr. Belt, 

 however, in thinking that the land species " have at 

 least equal means of dispersion, compared with the 

 sluggish, mud-loving, water-shells of our ponds and 

 ditches;"^ but, if this be so, how is it that the 

 species of the one group are generally confined to 

 small areas while those of the other are frequently 

 widely diffused ? 



In the first place it may be remarked that the 

 wonderfully restricted distribution of many terrestrial 

 molluscs cannot be taken as implying an absence of 

 means for occasional transport, though, of course, 

 frequent or constant transportal might have largely 

 hindered divergence. It must not be supposed, Mr. 

 Darwin points out, that species which have the capacity 

 of crossing barriers will necessarily range widely, for 

 this implies, not only the power of crossing barriers, 

 but the more important power of being victorious in the 



^ S. P. Woodward, as cited by Tate, " Land and Fresh-water 

 Mollusks," 1866, p. 222. 



' L. Reeve, " Land and Fresh-water Mollusks," 1863, p. 252. 

 3 T. Belt, " Naturalist in Nicaragua," ed. 2, 1888, p. 334. 



