156 THE DISPERSAL OF SHELLS. 



with broken shells.^ Miss Warren has suggested to me 

 that small shells, such as those living at the roots of 

 grass, may sometimes stick to the muddy clay on the 

 feet of cattle, and it seems probable that the range of 

 a species within a given country may occasionally be 

 extended in this way. But of all animals, with the 

 exception perhaps of man (who has done a great deal 

 in recent times) the ranges of land-shells have certainly 

 been most affected by birds. The waders, it will be 

 remembered, as Mr. Darwin remarked,'- are great 

 wanderers being " occasionally found on the most remote 

 and barren islands of the open ocean ;" many sorts of 

 birds, it is hardly necessary to repeat, annually migrate 

 across considerable tracts both of land and sea, and 

 most kinds, as every one knows, are liable occasionally 

 to be blown by violent gales of wind to great distances 

 over the ocean ; almost every year, for instance, Mr. 

 Darwin states, one or two land-birds are blown across 

 the whole Atlantic, from North America to the shores 

 of the British Isles, and many, to give another instance, 

 even the smaller land kinds, are constantly blown 

 from Europe to the Azores, a distance of nearly a 

 thousand miles; indeed, according to Mr. Godman, 

 scarcely a storm occurs there in spring or autumn with- 

 out bringing one or more species foreign to the islands/* 



' W. W. Attree, in Menifield's " Sketch of the Nat. Hist, of 

 Brighton,'' p. 157, as quoted in the "Zoologist," (3), ii. (1878), 90. 



■' "Origin," p. 345. 



3 " Origin," pp. 326, 328 -9; " Island Life," p. 73, ed. 2, p. 75 ; 

 "Principles, ii. p. 368; F. Du Cane Godman, "Ibis," (n. s.), ii. 

 (1866), p. 105. 



