96 THE DISPERSAL OF SHELLS. 



struggle for life with foreign associates.^ When in free 

 intercommunication closely-allied species are not always 

 able to invade each other's territory ; two equally well 

 fitted for their own places seem likely to be able to 

 hold their separate habitats for almost any length of 

 time.- The Achatinellinae of the Sandwich Islands, 

 just referred to, so remarkable for limited specific areas, 

 have in all probability been occasionally, though 

 perhaps very rarely, carried by accidental means into 

 the midst of each other's districts, but, as their distribu- 

 tion clearly indicates^ they must generally have failed 

 to establish themselves in the new surroundings, being 

 unable, no doubt, to compete successfully with those 

 already in possession. Sir C. Lyell has expressed 

 surprise that " in the course of that vast lapse of ages 

 which has occurred since the Newer Pliocene period " 

 the shell-faunas of Madeira ajid the adjacent islet, 

 Porto Santo, have been so little interchanged ; ^ but in 

 all probability shells have been occasionally carried 

 from one island to the other by natural agencies ; if 

 both are well stocked, however, and if the respective 

 faunas are equally well able to hold their own as 

 against the other, they are not likely to be much 

 affected by the occasional transportal of a few 

 individuals. Some of the characteristic Porto Santo 

 species live in crevices of stone, and it is probable that 

 they have often been transported to Madeira by human 

 agency with the large quantities of stone annually 



^ " Origin," p. 358. " " Origin," p. 356. 



3 " Principles," ed. 12, ii. (1875), p. 433, 



