The land-snails inhabiting the various groups of islands scattered throughout the vast extent of the Pacific Ocean provide the richest and most ideal material for the demonstration of the principles of geographical distribution. In the Hawaiian Islands snails of the family of Achatinellidae occur in great abundance, and like the lizards of the Galapagos Islands different species occur on the different members of the group. Within the confines of one and the same island, they vary from valley to valley, and the correlation between their isolation in geographical respects and specific differences on the other hand, first pointed out by Gulick, makes this tribe of animals classical material. In Polynesia and Melanesia are found close relatives of the Achatinellidae, namely, the Partulae, which are thus in relative proximity to the Achatinellidae and not on the other side of the world. Furthermore, the Partulae are not alike in all of the groups of Polynesia where they occur; the species of the Society Islands are absolutely distinct from those of the Marquesas, Tonga, Samoan, and Solomon Islands, although they agree closely in the basic characters that justify their reference to a single genus. The geological evidence tells us that these islands were once the peaks of mountain ranges rising from a Pacific continent which has since subsided to such an extent that the mountain tops have become separate islands. Thus the resemblances between Hawaiian and Polynesian snails, and the closer similarities exhibited by the species of the various groups of Polynesia, are intelligible as the marks of a common ancestry in a widespread continental stock, while the observed differences show the extent of subsequent evolution along independent lines followed out after the isolation of the now separated islands. The principle may be worked out in even greater detail, for it appears that within the limits of one group diverse forms occupy different islands, evolved in different ways in their own neighborhoods; while in one and the same island, the populations of the different valleys show marked effects of divergence in later evolution, precisely as in the case of the classic Achatinellidae of the Hawaiian Islands.