The interesting point is the extreme restriction of the species and varieties.

The average range of each species is only five or six miles, while some are restricted to but one or two square miles, and only a very few range over a whole island.

The forest region that extends over one of the mountain-ranges of the island of Oahu, is about forty miles in length and five or six miles in breadth; and this small territory furnishes about 175 species, represented by 700 or 800 varieties.

Mr.
Gulick states, that the vegetation of the different valleys on the same side of this range is much the same, yet each has a molluscan fauna differing in some degree from that of any other.

"We frequently find a genus represented in several successive valleys by allied species, sometimes feeding on the same, sometimes on different plants.