2 INTRODUCTION. 



the almost precipitous cliffs on the other side of the backbone of the 

 island, is the secret home of the very rare and beautiful Achatinella 

 versipellis ." * 



My mind was constantly seeking an answer as to why many of the 

 species of these Hawaiian genera of snails should have an area of distri- 

 bution not more than a mile or two in length, while, in the case of some 

 species of terrestrial mollusks in other parts of the world, the district 

 occupied is a thousand miles or more in length. Again, of Hawaiian 

 species, why should those living continuously in the trees, without 

 descending to the ground even for breeding, occupy on the average 

 areas much smaller than those occupied by species living contin- 

 uously on the ground? 



The mystery was only intensified when I observed a certain corre- 

 lation between the form of the island on which the species had origi- 

 nated and the method of grouping and distribution of the species and 

 varieties. In the first place, the forest species on one island are never 

 completely intergraded with those on another island. Again, the 

 intergrading of nearly allied species on one island usually relates to 

 species found in contiguous valleys ; while the most divergent forms 

 are found in the districts that are most widely separated. On West 

 Maui, which is a single conical mountain, deeply furrowed with val- 

 leys and gorges radiating from one center, we find each group of 

 species lying in a circle around the mountain, each species occupying 

 its own district, though intergrading with those of adjoining districts, 

 and no one of the species strongly divergent from any of the others 

 of the same group a distribution that seemed symmetrical and 

 impressed me as strikingly similar to the distribution of groups of 

 birds and mammals around the North Pole ; for example, the distribu- 

 tion of species of bears throughout the Northern Hemisphere. But, 

 in strong contrast with this, is the distribution of species of snails on 

 the island of Oahu. Here the forest region, in which' the snails are 

 mostly found, is not spread in a circular form over a group of radiat- 

 ing valleys, but lies in a strip about 35 miles in length and from 2 to 

 6 miles in width, upon a mountain range; and the forms of one 

 closely related group are distributed in two parallel series of species 

 on opposite sides of the ridge, the most divergent forms being those 



* In Plate II, figs. 11-25, are given 15 species of Achatinella, distributed in the 

 groves of five valleys, and, therefore, limited to an area less than 5 miles in 

 length and not more than 2 miles in width. Of these 15 species there are nearly 

 a hundred easily distinguished varieties. Moreover, several other much rarer 

 forms of Achatinella found on the vegetation of the same district have been de- 

 scribed as separate species; and of 6 other genera of the family Achatinellidae 

 there are, within the same limits, 17 or 1 8 strongly marked species. 



