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 
verstpellis.’’* 
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 milesin 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 Achatinellide 
there are, within the same limits, 17 or 18 strongly marked species. 
