20 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
drawn as to the relationships and recent migrations of the diverse types so dis- 
tinguished. Thus the geographical and the biological elements of the situation as 
it is to-day will be established, to be available for the investigation of the same 
problems by some student of the future. 
The collated facts of the preceding chapter show that the northern and 
southern halves of the island are somewhat sharply contrasted; the former is a 
relatively simple plateau with an extensive growth of vegetation suitable for 
Partulz, while the southern part is mountainous and broken, and its areas of high 
vegetation are scattered and disconnected for the most part. The snails vary in 
different localities of the island, and hence the naive conclusion would be that the 
local geographical and ecological differences are responsible for the biological 
differentiation of the organisms. While the contrary is the truth, nevertheless the 
geography of the areas of collection is indispensable for the description of the exact 
facts of distribution as well as for the discussion of the possibility that the “‘ environ- 
ment” is causal with respect to differentiation. 
An ideal study of Guam, unhampered by limitations of time and energy, 
would divide the island into areas one mile square, and would determine the forms 
living in each of these component territories. While this procedure has not been 
possible, yet sufficient material was obtained from all of the various ecological 
districts for a detailed colonial analysis and for wide comparisons. In Saipan the 
areas of collection were limited to a series of eight stations on the western side of the 
island; while this material is less comprehensive, it proves to be valuable not only 
for the comparison with the identical species of Guam, but also because of its 
diversification, which is all the more remarkable in view of the small number of 
localities from which snails were obtained. 
Partule were obtained from 39 localities of Guam, which are indicated in 
plate 7; while orthographic conventions differ, the names given are in current usage 
and clearly identify the places in question. These localities are representative of a 
series of districts into which the island may be arbitrarily divided for purposes of 
convenience and description. 
The Northeast Region, so called, occupies the greater part of the extreme upper 
portion of the whole island; its vegetation is broken here and there by plantations 
and clearings, but for the most part it consists of typical forests, or of forests from 
which many of the highest trees have been cut. The Tarague collection was taken 
from the bush of the high plateau near the coast, while the snails from Santa Rosa 
and Asados represent the inland associations. The first-named locality was not 
visited personally, the collection being secured by a trustworthy assistant. 
In the East-Central Region the Lolo locality is like Tarague above, Barrigarda 
is in line with Asados and Santa Rosa, while the collections from Ukudu, Dededo, 
and Saucio are typical of the populations of the western bush of the high limestone 
plateau. In ecological respects this region is virtually the same as the northeast 
region and is separated mainly for convenience. 
What is distinguished as the Coast Central Region includes the localities of 
lower levels, from Tumon to the Presidio. At most of these places, the vegetation 
