GENERAL FEATURES OF POLYNESIAN REALM IN RELATION TO DISTRIBUTION. 23 
points on the windward side of the island the coast may actually be reached. When 
the valley opens widely upon the alluvial plain the dry peripheral zone may extend 
centrally for many miles before grading insensibly into the moist area. 
Near the shore-line the ridges are as dry and as hot as the coastal shelf itself; 
passing towards the central peaks more rain will be encountered, but this is so 
rapidly shed into the intervening valleys that the dry region penetrates far into the 
interior. Only in the highest parts do the ridges revert to the moist region. ‘There- 
fore the two principal regions interdigitate with one another, for the outward 
radiations of the central high and wet regions continue into the low valley bottoms 
and alternate with the ridges that in an ecological sense are centripetal extensions of 
the low, dry, and hot coastal plain. 
BOTANICAL AND ZOOLOGICAL ASSOCIATES OF PARTULZ. 
The limits of the two climatological regions distinguished in the foregoing sec- 
tion are further emphasized by the diverse botanical associations existing in them. 
As Partule are restricted to a portion of the inner moist area, the plant types of 
this are of special interest. 
The dry coastal shelf and the lower parts of the large valleys are grassy plains 
bearing groves of coconut palms, bread-fruit (4rtocarpus), and various forms of 
tropical Leguminose, such as Mimosa and Erythrina (“‘flamboyant’’). The orange, 
lemon, and guava (imported in 1808) also thrive in such places (plate 60), as well as 
other exotics like the mango and the lantana—a bush that has spread with great 
rapidity, so as to be a serious hindrance to island agriculture. Upon the ridges 
between the outward parts of the valleys (plate 72), trees and low shrubs are prac- 
tically lacking, so that only grasses and a few suitable species of ferns form the 
whole plant association. 
Penetrating the valley, one usually finds Partule only where the guava thickets 
give place to a mixed forest of large trees (figs. 11 and 12), beneath whose shade the 
smaller succulent shrubs and herbs find a sufficiently moist habitat. Among the 
trees of forest growth are the tamanu (Callophyllum), hutu (Barringtonia), miro 
(Thespesia), and mape (Inocarpus). Sometimes Partule are found upon the trunks 
of such trees and upon their saplings and seedlings. More frequently, however, 
they appear on the under side of leaves of the banana and wild plantain (fei, Mussa), 
caladium (ape, Colocasia), turmeric (Curcumaria), wild ginger, and Dracena (ti). 
Various species of Pandanus also grow in the interior, sometimes in a well-marked 
zone along the valley-wall at the edge of the true forest and just below the treeless 
grassy ridges. The Pandanus belt marks the lateral boundaries of the area inhabited 
by Partule; above this the drier open area acts as a barrier to migration. While 
the suitable nurse-plants extend far toward the center of the island, where dense 
groves of plantain and other succulent plants are to be found (plate 8), the snails 
are prevented by the lowered temperature of high altitudes from reaching the 
central area across which they could otherwise make their way to another valley. 
Furthermore, the high, precipitous cliffs so frequently encountered at the head of a 
valley afford no foothold for the thick plant-growth that is necessary for the snails’ 
habitation. 
