18 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
The prevailing winds in the Society Islands are the strong southeast trades 
that blow almost uninterruptedly for the greater part of the year. At times the 
wind may veer to the northeast, and to the south at the other extreme, but with 
lesser power when it does so. Hence the wind-currents pass from Tahiti toward 
Moorea and the leeward islands of the group. Likewise the main ocean-currents 
sweep from Tahiti and the southeast toward the northwest, although far beyond 
the Society Group the lines of flow straighten out toward Samoa in a due westerly 
direction. Conceivably, then, Tahitian species of Partule may have been carried 
passively to the shores of the other members of the Society Group, and even to 
more distant islands in other groups. Nevertheless the biological characters and 
ecological relations of these snails are such as to render such a view untenable. 
The tides of the Society Islands are almost unique in the world, by reason of 
the fact that high water occurs approximately at mid-day and mid-night through- 
out the year, owing to a peculiar combination of circumstances now well understood. 
Naturally the tides are of no effect on the distribution and transport of Partule, 
because the snails do not reach the shores in a living condition. 
TEMPERATURE, RAINFALL, AND BAROMETRIC PRESSURE IN RELATION TO THE 
INTRA-INSULAR DISTRIBUTION OF PARTULZ. 
The three specified forms of environmental influence exert direct effects upon 
the distribution of Partule, although the question as to their values in relation to 
intrinsic characters is an entirely independent one. In general, the situation 
throughout the area inhabited by these snails is one of high temperature, much 
moisture, and low barometer; furthermore, these conditions are maintained with 
the greatest constancy throughout the year. 
The “heat-equator” of the globe traverses the area occupied by the genus 
immediately north of the Society Islands. “Tahiti is only a few hundred miles below 
this line, which here dips southward of the geographical equator to a distance of 10°. 
At the coast, the thermometer rarely rises during the day above 33° C. (93° F.), 
while at night, owing to the air-currents that flow downward from the high central 
peaks, the temperature drops, but rarely falls below 16° C. (60° F.) at any time. 
Variations in the ocean winds affect the temperature of the islands, always within 
narrow limits. The months from December to May are the hottest, but their 
extremes of temperature are not infrequently equaled during the alternative period. 
As a basis for the present analysis of temperature conditions we have the 
official figures taken at Papeete, the principal town. Tables 4 and 5 give the funda- 
mental data. But two facts must be borne in mind: first, that the station where 
observations are recorded is on the flat coastal plain; second, that it lies upon the 
northwest or leeward side of the island, sheltered by the high mountains from the 
full force of the trade-winds. ‘Therefore, while the figures for Papeete may be taken 
as typical for the island in certain respects, in the nature of the case they can not 
adequately represent the temperatures met by the snails in the valleys and upon 
the higher ground near the central part of the island. Were it practicable, recording 
instruments should be placed at other points along the coast, in the valleys, and 
