22 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
the clouded period for a given valley-head, the total number of hours of rain would 
be 1,500. At Papeete there is a deposition of 0.68 inch for each hour of rainfall, 
on the average (table 7); hence 1,020 inches would fall every year on the given 
inland area! From personal experiences in the interior, this enormous total does 
not seem too far beyond the actual amount; ten days’ continuous rain has been 
witnessed, even near the coast on the windward side, while in Samoa on one occa- 
sion 4.5 measured inches of rain fell in 2.5 hours. In any case, the just conclusions 
from the available data are: (1) that the windward side of Tahiti is far wetter than 
the opposite part; (2) that the interior is more moist than the coast; (3) that large, 
deep valleys are more humid than small or shallow ones, which collect less water 
in their streams or which soon lose much of their moisture through evaporation. 
TaB Le 8.—Statistics of humidity (in percentages) at Papeete, Tahiti. 
1901 | 1902 | 1903 | 1904 | 1905 
Absolute maximum 97 99 
Absolute minimum 65 61 
Average, 8 a.m Ac : : 84.3 
Average, 4 p.m Aid ; : 78.9 
Snails of the genus Partula are somewhat definitely conditioned as to their 
distribution by the amount of moisture in the atmosphere. The drier coastal areas 
of the leeward side are devoid of them, and this is true of the peripheral parts of the 
valleys. On the windward side, however, the region of high humidity extends 
down toward the shores, almost reaching these in some places; hence the snails 
are to be found at far shorter distances from the sea. 
Barometric pressure remains remarkably constant throughout the entire cal- 
endar day, being on the average (1903) 30.40 inches at 8 a. m. and 30.34 inches at 
4 p.m., Papeete records. ‘The pressure lessens rapidly and regularly as one leaves 
the coast in ascending the mountains and valleys. Partule are found as a rule only 
in the valleys at some distance from the coast, but their seeming relation in dis- 
tribution to barometric influence is only indirect, through their real restriction by 
temperature and moisture conditions that are usually realized only at upper levels. 
In brief, then, topographical and climatological conditions at Tahiti are such 
that two distinct regions are established, one being the peripheral coastal plain with 
relative lower rainfall and higher temperature, which is devoid of Partule; this 
encircles the second and contrasted area of excessive moisture, somewhat lower tem- 
perature, and much lower barometric pressure, which centers around the high moun- 
tain peaks that are protected from the sun’s rays by the dense clouds. The latter 
region extends in all directions down the larger valleys towards the coast-line, to a 
distance that is determined by the extent of the daily cloud-cap, by the amount of 
moisture shed from the high ridges into the valley streams, and by the height and 
slope of the side-walls of the valleys. A valley that runs east and west will naturally 
be drier and hotter than one trending north and south; in those valleys whose walls 
rise precipitously, so as to shut out most of the sun’s rays during the greater part of 
the day, the moist area may extend almost to the coastal plain; indeed, at some 
