42 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
Restricting our attention to Tahiti nul, we may group the valleys according 
to their location in one or another of the four primary quadrants, which, it has been 
shown earlier, constitute four well-marked ecological regions. The figures prove 
that hyalina is most abundant in the northern quadrant (4.62 per cent); the eastern 
section has 1.56 per cent, the western 1.28 per cent, and the southern section bears 
but 0.97 per cent of this species. The differences indicated might conceivably be 
due to a relative scarcity of other species in the northern quadrant and to their 
relative abundance in the southern section, where jyalina forms a smaller per- 
centage of the total population, even though the latter might actually be uniform 
in its frequency throughout the island. ‘This possibility, however, can be set aside 
inasimpleway. Asa given collecting trip in any valley was approximately equal in 
length of time to that made in another valley, and as the conditions of weather were 
sufficiently uniform to be disregarded as qualifying factors, we may determine the 
average number of hyalina procured in a collecting trip of average duration in each 
of the four main sections. ‘The data are as follows: 
Average 
Collect | number 
ing trips.) for one 
trip. 
Region. 
hyalina. 
North quadrant.... 280 21 13. 
West quadrant 67 14 
South quadrant.... 67 22 
East quadrant 57 10 
Tahiti nui 471 66 
Taiarapu 118 13 
Whole island 589 79 
The figures render it certain that in absolute numbers and not merely in relative 
frequency the species is most abundant in the northern quadrant, and least abundant 
in the same sense in the moist southern section of Tahitinui. Particular emphasis is 
laid upon these relations on account of the exactly opposite conditions exhibited by 
P. clara described in Chapter IV. 
A still closer scrutiny of the data reveals a much more striking lack of uniformity 
in distribution. When the relative numbers of /yalina occurring in different valleys 
are compared, great variation is found to exist even among localities in one and the 
same quadrant; in fact, parallel and neighboring valleys often exhibit most signifi- 
cant contrasts. Certain examples may be cited. In Fautaua Valley this species 
exists in greatest relative abundance, forming about 12 per cent of the whole popu- 
lation; it is interesting to note in passing that this locality is also the headquarters 
of the most generalized varieties of the complex species, P. otaheitana. In the 
neighboring valleys of Hamuta, a half mile or less to the east of Fautaua, and Tipaerui, 
2 miles to the west, hyalina forms only 2.06 per cent and 2.29 per cent of the popu- 
lations, respectively. Collections from the great valley of Papenoo contain 7 per 
cent of hyalina, while in Faaripoo, the next valley to the west of Papenoo, only 1 
hyalina was obtained in a series of over 300 snails. Again, Paraura Valley in the 
eastern quadrant bears 3.23 per cent of hyalina, but in the nearest neighbor, Vahiui, 
