140 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
The statistical comparison of the annual series is given in detail in tables 73 
and 74, which also include the constants for the whole population, to be used in 
the summary. The results are extremely interesting. While the proportionate 
measures and the tooth remain about the same, there is a regular and progressive 
diminution in the absolute dimensions of the whole shell and of the aperture; even 
the short series of banded shells for 1906 and 1908 exhibit precisely the same statis- 
tical differences as the larger series of plain individuals. ‘This result is in exact 
accord with the findings in the case of P. filosa, the small dextral species confined to 
this valley. Why the snails of both species should be absolutely smaller in the 
more favorable higher portion of the valley, and larger where the conditions are 
adverse, is difficult to understand. The only reasonable suggestion, to my mind, 
is that selection weeds out the smaller snails in the coastward area, leaving only the 
larger and more vigorous individuals; the absence of so rigid an eliminative process 
higher up leads to a reduction of the average measures through the survival of 
smaller individuals. This species is more abundant inland, which lends some 
support to the tentative interpretation given. 
Tasie 74.—Partula otaheitana amabilis, Pirai Valley. Statistics of tooth development. 
Standard 
deviation. 
Series. 
Sinistral, plain, 1906........ 176 14 26 108 23 5 2.8807 = .0424 0.8342 .0300 
OW[ecsuneos 250 14 41 179 10 6 2.8120 .0298 -6990= .0211 
INN sogc000 75 2 13 47 9 4 3.0000+ .0611 . 7831 .0432 
Os c00000c 360 20 50 239 33 18 2.9417 .0286 - 8058+ .0202 
Sinistral, plain, all years..... 861 50 130 573 75 33 2.8966 .0180 . 7829 .0127 
0 2.7500+ .1032 .4330= .0730 
0 2.8750 .0788 . 3306+ .0557 
Sinistral, banded, all years!.. 19 0 3 15 1 0 2.8948 .0691 .4466= .0488 
Sinistralalllrrererereretererer 880 50 133 588 76 33 2.8966+ .0177 .7772 .0125 
1Includes 1 banded snail of 1907 and 2 of 1909. 
CoMPARISON OF THE CoLorR-CLASSES. 
In comparing statistically the several distinguishable color-groups (table 75) it 
is obligatory to disregard the series of 1906, as deteriorated preservative altered 
the colors of Classes II and III; the series of 1908 may be also omitted on account 
of its smallness. There remain the groups of 1907 and 1909, in both of which the 
Class I shells greatly predominate. The figures show how the specimens assigned 
to the darker classes depart from those of Class I. 
As we have seen, the inland group of 1909 comprises individuals that are 
smaller than those of the lower valley, while at the same time Class I is relatively 
more abundant. It is true that this group is made up of smaller shells in the coast- 
ward section, and it might be supposed that the reduced averages of the 1909 series 
would be due simply to the greater prevalence of small, light-colored shells; but 
this is not so. The snails of Class II are also reduced in size in passing inland, and 
in addition the statistics relating to the banded examples, given in table 73, show 
