116 A First Study of Natural Selection in Clausilia laminata 
TABLE I. 
Mean Length of Peripheral Radius and Mean Angular Distance from the Columellar 
Plane, for each group of measures of 100 adult Clausilia laminata. {Every 
mean value determined from 100 observations, unless otherwise stated.) 
(Angular distances above the plane of the standard columellar radius, towards the 
apex of the shell, are called negative.) 
Angular Distance from 
Standard Columellar 
Mean Radial Length (mm.) 
Radius 
-11-4148 xlSO" 
0-4595 (37 observations) 
-10-4906 xl80° 
0-6254 (98 „ ) 
- 9-50528 X 180° 
0-8807 
- 8-50528x 180° 
1-1237 
- 7-50528x 180° 
1-3873 
- 6-50528x 180° 
1-6813 
- 5-50528 X 180° 
1-9978 
- 4-50528x 180° 
2-3443 + 0-00815 mm. 
- 3-50528x 180° 
2-7314+0-00902 mm. 
- 2-50528x 180° 
3-1 504 + 0 00905 mm. 
- 1-50528x 180° 
3-6238 + 0 00986 mm. 
- 0-50528x180° 
4-1366 + 0-01054 mm. 
+ 0-49472x 180° 
4-7206 + 0-01147 mm. 
+ 1-49472x 180° 
5-3599 
+ 2-49472 x 180° 
6-0885 
+ 3-49472 X 180° 
6-9119 
+ 4-49472x 180° 
7-8879 
+ 5-49472x 180° 
9-0029 
+ 6-49472x180° 
10-3057 
+ 7-47936x180° 
11-6877 (84'ob.servations) 
+ 8-56985x180° 
12-4938 (26 „ ) 
+ 9-66845x180° 
13-5175 (4 „ ) 
The regularity of the values obtained for the mean peripheral radius of the 
adult spiral made it seem that even so small a number of individuals might serve 
as a basis for comparison between young and mature individuals of the race. The 
way in which these shells are forined makes it less difficult to compare the mean 
magnitude and the variability of a character in young and in mature shells than it 
is in the case of most animal organs. For it is well known that the outer layers of 
the shell in Clausilia and other pulmonates are secreted by the growing edge of 
the mantle once for all ; and that when the mantle edge has secreted these outer 
layers, the only subsequent change which can be effected is due to the deposition 
or absorption of the deepest layer, involving change in the thickness of the shell, 
but no change in its external shape. The only changes which the outer layers 
