A FIKST STUDY OF NATUEAL SELECTION IN 
CLAUSILIA LAMINATA (MONTAGU). 
By W. F. R. WELDON. 
Among the terrestrial moUusca ot" Europe are many species which have 
certainly inhabited their present areas of distribution for a very long time*. 
The shells of some species are and have been exceedingly variable, while others 
are, and have apparently for long periods remained, almost invariable. 
The problems presented by species belonging to either category are of great 
interest in connection with the theory of natural selection ; and the characters 
which can be studied in recent and in fossil shells alike are often well fitted for 
numerical treatment. Such characters as the shape of the spire, the number of 
ridges and furrows on a given whorl, the size and shape of the aperture, can often 
be expressed in such a way as to admit of numerical comparison between in- 
dividuals; these are the characters used in the discrimination of "species"; and 
in the present state of our knowledge they atford excellent examples of specific 
characters which appear to us " useless." 
A typical example of a species which appears to have varied very little during 
a long period of time is Glausilia laminata (Montagu): and I wish now to record 
an attempt to determine the variability of one element of its spire. 
If the shell of an adult Glausilia laminata be carefully ground upon a soft 
stone, it is possible to expose a section passing with sensible exactness through the 
plane which contains the axis of the spire. The columella is in many cases 
approximately straight, except near its lower end, where it bends abruptly and 
supports the clausilium. This bend in the columella is associated with a change 
in the character of the spiral, leading to the formation of the oblique aperture of 
the finished shell, but above the point where the bend occurs the columella may 
without serious error be treated as a conical tube, some ten or twelve millimetres 
long, and about half a millimetre in diameter at its lower (broader) end. If a 
shell be ground on a flat stone, until the middle of this long and narrow tube is 
* For a discussion of the history of land shells in the Palaearctic Region, see Kobelt ; Stiidien zur 
Zootjco<jraphie, Wiesbaden 1897-98. 
