MEASUREMENT OF NATURAL SELECTION 531 



Ailanthus silk-worm moth^' I will pass by without discussion, since I 

 believe he is soon to publish further observations on the same subject. 



Weldon's work on natural selection was not limited to Crustacea, but 

 extended to the mollusca as well. 



The shells of certain snails, such as ClausUia and Helix, is essen- 

 tially a tube increasing in size as the animal grows older and wound in 

 a spiral, or more properly a helix, around a central axis with the suc- 

 cessive coils in contact. If one of these shells be cut longitudinally, the 

 central cone, or columella, as it is technically called, will be laid open 

 and will appear as a narrow conical tube extending the whole length 

 of the shell, while the tube which contained the animal will be cut 

 across twice in each complete revolution and will appear in cross sec- 

 tion. 



This point is made quite clear by an examination of the three dia- 

 grams (Fig. 4). 



Fio. 4. LONGiTUDiJfAL SECTIONS OF Clausilia latninata (A), ClausUia itaUa (B), 

 and Helix arbustorum (C), after Weldon and Di Cesnola in Biometrika. 



By deft manipulation such sections can be prepared. A shell may 

 be ground upon a soft stone until a plane which extends almost exactly 

 through the central columella is exposed.^* From such a preparation 

 it is quite possible to make the measurements which determine the pitch 

 and several other characteristics of the spiral. 



The shell of these snails is a permanent structure. In the adult 

 the whorls first laid down by the young animals can be measured. Now 

 it is clear that one can compare the properties of the portions of the 

 spiral already laid down in the shell of a young snail with the same 

 portions in the shell of an adult. In snails, as in other animals, not all 

 individuals survive to adult life. The problem is to find out whether 

 the properties of the first-formed portions of the spirals of the shells of 



" Crampton, iJ. E., * * Experimental and Statistical Studies upon Lepidop- 

 tera. I. Variation and Elimination in Phylosamia cynthia," Biometrika, Vol. 

 III., pp. 113-130, 1904. 



"To be sure, the work is exceedingly tedious and many shells are acci- 

 dentally spoiled, but four or five may be prepared in a day's work and in the 

 course of time a number sufficient for statistical work may be secured. 



