172 THE AMERICAN NATURALIST. [Vol. XXXVII. 



tions are true, and whether evolution is taking place in this 

 introduced species. These results, if trustworthy, would be of 

 very considerable value at this time, because the number of 

 quantitative determinations of the present rate of evolution that 

 have been made heretofore are very few indeed, and students 

 of the subject are seeking to accumulate sufficient data to 

 remove the theory of evolution from the realm of inference 

 to that of observed fact. 



It was for this reason that the authors planned a statistical 

 study of L. litorea. We proposed to compare variability and 

 type in two generations of shells which had been subjected to 

 their environments for different lengths of time. We hoped to 

 find material for this purpose in measurements of some charac- 

 ter in young shells and of the same character in the upper part 

 of fully grown specimens, as Weldon ('01) had done in his study 

 of Clausilia. 



Thus by comparing the type and variation of the same charac- 

 ter in the present young shells and in the survivors of a former 

 generation of young shells we might expect to discover any 

 selection that may be taking place with respect to that charac- 

 ter and to determine whether the result is progressive evolution 

 or is simply periodic, provided of course, that the character 

 chosen is knotun to remain constant during the life of the indi- 

 vidual. The only available character to measure seemed to be 

 the angle at the apex of the shell. 



For this investigation a large number of living specimens of 

 all sizes were collected at W'inthrop, Mass., from spaces between 

 the rocks at low tide, during the month of February, 1902. 



Many of the oldest shells were plainly much eroded at the 

 apex and unfit for our purpose. But all of the smaller shells and 

 many of the large ones appeared to be in good condition. It was 

 noticeable, however, that the larger shells were generally less 

 acute at the apex than the very small ones. The suspicion that 

 this fact aroused was further increased by examination with the 

 hand lens, which showed that near the apex of all the shells the 

 surface was roughened with fine depressions like minute pin- 

 holes. Evidently some process of erosion was at work, and it 

 would be useless to proceed with our investigation unless it 



