532 THE POPULAR SCIENCE MONTHLY 



individuals which have survived to adult life differ from those portions 

 of the shells of young snails, part of which will survive and part be 

 eliminated. If differences do occur they are most easily explained as 

 due to a selective mortality in the animals during their period of growth. 



Details of measurement and calculation are entirely too elaborate 

 and complicated for explanation here. In two studies, one by Weldon" 

 on Clausilia laminaia from Gremsmiihlen in Holstein and one by Di 

 Cesnola^" on Helix arbustorum from the banks of the Isis near Oxford, 

 the authors found that while there is no difference between the mean 

 characters of young and old shells there is a distinct difference in 

 variability. This kind of selective elimination which recurs every 

 generation and by which the existing type is maintained (without 

 necessarily giving rise to any progressive change) Weldon designates as 

 periodic, in accordance with Pearson's terminology.^^ 



The reader must not conclude from what has just been said that 

 Weldon regarded variation in the peripheral radii as the direct cause 

 of the selective elimination. " Such selection is, of course, * indirect,' 

 that is to say, the life or death of the individual is determined in each 

 case by the value of a (probably large) number of correlated characters, 

 of which the length of the peripheral radius is only one." "With justice 

 Weldon emphasizes the minuteness of the structural differences which 

 seem to mark the boundary between fitness and unfitness for survival in 

 Clausilia laminata. 



The results of Weldon's investigation-^ of Clausilia italia did not 

 agree with that of his former study of C. laminaia. By some readers 

 this fact will be interpreted as vitiating entirely any conclusion to be 

 drawn from all this laborious work on shells. To my mind this atti- 

 tude is quite wrong. Laying aside the fact that Weldon has suggested 

 biological reasons which may explain why no change in variability was 

 observed between young and old individuals in C. italia, we must bear 

 in mind the fact that there is no justification whatever for assuming 

 that natural selection, either secular or periodic, is to be observed at all 

 times in all species. Naturally contradictory results call for repetition 

 and amplification and for more refined control of conditions — but these 

 are the things which make for the advancement of science. Only the 

 merest beginning has been made in the study of selective elimination, 

 but Weldon has shown us the way in which the problem may be at- 

 tacked in two large groups of invertebrates. If other workers with his 

 patience are ready to volunteer their service to this phase of evolution, 



■ Weldon, W. F. R., * * A First Study of Natural Selection in Clausilia lam- 

 inata," Biometrika, Vol. I., pp. 109-124, 1901. 



"Di Cesnola, A, P., "A First Study of Natural Selection in Helix arh%u- 

 torum," biometrika, Vol. V., pp. 387-399, 1907. 



"Pearson, K., "Grammar of Science," 2d ed., pp. 413-414, 1900. 



"Weldon, W. F. R, "Note on a Race of Clausilia Italia," Biometrika, Vol. 

 III., pp. 299-307, 1904. 



