Page:Popular Science Monthly Volume 78.djvu/542

532 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 laminata from Gremsmühlen 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. laminata. 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 attitude 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 attacked in two large groups of invertebrates. If other workers with his patience are ready to volunteer their service to this phase of evolution,