Page:Popular Science Monthly Volume 78.djvu/539

Rh By washing the stones under which the crabs live along the beach Professor Weldon obtained a silt of a finer texture than the china clay he had been able to use. This was employed in experiments of the same kind, with identical results.

There seems no reason to suppose that the relation of the crabs to the mud on the beach is different from that in the aquarium. Whenever the fine sediment is stirred up a selective elimination of crabs must occur. It is this selective elimination which "Weldon regarded as furnishing the explanation of the decrease in frontal breadth observed in the measurements in 1903, 1905 and 1908.

Not content with these experiments, Weldon tried to obtain evidence of an entirely different kind. He arranged several hundred aquaria in each of which a young crab from the beach was kept in clear running sea water—and so entirely free from the influence of the mud. They were allowed to moult and grow and harden new shells. When measured they were found to be unmistakably broader than wild crabs of the same length. This is precisely the result to be expected if a selective elimination of broad-fronted individuals occurs in nature.

The source of this difference in capacity for survival seems to lie in the way in which the crabs filter the water entering their gill chambers. Professor Weldon found that a narrow frontal breadth renders one part of the process of filtration of water more efficient than it is in crabs of greater frontal breadth. The gills of the crabs which died during the experiments were covered with fine white mud, and this was not found in the gills of the survivors.

The labor of these experiments—the daily care of hundreds of animals, the thousands of measurements and the drudgery of calculation—was excessive. Most discouraging of all, perhaps, were the sterile and hostile criticisms which are so often the portion of a pioneer.

Besults which may be logically attributed to the action of natural selection but which by reason of the possibility of other explanations are not conclusive evidence for its potency, have sometimes been secured by biometricians concerned with other problems. For instance, Warren adduces "the elimination of the physically unfit" as one of the factors to account for the difference in variability of the termites of the same nest at different seasons. Possibly this factor may also account in part for differences in variability from nest to nest, but of course much more extensive and direct evidence must precede any final conclusions.

Another study of variation in insects, social and otherwise, is that