Page:Popular Science Monthly Volume 78.djvu/536

526 their mortality from predaceous enemies ought to be lower, but from a rather large series of observations in his experimental poultry plant only negative results are secured. The actual figures are:

It will be noticed that when the chicks eliminated chiefly by predaceous birds are examined alone, the proportion of self-colored birds is a little higher, but without further statistics no significance could be attached to the difference.

The comparisons in the preceding section were drawn between well-marked color varieties. Many more experiments of this kind are desirable, but if natural selection be a factor of the potency required to account for the origin of specific characters by the accumulation of small variations, it must be shown that the peculiarities of form or color which separate one individual from another are of significance in determining the ability to more than hold its own in competition with its fellows. So far as I am aware pertinent data are available for structural characters only.

Experiments with Crabs

The pioneer in the measurement of the intensity of natural selection was W. F. R. Weldon. His first attempt to determine whether survival may depend upon definite physical characters was made with the common shore crab, Carcinus mænas.