Page:Popular Science Monthly Volume 78.djvu/536

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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:

Class of Birds Number in
Original Flock
Eliminated by
Known Enemies,
Almost all Rats
Eliminated by
Unknown Enemies,
Chiefly Preda-
ceous Birds
All Eliminated
Self-colored 336 6 29 35
= 1.79% = 8.63% = 10.42%
Barred 3007 68 222 290
= 2.26% = 7.38% = 9.64%
Totals 3343 74 251 325

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.[1]

B. Structural Characters in Relation to Survival

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.[2]

  1. It would be very interesting if data could be obtained from flocks of young chickens in a diversified environment—i. e., one in which there is a variety of underbrush, weeds, stones, etc., giving wider opportunity for hiding. Davenport's chicks were on a "well-cropped pasture" and Pearl's birds "ran together on the same open, turf-covered range." Now it is quite possible that barring might afford no protection on open turf, and yet be most advantageous in a thicket. Some poultry man could do a very good service to science by appropriating a few hundreds of young birds to the hawks and crows, allowing them to have the run of a lot affording a diversity of shelter. Only where the habitat simulates closely the kind in which animals are found in nature can an experiment of this kind be really critical.
  2. Weldon, W. F. R., "An Attempt to Measure the Death-rate due to the Selective Destruction of Carcinus mænas with Respect to a Particular Character" (Report of the Committee for Conducting Statistical Inquiries into the