Page:Popular Science Monthly Volume 78.djvu/535

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525
MEASUREMENT OF NATURAL SELECTION

think we must regard Di Cesnola's evidence as rather strongly indicating a real protective value in the color dimorphism.

For vertebrates two papers are available. Davenport[1] and Pearl[2] have made observations on the relative number of self-colored and of barred or pencilled birds killed by enemies. Davenport finds that out of 24 chicks from five to eight weeks old killed on one afternoon by three crows only a single one was other than self-colored, although twenty per cent, of the flock of about 300 chicks "had a pencilled or

 

PSM V78 D535 Daily estimate of mantis survival on brown and green vegetation.png

Fig. 1. Number of Insects surviving day by day in series of Brown and Green Mantis exposed on Brown and Green Vegetation. Ordinates = dates; abscissæ = number of individuals. Similar color in insect and environment represented by heavy dots; dissimilar combinations by circles.

 

striped marking more or less like that of the female jungle fowl or ordinary game." He concludes, "this fragment, then, so far as it goes, indicates that the self-colors of poultry which have arisen under domestication, tend to be eliminated by the natural enemies of these birds, and the pencilled birds are relatively immune from attacks because relatively inconspicuous."

Photographs by Pearl show that the barred birds are much less conspicuous in their surroundings than are self-colored ones. Theoretically,

  1. Davenport, C. B., "Elimination of Self-coloured Birds," Nature, Vol. LXXVIII., p. 101, 1908.
  2. Pearl, R., "Data on the Relative Conspicuousness of Barred and Self-colored Fowls," Amer. Nat., Vol. XLV., pp. 107-117, figs. 1-4, 1911.