MEASUREMENT OF NATURAL SELECTION 



525 



think we must regard Di Cesnola's evidence as rather strongly indi- 

 cating a real protective value in the color dimorphism. 



For vertebrates two papers are available. Davenport' and Pearl* 

 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 



Fig. 1. NuMBKR of Insects scbvivino day by day in series of Bbown and 

 Gbeen Mantis exposed on Brown and Green Vegetation. Ordlnates = dates ; 

 absciss« = number of individuals. Similar color in insect and environment repre- 

 sented 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 do- 

 mestication, tend to be eliminated by the natural enemies of these birds, 

 and the pencilled birds are relatively immune from attacks because rela- 

 tively inconspicuous." 



Photographs by Pearl show that the barred birds are much leas con- 

 spicuous in their surroundings than are self-colored ones. Theoretically, 



* Davenport, C. B., ' ' Elimination of Self-coloured Birds, ' ' Nature, Vol. 

 LX XVIII., p. 101, 1908. 



* Pearl, R, ' ' Data on the Belative Conspicuousness of Barred and Self- 

 colored Fowls," Amer. Nat., Vol. XLV., pp. 107-117, figs. 1-4, 1911. 



