526 



THE POPULAR SCIENCE MONTHLY 



their mortality from predaceous enemies ought to be lower, bui" 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 Bats 



Eliminated by- 

 Unknown Enemies, 

 Chiefly Preda- 

 ceous Birds 



All Eliminated 

 Birds 



Self-colored 

 Barred 



336 

 3007 



6 



= 1.79% 



68 



= 2.26% 



29 



= 8.63% 



222 



= 7.38% 



35 



= 10.42% 



290 

 = 9.64% 



Totals 



3343 



74 



251 



325 



It will be noticed that when the chicks eliminated chiefly by pre- 

 daceous 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. 5 



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 de- 

 sirable, 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 selec- 

 tion was W. P. E. Weldon. His first attempt to determine whether 

 survival may depend upon definite physical characters was made with 

 the common shore crab, Carcinus moenas. 6 



5 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. Daven- 

 port's chicks were on a "well-cropped pasture" and Pearl's birds "ran to- 

 gether 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. 



6 Weldon, "W. F. E., "An Attempt to Measure the Death-rate due to the 

 Selective Destructior of Carcinus moenas with Eespect to a Particular Char- 

 acter" (Eeport of the Committee for Conducting Statistical Inquiries into the 



