200 DARWINISM AND HUMAN LIFE 



tortoise-shell butterfly {Vanessa urticce) to nettles, 

 tree-trunks, fences, walls, and so on. At Oxford 

 there was a mortality of 93 per cent., pointing to 

 an extremely high ehmination-rate, and the only 

 pupae that survived were on nettles, where they 

 were least conspicuous. At St. Helens, in the Isle 

 of Wight, the elimination was 92 per cent, on fences 

 where the pupae were conspicuous, as against 

 57 per cent, among nettles where they were incon- 

 spicuous. Here, again, there is definite evidence of 

 discriminate ehmination. 



Another illustration is to be found in the late 

 Prof. Weldon's ' well-known experiments on crabs. 

 He placed 248 male shore-crabs {Carcinus mcenas) 

 in a vessel of sea-water containing in suspension 

 a quantity of china-clay ; and 94 survived. It was 

 found that the mean of the frontal breadths of 

 the survivors was distinctly smaller than that of 

 the ehminated. " A difierence in the mean value 

 of a character between survivors and eliminated, 

 when both have been exposed to identical environ- 

 mental conditions, is proof that the character is 

 being acted upon by natural selection. . . ."' That 

 is, by the ordinary " secular selection," for there 

 is another mode — " periodic selection," in which 

 the mean value of the character is not changed, 

 but extreme deviations from the mean are lopped 

 off. " Periodic selection " can be detected by the 

 decrease in the range of variabiUty. 



Measuring small specimens (10-15 mm.) of 

 shore-crab taken from Plymouth Sound, in the 



1 "Proo. Eoyal Soo." (1895), voL Ivii. pp. 360-79. Also Naiure 

 (1898), vol. Iviii. pp. 499-506 and 595-6. 



' See " The Evidence of Natural Selection," by E. S. Kussell, 

 in Riviela di Scienza (1908), voL iii. 



