NATURAL SELECTION 199 



(iii) There are no control measurements given of the 

 wild population in silt-free conditions from which 

 one could see if the changes do or do not occur 

 there. 



(iv) The control experiments are criticised by Cunning- 

 ham (I.e. p. 196). As regards the first, he points 

 out that it is not stated that the survivors were, 

 on the average, of the same carapace length as 

 the dead. As regards the second series, in which 

 the M.F.W. under silt-free conditions was larger 

 than in the wild population, it is rather difficult 

 to give the facts in a condensed form, because 

 there was a preliminary mortality due (?) to the 

 presence of putrescence in the water, and the 

 shells of the survivors at the first moult were less 

 than those of wild forms, which Weldon put 

 down to the fact that those of greater M.F.W. were 

 selectively eliminated. Cunningham makes it 

 amply clear : (a) that necessary comparisons were 

 not made, and (b) that Weldon omitted to con- 

 sider the effect of food-supply and temperature 

 on the size of the experimental animals. 



On the whole the objections raised as to Weldon's results 

 are so serious that the latter cannot be accepted as good 

 evidence for the efficacy of selection. 



(2) Harrison (ig2o). 



In the Cleveland district of Yorkshire a colony of the moth 

 Oporabia autumnata was originally broken into two parts, one 

 ultimately inhabiting a coniferous wood, the other a birch 

 wood. The colour of the two colonies was found to differ, those 

 moths living in the birch wood being paler (no statistics given). 

 Harrison attributes the difference to the elimination by noc- 

 turnal birds and bats of the pale forms in the coniferous wood, 

 on the assumption that these moths are more conspicuous. His 

 proof is that of 15 pairs of wings (remains of moths attacked 

 by enemies [?] ) found on the ground in that wood the majority 

 (numbers not given) are pale, though in the total population 

 the dark forms outnumber the pale in the ratio of 25 : 1. He 

 states that owls, nightjars and bats are numerous in the pine 



