( Ixii ) 



diflSculty or loss be made to assume a form approaching that 

 of the spring emergence, but different from it. In both this 

 double-brooded insect, and the single-brooded autumnaria and 

 alniaria, the effect of cooling or icing the pupa was to darken 

 the colour of the moth. The imago of the summer brood of 

 iU.ustraria became very weakly after fifty-five days' icing, but 

 six out of seven survived icing for that period. The experi- 

 ments proved conclusively what had been suspected last year, 

 that in autumnaria the male was habitually longer in the 

 pupal state than the female ; in the forced pupae no male was 

 less than 16|- days, no female more than 15 days, the average 

 period in the male being 17 days, in the female 14^ days ; in 

 alniaria the male averaged 4 days more than the female. In 

 some summer illustraria the male pupal period averaged 12-2 

 days, the female 10-6 days. There appeared to be a very slow 

 progress of development during the icing period, 120 days in 

 the latter representing perhaps 2 or 3 at the ordinary tempe- 

 rature. Some incidental observations were added as to the 

 deterioration of illustraria by breeding ; this was not noticed 

 until the third generation, and had been reversed in some 

 cases where eggs were sent to Wimbledon, and produced 

 moths considerably larger than their parents ; the causes were 

 obscure, and rather pointed to the desirability of a change of 

 diet in successive generations. The experiments did not lend 

 any support to the view that illustraria could pass a long 

 winter in any other than the pupal stage. 



Lord Walsingham observed that it appeared that exposure 

 to cold in the pupa-state produced darker colouring in the 

 imago, and that forcing in that stage had an opposite effect ; 

 that insects subjected to glacial conditions probably derived 

 some advantage from the development of dark or suffused 

 colouring, and that this advantage was, in all probability, the 

 more rapid absorption of heat. He said he believed that an 

 hereditary tendency in favour of the darker forms would be 

 established under glacial conditions, and that this would 

 account for the prevalence of melanic forms in northern 

 latitudes and at high elevations. 



Mr. Elwes, Mr. Jenner Weir, Dr. Sharp, and others con- 

 tinued the discussion. 



