AND OF LIGHT. 241 



in almost all the observations above referred to, exposed 

 his pupse to warmth for about three days, and then kept 

 them at the room temperature until they emerged, this 

 generally occurring 4 to 10 days later. The exposure 

 to cold generally extended to about 30 days, and emer- 

 gence took place after about 11 days more at room tem- 

 perature. As the effects obtained by him are just as 

 great, if not greater, than those obtained by other ob- 

 servers, it would seem quite clear that in the forms he 

 employed the critical time for temperature is cer- 

 tainly during the first portion of the pupal period. 

 However, Merrifield, * in his observations on the sum- 

 mer and winter forms of P. napi, found the critical time 

 to be in the last days of the pupal period, a directly 

 opposite result to that of Weismann for the same insect. 

 Weismann f explains the apparent contradiction by 

 supposing that in P. napi adaptive and direct sea- 

 sonal dimorphism are mixed. The species may have 

 adapted itself to the seasons of the year by a double 

 protective colouring, and the critical period for the de- 

 termination of the adaptive form may be at the begin- 

 ning of the pupal period. The direct reaction of the 

 species to temperature may, however, as Merrifield 

 found, be determined only at the end of the pupal 

 period. 



In his experiments with P. phlceas, Merrifield found 

 that pupse kept at 0.5 C. for ten weeks, and then ex- 

 posed to a temperature of 32 for six days, gave speci- 

 mens with features very similar to those obtained from 

 pupae kept throughout at a temperature of 27 to 32. 



" * Trans. Ent. Soc., 1893, p. 55. 

 f The Entomologist, 1896, p. 240. 



