ES 
much attention to the pupal stage, and I will first address myself 
to that stage. Some of the most striking of these effects have been 
caused by the application of extreme temperatures, such as the 
organism would rarely or never be exposed to in nature, and, 
though the results have been exceedingly valuable in, it may be 
said, helping to establish the mutual relations of diverse species, 
and in affording a basis for theories as to their phylogeny and 
their future evolution, I submit that this class of experiment can 
throw but an insufficient light on the processes by which, with 
normal temperatures proper to the seasons of the year, seasonal 
dimorphism has actually been caused. 
PUPAL STAGE IN A. levana. 
Great changes of aspect, as is well known, can be caused by 
subjecting the pupa of the summer phase of A. levana to a low 
temperature; by this treatment the imago may be completely 
converted in general aspect from the frorsa to the levana form, or 
to the intermediate form called porima. But as regards life- habit, 
— that is the conversion of it by such means, if applied only while 
in the pupal stage, into the winter phase with ¿ts characteristic of 
the normal long pupal period, — this has not that I know of been 
found practicable; it is certainly extremely difficult, and has 
alwavs failed with me. The actual pupal period has indeed in such 
cases been greatly lengthened by the low temperature, but it has 
never in my experience produced that state of resolute resistance 
when afterwards exposed to a warmer temperature, — a resistance 
often to the death (1), — which the normal winter phase shows. 
And as to the pupa of the winter phase, the conversion of that 
into even the summer phase aspect or facies, that of prorsa, 
presents great difficulty. 
PUPAL STAGE IN S. bilunaria. 
I think I may say the same of S. bilunaria. Cooling the sum- 
mer phase protracts the pupal period and, if considerable in degree 
(1) See my presidential address, January 1006. (« Proc. Ent. Soc. Lond. », 
1905, XCII, XCIV-XCVI, CX, CXI.) 
