2 8 Studies in the Theory of Descent, 



of pupation, and in the next place immediately 

 after that period. His experiments were made, 

 however, with such a small number of specimens 

 that scarcely any safe conclusion can be founded 

 on them ; still, this conclusion may be correct, in 

 so far as everything depends on whether, from 

 the beginning, the formative processes in the 

 pupa tended to this or that direction, the final 

 result of which is the Prorsa or Levana form. If 

 once there is a tendency to one or the other 

 direction, then temperature might exert an accele- 

 rating or a retarding influence, but the tendency 

 cannot be further changed. 



It is also possible indeed, probable that a 

 period may be fixed in which warmth or cold 

 might be able to divert the original direction of 

 development most easily ; and this is the next 

 problem to be attacked, the answer to which, now 

 that the main points have been determined, should 

 not be very difficult. I have often contemplated 

 taking the experiments in hand myself, but have 

 abandoned them, because my materials did not 

 appear to me sufficiently extensive, and in all 

 such experiments nothing is to be more avoided 

 than a frittering away of experimental materials 

 by a too complicated form of problem. 



There may indeed be a period most favour- 

 able for the action of temperature during the first 

 days of the pupal stage ; it appears from Experiment 

 No. 12 that individuals tend in different degrees 



