22 Studies in the Theory of Descent. 



have shown, however, that this is not the case, but 

 rather that the last generation bears an ineradi- 

 cable tendency to take the Levana form, and is 

 not susceptible to the influence of warmth, however 

 long continued ; while both summer generations, 

 on the contrary, show a decided tendency to assume 

 the Prorsa form, although they certainly can be 

 made to assume the Levana form in different 

 degrees by the prolonged action of cold. 



The conclusion seems to me inevitable, that the 

 origination of the Prorsa form was gradual that 

 those changes which originated in the chemistry 

 of the pupal stage, and led finally to the Prorsa 

 type, occurred very gradually, at first perhaps 

 remaining completely latent throughout a series 

 of generations, then very slight changes of mark- 

 ing appearing, and finally, after a long period 

 of time, the complete Prorsa type was produced. 

 It appears to me that the quoted results of the 

 experiments are not only easily explained on the 

 view of the gradual action of climate, but that this 

 view is the only one admissible. The action of 

 climate is best comparable with the so-called 

 cumulative effect of certain drugs on the human 

 body ; the first small dose produces scarcely any 

 perceptible change, but if often repeated the effect 

 becomes cumulative, and poisoning occurs. 



This view of the action of climate is not at all 

 new, most zoologists having thus represented it ; 

 only the formal proof of this action is new, and 



