1 8 Studies in the Theory of Descent. 



duration of the pupal stage the Levana form must 

 result. On one occasion the chrysalis stage was 

 considerably shortened in this brood by the 

 continued action of a high temperature, many 

 specimens thus having their period of develop- 

 ment reduced from six to three months. The 

 supposed explanation above given is, however, in 

 reality no explanation at all, but simply a restate- 

 ment of the facts. The question still remains, why 

 the third brood in particular has no tendency to 

 be accelerated in its development by the action of 

 heat, as is the case with both the previous broods ? 

 The first answer that can be given to this 

 question is, that the cause of the different action 

 produced by a similar agency can only lie in the 

 constitution, i. e., in the physical nature of the 

 broods in question, and not in the external in- 

 fluences by which they are acted upon. Now, 

 what is the difference in the physical nature of 

 these respective broods ? It is quite evident, as 

 shown by the experiments already described, that 

 cold and warmth cannot be the immediate causes 

 of a pupa emerging in the Prorsa or Levana form, 

 since the last brood always gives rise to the 

 Levana form, whether acted on by cold or warmth. 

 The first and second broods only can be made to 

 partly assume, more or less completely, the Levana 

 form by the application of cold. In these broods 

 then, a low temperature is the mediate cause of the 

 transformation into the Levana form. 



