156 Appendix. 



the generation which formerly hibernated being made 

 to recede step by step towards the autumn and the 

 summer. By a lengthening of the summer there occurs 

 therefore a retrogressive interruption of generations. 



The exact opposite occurs if the summer should become 

 shortened. In this case the last generation would no 

 longer be so far developed as formerly ; for instance, it 

 might not reach the pupal stage, as formerly, at the be- 

 ginning of winter, and would thus hibernate in a younger 

 stage, either as egg or larvae. Finally, by a continual 

 shortening of the summer it would no longer appear at 

 the end of this period but in the following spring ; in 

 other words, it would be eliminated. By a shortening of 

 the summer accordingly the interruption of generations 

 occurs by advancement. 



The following considerations, which submit themselves 

 with reference, to seasonal dimorphism, are readily con- 

 ceivable, at least, in so far as they can be arrived at by 

 purely theoretical methods. Were the summer to be- 

 come shorter the generation which formerly hibernated 

 in the pupal stage would be advanced further into the 

 spring. It would not thereby necessarily immediately 

 lose the winter characters which it formerly possessed. 

 Whether this would happen, and to what extent, would 

 rather depend upon the intensity of the action of the 

 summer climate on the generation in question, and on 

 the number of generations which have been submitted 

 to this action. Hitherto no attempts have been made 

 to expose a monomorphic species to an elevated tem- 

 perature throughout several generations, so as to obtain 

 an approximate measure of the rapidity with which 

 such climatic influences can bring about changes. For 

 this reason we must for the present refrain from all 

 hypothesis relating to this subject. 



The disturbance of generations by the shortening of 

 summer might also occur to a species in such a manner 



