1 54 Appendix. 



one of the generations now supposed to be originating, 

 or as one now supposed to be disappearing. The 

 latter may be admitted. Somewhat further north this 

 generation would be entirely suppressed, and the third 

 brood would hibernate, either in the imago state or as 

 pupse or caterpillars. In Virginia this third generation 

 consists of both forms. We may expect that further 

 north, at least, where it hibernates as pupae, it will con- 

 sist entirely, or almost entirely, of the var. Umbrosa. Still 

 further north in the Catskill Mountains, as Edwards 

 states from his own observations, the species has only two 

 generations, and one might expect that the var. Um- 

 brosa would there occur as the winter generation. 



Should the foregoing be correct, then the fact that the 

 second generation assumes the Umbrosa form by the 

 action of cold, as was the case in Edward's experiments, 

 becomes explicable. The new marking peculiar to this 

 form produced by this means must be regarded as a com- 

 plete reversion to the true winter form, the characters 

 of which are becoming partly lost as this generation is 

 no longer exposed to the influence of winter, but has 

 become advanced to the beginning of summer. 



The foregoing explanation is, of course, purely hypothe- 

 tical ; it cannot at present be asserted that it is the correct 

 one. Many investigations based on a sufficiently large 

 number of facts are still necessary to be able to attempt 

 to explain this complicated case with any certainty. 

 Neither should I have ventured to offer any opinion on 

 the present case, did I not believe that even such a pre- 

 mature and entirely uncertain explanation may always 

 be of use in serving the inventive principle, i. ^., in 

 pointing out the way in which the truth must be 

 sought. 



As far as I know, no attempt has yet been made to 

 prove from a general point of view the interpolation of 

 new generations, or the omission of single generations 



