POLYMORPHISM IN SPECIES. 187 



matter, which, possessing a continuous life itself, 

 develops periodically into the different forms of 

 successive generations of butterflies. We have seen 

 that the periodicity of the root-pressure does not 

 cease immediately upon the removal of the stimuli 

 which originally produced it ; in the same way we 

 observe that the periodic development of the winter 

 form of the butterfly persists in spite of the applica- 

 tion of heat. The fact, however, that the winter 

 form, as we have seen, may be somewhat affected 

 by heat, strengthens the belief that the periodic 

 difference of development is due to co-ordinations 

 of a trophic-nervous nature, which have become in- 

 grained in the living continuity of matter, by the 

 long exposure to the successive changes of summer 

 and winter. Whether the periodicity could be 

 changed by a long exposure to a different succes- 

 sion of stimuli, as was done with the root-pressure, 

 is an interesting though difficult problem ; but, 

 reasoning from analogy, we might look for a result 

 similar to that of the experiment on the root- 

 pressure. 



In the case of the dimorphic butterflies which we 

 have been considering, if the chrysalids which are 

 about to produce the summer form be subjected to 

 different degrees of cold, there will be developed 

 several forms intermediate in appearance between 



