On the Seasonal Dimorphism of Butterflies. 3 7 



A. Levana, for, as already explained, not only 

 may the development of the winter form be forced 

 to a certain degree by artificial warmth, but the 

 summer generation frequently produces reversion- 

 forms without protraction of development. The 

 intermediate reversion-form Porima was known 

 long before it was thought possible that it could 

 be produced artificially by the action of cold ; 

 it appears occasionally, although very rarely, at 

 midsummer in the natural state. 



If, then, my explanation of the phenomena is 

 correct, the winter form is primary and the 

 summer the secondary form, and those individuals 

 which, naturally or artificially, assume the winter 

 form must be considered as cases of atavism. The 

 suggestion thus arises whether low temperature 

 alone is competent to bring about this reversion, 

 or whether other external influences are not also 

 effective. Indeed, the latter appears to be the 

 case. Besides purely internal causes, as previously 

 pointed out in P. Ajax, warmth and mechanical 

 motion appear to be able to bring about rever- 

 sion. 



That an unusually high temperature may cause 

 reversion, I conclude from the following observa- 

 tion. In the summer of 1869 I bred the first 

 summer brood of A. Levana; the caterpillars 

 pupated during the second half of June, and from 



to the 6th of June, fifty-eight butterflies of the form Marcellus, 

 one, of Walshii, and one of Telamonides. 



