The Transformation of the Mexican AxolotL 581 



have been developed in captivity in certain in- 

 stances from Siredon Mexicanus (S. Pisct/brmts) , 

 as well as from the Paris Axolotls, are not pro- 

 gressive, but reversion forms ; I believe that the 

 Axolotls which now inhabit the Mexican lakes 

 were Amblystomas at a former geological (or 

 better, zoological) epoch, but that owing to 

 changes in their conditions of life, they have 

 reverted to the earlier perennibranchiate stage. 



I was undoubtedly first led to this conception 

 by the results which arose from my studies on the 

 seasonal dimorphism of butterflies."* In this case 

 we were also concerned with the two different 

 forms under which one and the same species 

 appears, and of which it was shown to be probable 

 that the one is phyletically older than the other. 

 The younger summer form, according to my view, 

 has arisen, through the gradual amelioration of the 

 climate, from the winter form, which at an earlier 

 zoological epoch was the only one in existence ; 

 but the latter, the primary form, has not for this 

 reason ceased to exist, but now alternates in each 

 year as a winter form with the secondary summer 

 form. 



Now with seasonally dimorphic butterflies, it 

 was easily possible to induce the summer brood to 

 assume the winter form by exposing their pupae 

 for a long time to a low temperature ; and it was 

 shown to be highly probable that this abrupt and 

 " See Part I. of this volume. 



