ON CHANGE OF MEXICAN AXOLOTL TO AN AMBLYSTOMA. 361 



that through alterations in their conditions of life they have sunh hack to 

 their earlier stage of the Perennibranchiates. 



Without doubt I was first led to this intepretation by the results I 

 had reached in my studies ou the season-dimorphism of butterflies.* 

 There also we have the question of two different forms under which ono 

 and the same species appears, and of which it may be proved as prob- 

 able that one is phyletically the older, the other the younger. The 

 younger summer form has in my view proceeded through the gradual 

 warming of the climate from the winter form that in an earlier zoolo;4,i- 

 cal epoch was the only one; but this latier, the primary form, has not 

 on that account ceased to exist, but to-day still alternates every year 

 with the secondary, the summer form. 



Now it is easy with the season-dimorjUious butterflies to make the 

 summer brood assume the winter form, and that by exposing their 

 pupae rather a long time to a lower temperature; and it may be made in 

 the highest degree probable that this suddenly appearing variation or 

 transformation, often very far reaching, is sudden in appearance only, 

 and is but apparently the result of cold actiug upon this generation. 

 Much rather in truth the variation depends upon a reversion to the 

 primary form of the species, and therefore the cold that appears but 

 once is only the impulse to the reversion, but not the true cause of the trans- 

 formation. This cause must be sought in the long-continued operation 

 of the cold, to which thousands of generations of the ancestors of our 

 living butterflies were subjected, and whose final result was just the 

 winter form. 



If we now assume for a moment that my interpretation of the Axolotl's 

 transformation-history as just given is correct, we have relations here 

 which are in many respects analogous to those of the season-dimorphism. 

 To be sure the two forms in this case no longer alternate regularly with 

 each other, but the primary form may occasionally appear in place of the 

 secondary, and this result from the shock of external circumstances. 

 As there we succeed in stimulating the summer brood by the action of 

 cold to lay aside the summer form and adopt the winter dress, so here 

 we are able to lead the Axolotls iuto the Amblystoma state by com- 

 pelling them at a certain age to bre ithe air. 



Further, as in the season-dimorphism, it may be shown that this 

 transformation called forth by artificial means is only in appearance a 

 sudden new formation, but in truth a reversion to the much older win- 

 ter form, so here we also had to do, not with an actual new formation of 

 the species, but only with an apparent one, a reversion to the phyletically 

 older form of the species. To be sure that sounds very paradoxical if 

 here is a form that must have come by reversion, and yet it must 

 undoubtedly be accounted as the more highly developed. But I believe 



• Studien zur Desceudenztheorie, I. Ueber den Saison -Dimorphism der Schmetter- 

 linge (Studies on the Doctrine of Descent, I. On the Season-Dimorphism of Butter- 

 flies), Leipsic, 1875. 



