THE AM ERIC AX NATURALIST. [Vol. XXXVI 1. 



prolong the larval, gill-bearing stage ; while unfavorable condi- 

 tions for aquatic life — e.g., the drying up of ponds, with forced 

 aerial respiration — are thought to be the regular causes of meta- 

 morphosis. Heat and light arc regarded by many as subsidiary 



This explanation of the metamorphosis of Amblystoma — and 

 of allied forms — as due to a direct response to changing condi- 

 tions of environment is traditional. It has received further sup- 

 port from casual observations of naturalists, who have seen these 

 animals undergoing metamorphosis on the mud of evaporating 

 ponds. But the final sanction which raised the hypothesis into 

 an almost universally accepted datum of science was Weissman's 

 great article, " On the Change of the Mexican Axolotl to 

 an Amblystoma." 1 This article was based upon Marie von 

 Chauvin's experiments with five larva; only. Later and much 

 more extensive experiments by the same person were much less 

 favorable to Weissman's conclusions. Indeed a careful study of 

 their methods and results seems to the writer rather to cast 

 doubt upon the entire conclusion that enforced air breathing 

 caused the metamorphosis of these supposed Axolotls. But a 

 consideration of these experiments in detail is bevond the scope 



in the vicinity of Crete, Saline County, Nebraska. 



First, metamorphosis, in the writer's vicinity, occurs rarely if 

 at all as the obvious result of enforced air breathing through the 

 drying up of ponds. Diligent search has been made; in one 

 summer over one hundred ponds and larger pools were examined. 

 Many contained larva? ; but none were found in ponds or pools 

 that were less than one foot in depth. The last remnants of 

 large ponds, where larva? had been abundant a few weeks before, 

 showed no trace of them, although the water might be alive 

 with the tadpoles of the common frog. Experiment showed 

 that Amblystoma larva? could not usually withstand the temper- 

 ature of very shallow water exposed to Nebraska sunlight in 

 June, while the tadpoles of Rana were unharmed by it. 



