ON CHANGE OF MEXICAN AXOLOTL TO AN AMBLYSTOMA. 369 



lu geueral it may be presumed that the same external influences 

 demanded the reversion ichich at an earlier time called forth the formation 

 of the Percnnibranchiate stage. 



In favor of this conjecture we may adduce the experiments here com- 

 municated, for evidently it is the stimulus of the air breathing which 

 brings the young Axolotls to the reversion to the Amblystoma form, 

 i. e., the same stimulus under whose dominating influence the Ambly- 

 stoma form must have arisen. 



But the case is quite similar with the season-dimorphous butterflies. 

 There a reversion of the summer brood to the winter form is most easily 

 called forth by the operation of cold, i. e., by the same influences under 

 whose control the winter form was developed. We know, at any rate, 

 that reversion may arise also from the crossing of races and species, 

 and I sought to show that reversion in butterflies may also be called 

 forth by other influences than cold. But the most probable presumption 

 is evidently the one that the reversion was induced through a recurrence 

 of the same cause which to a certain extent produced the Perenni- 

 brauchiate form. That this form has been shaped under the influence 

 of life in the water admits of no doubt; and so my conjecture is that the 

 hypothetical Amblystoma Mexicamnn, the supposed ancestral form of 

 the present Axolotl of the Lake of Mexico, must have been brought to 

 revert to the Perennibranchiate form by the circumstance that the possihility 

 of going on land was tahen from it and it was constrained to stay in the 

 water. 



I will not reject beforehand every other opinion. We must carefully 

 distinguish between the mere provocations which are able to produce 

 sudden reversion and actual causes of variation which have for their 

 result, directly or indirectly, the molding of a species. So it would 

 not be inconceivable a priori that reversion should occur through the 

 working of an impulse which has nothing to do with the origin of the 

 phyletically older form. Certainly temperature has had no share, or 

 only a very slight one, in the fashioning of the Perennibranchiate form. 

 Yet cold, in and for itself, might quite well be one of the provocations 

 which one day caused the Amblystoma form to revert to the Siredon 

 form ; and one could not a priori contradict De Saussure when he 

 expresses the opinion that the low temperature of the Mexican winter 

 must hinder the transformation (of the Axolotl to the Amblystoma) 

 which had taken place " in the hot reptile room'- of the Paris Jardin des 

 Plantes. He supports his view by the fact that " Tschudi has found 

 the Amblystoma" (of course another species) "in the hottest part of the 

 United States." " On the plateau of Mexico, however, it snows every 

 winter : and although the lake does not freeze, its temperature must sink 

 very low near the surface." 



But although no theoretical considerations oppose this view, yet I do 

 not think it correct. I question very much that it was the temperature 

 which induced the change from the Amblystoma back to the Axolotl, 

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