602 Studies in the Theory of Descent. 



the influence of an aquatic life there can be no 

 doubt, and thus, in accordance with my supposition, 

 the hypothetical Amblystoma Mext'canum, the 

 supposed ancestral form of the Axolotl of the 

 Mexican Lake, might have been caused to revert to 

 the perennibranchiate form by a reduction in the 

 possibilities of its living upon land, and by its being 

 compelled to frequent the water. 



I will not here return to the consideration of 

 every other opinion ab initio. It is very advisable 

 to distinguish between the mere impulses which 

 are able to produce sudden reversion, and between 

 actual transforming causes which result directly or 

 indirectly in the remodelling of a species. Thus, 

 it is conceivable h priori that reversion may occur 

 by the action of an inciting cause having nothing 

 to do with the origin of the phyletically older form. 

 Temperature can certainly have played no part, or 

 only a very small part, in the formation of the 

 perennibranchiate form ; nevertheless cold may 

 well have been one of the inciting causes which 

 induced the Amblystoma at one time to revert 

 to the Siredon form, and we cannot at present 

 consider De Saussure to be incorrect when he 

 maintains that the low temperature of the Mexican 

 winter might prevent that transformation (of the 

 Axolotl into the Amblystoma) which would occur 

 " in the warm reptile-house " of the Jardin des 

 Plantes. He supports this view by stating that 

 " Tschudi has found the Amblystoma " (of course 



