6o8 Studies in the Theory of Descent. 



of individuals had their metamorphosis delayed 

 till the second summer ; notwithstanding this, 

 transformation then occurred. 



It cannot be objected to my reversion hypothesis, 

 that it opposes on the one side what on the other 

 it postulates, viz. zpersaltum change of structure. 

 Reversion is characterized by the sudden acquisi- 

 tion of an older, i.e. a formerly existing phyletic 

 stage. That reversion occurs is a fact, whilst 

 nobody has hitherto been able to prove, or even to 

 make probable, that a stage of the future (sit 

 venia verbo) has been attained at once (per 

 salt urn}. 



Now if it is possible to find influences in the 

 present conditions of life of the Axolotl which make 

 it difficult or quite impossible for it to live upon 

 land, and which therefore appear as incentives to 

 the reversion to the Ichthyodeous form, the other 

 portion of my hypothesis the assumption that the 

 Axolotl had become an Amblystoma at a former 

 period can also be supported by facts. 



We know from Humboldt 41 that the level of 

 the Lake of Mexico at a comparatively recent 

 period was considerably higher than at present. 

 We know further that the Mexican plateau was 

 covered with forest, which has now been destroyed 

 wherever there are human, and especially Spanish 

 settlements. Now if we suppose that at some 

 post-glacial period the mountain forests extended 

 41 See Miihlenpfordt's work already quoted, vol. i. 



