6oo Studies in tke Theory of Descent. 



prises the phyletically older stage to which the 

 reversion takes place. 



The hypothesis which interprets the transforma- 

 tion of the Axolotl as a case of reversion, thus holds 

 out the possibility of our being able to comprehend 

 the sterility of the Amblystomas arising in this 

 manner, whilst, on the other hand, for the adherents 

 of a phyletic vital force, not only is this observed 

 sterility as DumeYil expresses it " un veritable 

 6nigme scientifique," but an absolute paradox. 

 We should expect such a directive and inciting 

 principle to call into existence new forms having 

 vitality and not destined to perish, the more so 

 when it is concerned with a combination of 

 structural characters which, when originating in 

 another manner (viz. from other species of Siredon), 

 have long since shown themselves to have vitality 

 and reproductive power. We are indeed acquainted 

 with species of Amblystoma which propagate as 

 such, and each of which arises from an Axolotl-like 

 larva. Thus we cannot regard the sterile Ambly- 

 stomas produced by the Paris Axolotls as abortive 

 attempts of a vital force an interpretation which 

 is certainly in itself already sufficiently rash. 



Now if it be asked what change in the conditions 

 of life could have led to the reversion in the Lake 



of Mexico 35 of the Amblystoma to the Siredon 







** As we do not know the origin of the " Paris Axolotl " I 

 must restrict myself in the following remarks to Siredon Mexi- 

 canus (Shaw). 



