368 ON CHANGE OF MEXICAN AXOLOTL TO AN AMBLYSTOMA. 



the same time it involves a leap in an opposite direction, namely, a leap 

 away over a long succession of generations, back to an animal form, 

 which for a long time the species had not produced, which to a certain 

 extent had become foreign to it. 



We should therefore have here also the coincident eflFect upon tho 

 AxolotPs constitution of a widely different constitution, or — if one pre- 

 fers — the blending of two widely varying constitutions. 



Of course I am very far from wishing to give this explanation for an 

 exact one. It is nothing more than an attempt to point out the 

 force in which we are to seek for the cause of the varying extent to 

 which the power of reproduction is affected. To investigate more 

 deeply and to demonstrate specially in what manner this force comes 

 into operation must be left to a later time. For the present it must 

 suffice to have shown in this connection that in general an essential 

 difference exists between the two kinds of reversion, as well as to have 

 made it in some measure comprehensible that this difference may bo 

 the motive force in relation to the question of sterility. Perhaps the 

 law involved here may some day be formulated thus : Atavistic individ- 

 uals lose the potcer of reproduction the more compleiehj as the series of 

 generations of their ancestors is longer whose ontogenesis no longer contains 

 the ■phylctically older stage to which the reversion takes place. 



Consequently our hypothesis, which regards the transformation of tho 

 Axolotl as a reversion, offers us at once the possibility of learning to 

 understand the sterility of the Amblystomas produced from Axolotls. 

 So on the contrary the observed sterility of these Amblystomas is for 

 those who think that a phyletic vital energy was here exertf d — let it be 

 named whatever it may — not only " a veritable scientific enigma," as 

 DuMi^RiL expresses himself, but a complete parodox. Of such a design- 

 ing, impelling principle it ought to be expected that it would bring forth 

 new forms capable of life and not decayed to the point of dying out ; and 

 this so much the more since there is concerned a combination of struc- 

 tural peculiarities, which, when it has originated in another way (namely, 

 from other species of Siredon), has already long ago shoivn itself capahle of 

 life and reproduction. We know species of Amblystoma which reproduce 

 as such, and every one of which comes from a larva like the Axolotl. One 

 cannot, therefore, explain the sterile Amblystomas which the Paris Axo- 

 lotl produces as an unsuccessful effort of the vital energy— an explana- 

 tion, which to be sure, would be in and for itself sufficiently presump- 

 tuous. 



But now it may be asked, What change in the conditions of life could it 

 have been which made the Amblystoma in the Lale of Mexico* revert to 

 the Siredon form ? To be sure, I can only answer this question with con- 

 jectures, which can claim but a limited value so long as they cannot be 

 sustained by a more exact knowledge of the circumstances there and 

 the habits both of the Axolotl and the Amblystoma. 



*As we Ao not know the origin of the Paris Axolotl, I must confine myself in tlie 

 foUowins: to Siredon Mcxlcanus fcjhaw. 



