The Transformation of the Mexican Axolotl. 613 



form becomes untenable through external in- 

 fluences, instead of simply perishing since ad- 

 vancement in another direction does not appear to 

 be possible a reversion of the species to the 

 older and more lowly organized stage occurs. In 

 the second place, the older phyletic form may 

 not be abandoned while a newer form is being 

 developed therefrom, but the former may alternate 

 with the latter, as we see in the case of seasonally 

 dimorphic butterflies. It can hardly be objected 

 if I regard the alternation of the summer and 

 winter form in this case as a periodic reversion to 

 the phyletically older (winter) form. 



Although the reversion of an entire species, 

 such as I suppose to have been the case with the 

 Axolotl, may be of rare occurrence, this is cer- 

 tainly not the case with periodic or cyclical re- 

 version ; the latter plays a very important part in 

 the development of the various forms of alternating 

 or cyclical propagation. 41 



POSTSCRIPT. 



In the previous portion of this essay it was 

 pointed out that the causes to which I attributed 

 the reversion of the hypothetical Amblystoma 

 Mexicanum to the existing Axolotl, did not ap- 

 pear to me to amount to a complete explanation 



* In the province of botany such a case has already been 

 made known by Fritz Miiller (Botan. Zeitung, 1869, p. 226; 



