The Transformation of the Mexican Axolotl. 611 



is of a purely theoretical nature, and concerns a 

 corollary to the " fundamental biogenetic law " 

 first enunciated by Fritz M tiller and Haeckel. 

 This, as is well known, consists of the following 

 law : The ontogeny comprises the phylogeny, 

 more or less compressed and more or less modified. 

 Now according to this law, each step in phyletic 

 development when replaced by a later one, must 

 remain preserved in the ontogeny, and must there- 

 fore appear at the present time as an ontogenetic 

 stage in the development of each individual. But 

 my interpretation of the transformation of the 

 Axolotl appears to stand in contradiction to this, 

 since the Axolotl, which at a former period was an 

 Amblystoma, retains nothing of the latter in its 

 ontogeny. The contradiction is, however, only 

 apparent. As long as we are concerned with an 

 actual advance in development, and therefore with 

 the attainment of a new step never formerly 

 reached, the older stages will be found in the 

 ontogeny. But this is not the case when the new 

 stage is not an actual novelty, but formerly repre- 

 sented the final stage of the individual develop- 

 ment ; or, in other words, when we are concerned 

 with the reversion, not of single individuals, but of 

 the species as such, to the preceding phyletic 

 stage, i.e. with a phyletic degeneration of the 

 species. In this case the former end-stage of the 

 ontogeny would be simply eliminated, and we 

 should then only be able to recognize its fonm-r 



