ox CHANGE OF MEXICAN AXOLOTL 10 AN AMBLYSTOMA. 373 



briu^' iu the Siredon 2l€xlcanus as aa unwelcome addition under the 

 genus AjnhJijstoma. So long as there are not one only, but several species 

 of tSiredon ou the earth, which are regularly reproduced as such and only 

 as such, so long the genus exists. And if we would not quite rob the 

 systematic writers of the hope that some time these species of Siredon 

 would rise to be Amhlystomas, yet it corresponds better to the state of 

 things now existing ou the earth if we still allow the genus Siredonto stand 

 among the genera of fish-salamanders and reckon iu it all those species 

 which, like the Paris Asolotl, the Siredon Mexicamis Shaw, and proba- 

 bly also Siredon JicJicnoides, only take the Amblystoma form as an excep- 

 tion or by artificial influences, but without reproducing themselves in it. 



Ou the other hand, all those species may probably be added to the 

 genus Amblystoma which reproduce iu this state, and in which the 

 Perennibranchiate slage appears only as a larval condition. 



To make this distinction in the individual case will be chiefly the task 

 of the American naturalists, from whose ever-increasing activity we may 

 hope indeed for fuller details on the reproduction of the numerous spe- 

 cies of AmhJystoma iu their native land. I should rejoice if my expla- 

 nations here presented should give an impulse to such investigations. 



The second corollary to which I referred is of a purely theoretical 

 nature. It concerns an addition to the ^^fundamental late in the genesis 

 of life,'^ first set forth by Fritz Miiller and Haeckel. It is well known 

 that this is stated in the following proposition : The Ontogenesis contains 

 iu itself the Phylogenesis, more or less contracted, more or less modi- 

 fied. xVIthough the proposition cannot be rigidly proved, because we 

 have no means of seeing the phyletic development directly unfolded be- 

 fore our eyes, yet its correctness and general validity cau be made so 

 highly probable, in an indirect w^ay, that few naturalists of the present 

 time doubt it who have occupied themselves with the history of devel- 

 opment and comparative morphology. 



Xow, according to this proposition, every stage of the phyletic devel- 

 opment, when it is displaced by a later one, must remain included in 

 the Ontogenesis, and therefore come to light in the form of an ontoge- 

 netic stage in the development of every individual. Now, with this my 

 explanation of the Axolotl's transformation appears to stand in contra- 

 diction, for the Axolotl which had in former generations been an Am- 

 blystoma contains nothing of the Amblystoma in its Ontogenesis. Never- 

 theless the contradiction is only apparent. As soon as a further develop- 

 ment is actually in question, and therefore the attainment of a new stage 

 not yet realized, so soon the older stage is taken up into the Ontogen- 

 esis. But it is not so when the new is not actually new, but has at a 

 former time i^reseuted the final stage of the individual development, or, 

 in other words, when there is a reversion, not of the single individual, 

 but of the species as such to the preceding phyletic stage, and, therefore, 

 a -phyletic sinking back of the species. In this case the final stage of the 

 Ontogenesis is simply eliminated, it falls ont, and ice can only recognize its 



