5 70 Studies in the Theory of Descent. 



Salamandridce? published some years ago by 

 Strauch, 10 this author, following Cope, 11 gives 

 twenty species of Amblystoma as inhabiting North 

 America. Although some of these species are 

 based on single examples, and consequently, as 

 Strauch justly remarks, " may well have to be 

 reduced in the course of time," there must never- 

 theless always remain a large number of species 

 which live and propagate as true Amblystomas, 

 and of which the habitat extends from the latitude 

 of New York to that of New Mexico. There are 

 therefore true species of Siredon which regularly 

 assume the Amblystoma form under their natural 

 conditions of life, and which propagate in this 

 form, while, on the other hand, there are at least 

 two species which, under their existing natural con- 

 ditions of life, always propagate as Siredon. It is 

 but another mode of expression for the same facts 

 if we say that the Mexican Axolotl and the Paris 

 Siredon whether this is Lichenoides or some other 

 species stand at a lower grade of phyletic 

 development than those species of Amblystoma 

 which propagate in the salamander form. No one 

 can raise any objection to this statement, while the 

 alternative view maintained by all authors contains 

 a theory either expressed or implied which is, as I 

 believe, incorrect, viz. that the Mexican Axolotl 



10 Proc. Acad. Philadelph. xix. 1867, pp. 166 209. 



11 Mdm. Acad. Petersb. vol. xvi. 



