622 Studies in the Theory of Descent. 



woods surrounding the Mexican lakes were in- 

 habited by Amblystomas, which, as the lakes sub- 

 sequently became more and more dried up and the 

 air continually lost moisture, found it more diffi- 

 cult to exist on the land. They would at length 

 have completely died out, had they not again 

 become aquatic by reversion to the Ichthyodeous 

 form. It may perhaps be supposed that the 

 above-mentioned physical conditions desolate, 

 salt-incrusted shores co-operated in the pro- 

 duction of the reversion, by making it difficult for 

 the larvae to quit the water ; but we can only judge 

 with certainty upon this point when, by means of 

 experiment, we have discovered the causes which 

 produce reversion in the Amphibia. 



ADDENDUM. 



I have lately met with another interesting 



notice on the reproduction of the native North 



American Amblystomas. Professor Spence F. 



Baird, of Washington, has often observed the 



thereby indicating the period of excessive rainfall which, ac- 

 cording to Mr. Alfred Tylor, succeeded to and was a 

 consequence of the thawing of the great glaciers which ac- 

 cumulated during the last glacial epoch. There is abundant 

 evidence to show that during the latter period glacial action 

 extended in North America at least as far south as Nicaragua. 

 See Belt on "The Glacial Period in North America," Trans. 

 Nova Scotian Inst. of Nat. Sci. 1866, p. 93, and "The 

 Naturalist in Nicaragua," pp. 259 265. R.M.] 



