With respect to my question, whether we might not perhaps assume that at some post-glacial period the conditions of atmospheric moisture on the high plains of Mexico were essentially different from those at present prevailing, I recollected Dr. v. Frantzius and the above-quoted observation of Humboldt’s,[275] who discovered in the neighbourhood of the Lake of Tezenco (Mexico) distinct evidence of a much higher former level of the water. “All such elevated plains were certainly at a former period so many extensive water-basins, which gradually became filled, and are still filling up with detritus. The evaporation from such large surfaces of water must at that time have caused a very moist atmosphere, favourable to vegetation and adapted for the life of naked Amphibia.” From this side also my hypothesis thus receives support, and we may assume with some certainty that at the beginning of the diluvial period[276] the woods surrounding the Mexican lakes were inhabited by Amblystomas, which, as the lakes subsequently became more and more dried up and the air continually lost moisture, found it more difficult 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 production of the reversion, by making it difficult for the larvæ 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 development from the egg of various species, and especially of _Amblystoma Punctatum_ and _A. Fasciatum_. His observations do not appear to be as yet published, so that I was unable to discover any account of the development of _Amblystoma_ in existing literature.[277] I am authorized to extract the following brief data from a letter addressed to Dr. v. Frantzius. In order to deposit their eggs the Amblystomas go into the water, where the eggs are laid enclosed in a jelly-like mass, but never more than fifteen to twenty together. The spherical eggs are very large, perhaps a quarter of an inch in diameter. They soon develop into a _Siredon_-like larva, which remains several months in this condition. The gills then shrivel up, the creature begins to crawl, and gradually passes through the different transformations to the complete Amblystoma form. It appears from this communication that the Amblystomas lay much larger and much fewer eggs than the Axolotl, and that their development throughout resembles that of our salamanders.