First, with regard to more obvious results. If _Siredon Mexicanus_, Shaw, only by occasional reversion assumes the Amblystoma form, and never, or only exceptionally, propagates as such, but only as _Siredon_, the more recent systematists are not justified in striking out the genus _Siredon_ and in placing _S. Mexicanus_ as an undeveloped form in the genus _Amblystoma_. So long as there exists not one only, but several species of _Siredon_ which as such regularly propagate themselves, the genus exists; and although we would not deprive systematists of all hope of these species of _Siredon_ being one day re-elevated to _Amblystomæ_, it nevertheless better accords with the actually existing state of affairs if we allow the genus _Siredon_ to remain as before among the genera of _Salamandrina_, and to include therein all those species which, like the Paris Axolotl, _S. Mexicanus_, Shaw, and probably also _S. Lichenoides_, Baird, only exceptionally, or through artificial influences, assume the Amblystoma form, but without propagating regularly in this condition. On the other hand, we should correctly comprise under the genus _Amblystoma_ all those species which propagate in this state regularly, and in which the perennibranchiate stage occurs only as a larval condition. To arrive at a decision in single cases would chiefly concern the American naturalists, whose ever increasing activity may lead us to hope soon for a closer investigation of the reproduction of the numerous species of _Amblystoma_ of their native country. I should rejoice if the facts and arguments which I have here offered should give an impetus to such researches. The second consequence to which I may refer, is of a purely theoretical nature, and concerns a corollary to the “fundamental biogenetic law” first enunciated by Fritz Müller 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.