6 1 2 Studies in the Theory of Descent. 



existence by its occasional appearance in a re- 

 version form. Thus, under certain conditions the 

 Triton sinks back to the perennibranchiate stage ; 

 not in such a manner that the individual first 

 becomes a Triton and then undergoes perenni- 

 branchiate re-modification, but simply, as I have 

 already shown above, by its remaining at the 

 Ichthyodeous stage and no longer attaining to the 

 Salamander form. So also, according to my 

 hypothesis, the salamandrine Amblystoma Mexi- 

 canum, formerly inhabiting the shores of the 

 Lake of Mexico, has degenerated to the perenni- 

 branchiate stage, and the only trace that remains 

 to us of its former developmental status is the 

 tendency, more or less retained in each individual, 

 to again ascend to the salamander stage under 

 favourable conditions. 



The third and last consequence which my in- 

 terpretation of the facts entails, is the change in 

 the part played by reversion in organic nature. 

 Whilst atavistic forms have hitherto been known 

 only as isolated and exceptional cases, interesting 

 indeed in- the highest degree, but devoid of sig- 

 nificance in the course of the development of or- 

 ganic nature, a real importance in this last respect 

 must now be attached to them. 



I may assume that reversion can in two ways 

 be effectual for the preservation or re-establish- 

 ment of a living form. In the first place, where, 

 as in Axolotl, the new and organically higher 



