584 Studies in the Theory of Descent. 



systems of organs, with true reversion. The 

 latter is a return to a form which has already been 

 once in existence ; but in the former case, in spite 

 of all simplification of the organization, some 

 entirely new feature always comes into existence. 

 But I am not able to see any absurdity in the 

 assumption that even true reversion, whether of a 

 whole species or of the individuals of a certain 

 district, may be regarded as possible, and I require 

 no further concession. Why, for example, should 

 it be inconceivable that at a very remote period 

 the Axolotl was adapted to a life on land ; that 

 through the direct and indirect action of changed 

 conditions of life it gradually acquired the sala- 

 mander form, but that subsequently, through new 

 and unfavourable changes in the conditions of 

 life, it again relapsed to the older form, or at least 

 to one nearly related thereto ? 



At any rate such an assumption contains 

 nothing opposed to known facts, but can be sup- 

 ported in many ways, and finally it commends 

 itself, at least in my opinion, as offering the only 

 admissible explanation of the facts before us. 



The existence of a whole series of species of 

 Amblystoma, as already mentioned, at once shows 

 that species of Siredon can become elevated into 

 the salamander form, and can propagate regularly 

 in this state, and further, that this phyletic ad- 

 vance has already actually taken place in many 

 species. 



