214 AMPHIBIA 



only some of the young axolotls mentioned above as reared at 

 Paris underwent the metamorphosis into Amblystoma, the rest, 

 kept under apparently the same conditions, remained water- 

 breathing axolotls. 



Various theoretical explanations of the history of the axolotl 

 have been maintained by evolutionists. The most extreme of 

 them, namely that the change of the axolotl into the Ambly- 

 stoma is a new evolution happening for the first time must be 

 rejected at once, for a new form could not be expected to be 

 identical with one already existing. The axolotl must have 

 descended from normal Amblystomas and under certain condi- 

 tions have reverted to its ancestral form. Weismann went so 

 far as to maintain that the axolotl itself was to be regarded as 

 a reversion to the original aquatic ancestor of the Amphibia, 

 but we know that the larva in the possession of limbs of the 

 terrestrial type differs essentially from the ancestral fish. We 

 are thus forced to accept the view that the case is merely the 

 loss of the adult stage of development and the persistence of 

 the larval stage ; this phenomenon has been called neoteny from 

 neos young, and teino extend, the extension of youth, of the 

 young state. The term, however, while expressing the facts, 

 is no explanation, we have to consider what are the causes of 

 the phenomenon. The case of the axolotl is not unique, it is 

 known that the larvae of other Urodela occasionally grow to a 

 larger size and even become sexually mature without losing 

 their gills ; the larvae of the spotted salamander usually meta- 

 morphose when four centimetres long but occasionally they 

 reach a length of eight centimetres or about three inches in the 

 larval stage ; larvae of Triton have been found eight or nine 

 centimetres long with functional gills and sexually mature ; in 

 one lake in Lombardy such gill-breathing mature specimens 

 occur constantly, affording an instance of a European axolotl. 

 In the Anura also the larval stage may be prolonged, but no in- 

 stance is yet known in which they become sexually mature in this 

 condition. Further it has been shown that the metamorphosis 

 of axolotls can be determined or prevented by forcing them 

 to breathe air or preventing them from doing so. If they are 

 kept in shallow vessels in water insufficiently aerated they be- 

 gin to change into the perfect Amblystoma, if kept in deep 

 water with plenty of oxygen they remain axolotls. Thus we 



