II THE AXOLOTL 47 



the Axolotl its larva. If it is not formed, the latter is the 

 adult animal capable of sexual reproduction. 



Since, then, in the spotted Salamander and its allies exactly 

 the same metamorphosis in structure occurs in the passage 

 from larval life to the adult condition as in the transition 

 from the Axolotl to the Amblystoma, we have in these 

 amphibia again clearly revealed definite directions of evolution. 



Weismann has offered another explanation of the develop- 

 ment of Amblystoma out of Siredon, namely, that it is a 

 reversion to an earlier form. But no cogent reason for this 

 hypothesis seems to me to have been given, any more than 

 there is any reason to suppose that the adult Salamander is 

 to be regarded as a reversion to an earlier form. What is 

 true in the one case is reasonable in the other. The only 

 difference I can see in the two cases, besides the sexual 

 immaturity of the Salamander larva, is that in the spotted 

 Salamander and its nearest allies the metamorphosis into an 

 exclusively air-breathing animal has become the rule, \vhile 

 in Siredon it is only commencing, and occurs at first in rare 

 individual cases. 



If this be so, then, as I have said, the metamorphosis of 

 Siredon pisciformis into Amblystoma is evidently the most 

 perfect example of the occurrence of the evolution fer saltum 

 of a species of animal now living, in consequence of the action 

 of external conditions. 



My view of the meaning of the metamorphosis of the 

 Axolotl is supported by the experiments of Fraulein von 

 Chauvin. That lady reared larvae which were the progeny 

 of Amblystoma. These larvae were kept under conditions 

 in wliich larvae begotten by the Axolotl would never have 

 changed into Amblystoma. The larvae thrived well, and their 

 gills attained in abundantly aerated water an unusually great 

 development. In spite of this they came often to the surface 

 of the water to get air, and remained there for hours — a thing 



