2 4 3 CHAPTERS ON EVOLUTION. 



condition of their adult progenitors." On this reasoning, the Axolotl's 

 later history cannot be expected to coincide with that of the Ambly- 

 stoma. It is a larval form, which, apparently arrested in develop- 

 ment, has nevertheless, contrived to develop the lungs which mark 

 the full growth of all amphibia, whilst it likewise retains the gills of 

 early life. The relationship between the Axolotl and Amblystoma 

 presents, besides, one of the most effective refutations of that 

 common but ignorant remark that no one has yet adduced any 

 proof of the direct transmutation of one species into another. In the 

 case before us, not merely is the transformation one in which one 

 genus of animals apparently becomes another, but the near relation- 

 ship of two thoroughly distinct forms is thus proved to lie within the 

 province of exact zoological observation. 



It should be added that Dr. Weismann, of Freiburg, who has 

 devoted much attention to the metamorphosis of the Axolotl, main- 

 tains that the case in question is one, not of sudden advance in a 

 species, but of reversion to a lower stage. He believes that " those 

 Amblystomas which have been developed in captivity in certain 

 instances from Siredon Mexicanus (S. pisciformis), as well as from 

 the Paris Axolotls, are not progressive but reversion forms. " I 

 believe," concludes Dr. Weismann, "that the Axolotls which now 

 inhabit the Mexican lakes were Amblystomas at a former geological 

 (or better, zoological) epoch, but that owing to changes in their 

 conditions of life, they have reverted to the earlier perennibranchiate 

 (permanently-gilled) stage." One of the most interesting facts which 

 lend support to this view of the backward development of the Axolotl 

 is the discovery that the Axolotls possess a rudimentary intermaxillary 

 gland furnishing a glutinous secretion, and which serves to aid the 

 capture of insect-prey. Now, as this gland exists in a perfect shape 

 in all land amphibians, but is absent in gill-possessing forms, its 

 presence in the gilled Axolotls would certainly seem to indicate that 

 these animals retain the gland as a legacy from the higher or Ambly- 

 stoma stage from which they are believed by Weismann to have 

 descended and retrograded. In whatever light we regard the case of 

 the Axolotl, the bare facts of its curious development remain un- 

 altered. It seems justifiable enough, notwithstanding Dr. Weismann's 

 opinion, to regard the Axolotl as a " permanent larval form," and as 

 representing an arrest of the development of Amblystoma, producing 

 a literally new race of animals able to reproduce their like, and thus 

 evolving a new group of animals by an easily understood modifica- 

 tion of an existent species. The presence of the "intermaxillary 

 gland " may represent the initial stage or beginning of that gland's 

 development in the Axolotl race, instead of a degeneration from the 

 perfect gland of the Amblystoma. Inheritance alone, might account 

 for the development of this structure. But, indeed, whether we 



