YOUTH IN BIRDS AND LOWER ANIMALS 61 



to change into the adult amblystomas, the most successful method 

 being to keep them in very shallow vessels so that they had a 

 frequent opportunity of breathing air, and at the same time to make 

 the normal gill-respiration inconvenient by securing that the water 

 should have less than its proper quantity of dissolved air. 



The curious facts as to larval reproduction in the axolotl throw a 

 possibly new light upon the relations of the different groups of 

 batrachians to each other. It had been assumed that the Perenni- 

 branchiata, those which remained aquatic and had gills throughout 

 their life, were the representatives of a primitive stock, a ad that in 

 the same way the gilled larvae of the terrestrial adults represented 

 an ancestral stage passed through in the actual development of the 

 modern forms. It is clear, however, that the external gills do not 

 correspond with the primitive fish gills, and that the limbs with 

 fingers and toes correspond with terrestrial rather than aquatic 

 conditions. If the occasional metamorphosis of the axolotl had 

 not been discovered, the axolotl would have been classed with the 

 other perennibranchs. It is quite probable that the other perenni- 

 branchs are creatures which have actually permanently lost their 

 terrestrial adult condition, and so are degenerate rather than 

 primitive. It has been suggested even that the ancestors of the 

 living batrachians were terrestrial creatures, breathing by lungs 

 and with two pairs of limbs with hands and feet possessing fingers 

 and toes, and that the aquatic larvae with their external gills were 

 new interpolations in the life-history. If such a theory were 

 justified, then the perennibranchs, instead of being an ancestral 

 set of batrachians, would really be more modern than the terrestrial 

 forms, and their greater simplicity would be due to the loss of the 

 adult stage. 



The progress of evolution is not invariably associated with advance 

 in structure, and it is quite possible that some of the groups which 

 we now think of as being primitive and as possibly representing 

 ancestral stages in evolution are merely larvae, to which the power 

 of reproduction has been shifted backwards, and which in consequence 

 have permanently lost their adult stages. From this point of view 

 the curiosities of youth which I have been describing would have 

 a great importance in the theory of evolution. 



