224 METAMORPHOSIS AND 



dition. The same species in other parts of North 

 America normally goes through the metamorphosis, 

 like other species of the Urodela. It is evident, 

 therefore, that the Mexican Axolotls, although they 

 have been perennibranchiate for a great number of 

 generations, have not lost the hereditary tendency 

 to the metamorphosis which changes the larvae of 

 Amblystoma elsewhere into an air-breathing ter- 

 restrial animal. This may be regarded as evidence 

 that the conditions of life which prevent the meta- 

 morphosis in the Mexican Axolotl have produced no 

 hereditary effect. The fact, however, that Axolotls 

 require special treatment to induce metamorphosis 

 seems to show that they have distinctly less con- 

 genital tendency to metamorphosis than larvae of 

 the same species, Amblystoma tigrinum, in other 

 parts of North America, and this difference must be 

 attributed to the inherited effect of the conditions. 

 The most important of these conditions seems to 

 be abundance of oxygen in solution in the water, and 

 the next in importance abundance of food in the 

 water. Recently it has been shown that the meta- 

 morphosis may be induced by feeding Axolotls on 

 thyroid gland. But there is no reason to suppose 

 that a congenital defect of thyroid arising as a muta- 

 tion was the original cause of the neoteny, i.e. the 

 persistence of the larval or aquatic, branchiate 

 condition. Such a supposition would imply that 

 the association between Axolotls and the peculiar 

 Mexican lakes, supplied with oxygenated water 

 by springs at the bottom, was purely accidental. 

 Moreover, there is no evidence that there is any 

 deficiency of thyroid in the Axolotl. The secretion 

 of the thyroid gland is necessary for the normal 



