THE AMPHIBIANS 191 



Sunlight and warmth are apparent necessities for speedy 

 development. Tadpoles kept in captivity where the con- 

 ditions are generally unfavorable may require years to as- 

 sume the adult form. As mentioned above, the tiger sala- 

 mander (Amblystoma tigrinum) occurs in most parts of the 

 United States and Mexico. In the East this species drops its 

 gills in early life as other salamanders do, and assumes the 

 adult form, but in the cold water of high mountain lakes, 

 in Colorado and neighboring States, it may never become 

 adult, always remaining as in Fig. 111. This peculiar form 

 is locally known as axolotl. In this condition it breeds. It 

 is thus one of the very few examples of animals whose un- 

 developed larvae are able to produce their kind. Owing to 

 this trait it was at first considered a distinct species, and 

 many years elapsed before its relationship to the true adult 

 form was discovered. 



