THE AMPHIBIANS 183 
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 (Amédlystoma 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. 110. 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 larve 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. 
