THE ANIMAL CLIMAX-THE CHORDATES 



297 



Fig. 13-21. This normally occurring adult tiger salamander is in no way different from the one "artificially" pro- 

 duced in Fig. 13-20. 



knowledge to every school child (Fig. 

 13-22). He knows that frogs deposit their 

 eggs in the water much the same as fish do, 

 that the eggs hatch into tiny fish-like tad- 

 poles which breathe by means of gills, that 

 during the weeks and months that follow 

 the tadpole eventually loses its tail and 

 develops lungs and jumping legs, which en- 

 able it to move onto land. It took the fish 

 many million years to accomplish what the 

 tadpole now repeats in a few weeks. 



The amphibians are tied to water in vary- 

 ing degrees. Some species have tried to 

 divorce themselves completely from the 

 water, as, for example, the South American 

 toad, whose eggs brood in fluid-filled sacs 

 upon its back. Other species, such as the 

 mud-puppy, spend their entire life in the 

 water and cannot be forced to leave it. A 



curious intermediate is a variety of tiger 

 salamander (Ambijstoma tigrinum) which 

 normally spends its entire life in the larval 

 body form, but which, if fed thyroid extract 

 or high levels of iodine, can be made to 

 lose its gills, develop lungs, and come out 

 on land just as its relatives do (Fig. 13-20). 

 This tiger salamander larva was thought to 

 be a different species from the usual adult 

 (Fig. 13-21) and was called the axolotl. 

 It would seem that while the axolotl has 

 descended from forms that attained the 

 adult state, it "preferred" to retain its larval 

 body form, perhaps because of more abun- 

 dant food or for some other reason. Among 

 the amphibians, then, there are those which 

 attempt to leave the water altogether and 

 those which tend never to leave it. This is 

 exactly what would be expected if evolu- 



