352 



ANIMAL LIFE 



into Artemia saUna upon the gradual addition of fresh 

 water. The crustaceans and fish of the Mammoth Cave, 

 though differing greatly in certain features from the forms 



FIG. 394. Variation in the Catalpa caterpillar. Photograph natural size. 



in the waters just outside, give unmistakable evidence of 

 being their direct descendants. 



Other evidences of evolution are furnished by rudimen- 

 tary structures, which are present in all of the higher verte- 

 brates. Some of the lizards, having only one pair of legs, 

 have rudiments of the other pair beneath the skin. The 

 rudiments of the hind legs in the Python are apparent ex- 

 ternally. No functional hind limbs are present in any of 

 the whales, but several species possess small structures 



