296 



THE RISE OF ANIMAL LIFE 



Fig. 13-20. The common tiger salamander (Ambysfoma tigrinum) normally undergoes a typical amphibian metamor- 

 phosis resulting in the adult shown in Fig. 13-21. One variety of this species living in western North America, 

 particularly in the southwest, becomes sexually mature while still a larva and never reaches the adult stage. 

 The specimen on the extreme left is a young larva, while the one next to it is a sexually mature larva. This is 

 as far as development proceeds in nature. Several specimens similar to this one were placed in water with a high 

 level of iodine. During the next few weeks the "adult larvae" metamorphosed to typical adults as the next two 

 pictures show. 



leaving it. However, during these excur- 

 sions some may have found abundant food 

 near the water's edge, whereas others which 

 could not stand drought may have found 

 it more profitable to wander from pond to 

 pond in search of food. Again, it does not 

 take a great stretch of the imagination to 

 see how some might have found other 

 members of their own group to feed upon, 

 while others might have changed to a her- 

 bivorous diet ( as some we know did ) , since 

 vegetation was abundant. From such be- 

 ginnings the great variety of life among 

 land vertebrates appears to have devel- 

 oped. 



There were undoubtedly numerous un- 

 successful attempts by many groups of the 

 fishes to make the transition onto land. 

 The ancestors of the "lobe-finned" fish ap- 

 parently were successful, and gave rise to 

 the amphibians which include our present- 

 day frogs and salamanders. As the name 

 amphibian implies, these animals live both 

 in and out of the water. Their larval stages 

 are always spent in an aquatic environ- 

 ment, but the adults of most species are 

 able to live out of water, although they usu- 

 ally do not venture far from moist sur- 

 roundings. 



The life history of the frog is common 



