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AMPHIBIANS 



much about the way frogs have developed from fishes. 

 The tadpole breathes and eats like a fish ; but as soon as 

 lungs and legs are formed, it breathes and eats like a frog. 

 This same study of the tadpole also illustrates how ani- 

 mals may gradually have come to live on land. In the 

 early history of the earth there were hundreds of animals 

 and plants which are no longer known to science. The 

 skeletons, foot-prints, and whole bodies of many of these 

 are preserved in the rocks. Such remains are called fossils. 

 If all the animals, or one of each kind, had been pre- 

 served in the rocks, it would be easy to investigate these 



Figure 123. — Fossil Shells of Animals now Extinct. 



earlier animals and their relation to the living animals of 

 the present. But in our information there are great gaps, 

 which we are, however, gradually bridging. Apparently 

 unrelated animals have resemblances, so that in time we 

 may come to see that all animals are really related forms, 

 varying only in complexity of structure. One thing that 

 we must always keep in mind is that the plants and animals 

 which live now are but a small fraction of those which have 

 lived. The rocks have preserved the remains of only a 

 small part of the forms of the past. Many of the records 



