LIFE HISTORY OF THE TOAD 
95 
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 animals 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 preserved 
in the rocks, it would be easy to investigate these earlier 
Figure 81. — Fossils. 
On the right a fossil leaf that grew on a sassafras tree; on the left a group 
of fossil animals that once were abundant but have become extinct. 
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 ani¬ 
mals 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 
