CHAPTER X 



PREHISTORIC ANIMALS 



We are tolerably familiar with the structure and habits of 

 many animals which lived on the earth in prehistoric times ; 

 many of these have no living representatives, — they are extinct ; 

 others exist to-day almost identical in form with some which 

 lived ages ago ; still others have left descendants which have 

 become modified from age to age until they are represented 

 to-day by individuals very different from their earliest known 

 ancestors, with which, however, they are connected by a series 

 of links forming a complete chain. These animals are known 

 to us through their fossil remains, the study of which constitutes 

 the principal branch of the science of palaeontology. 



Although the number of fossils known is very large, they are 

 far from giving us a complete picture of the animal life of the 

 past, — in other words the palaeontological record is necessarily 

 very imperfect. There are several reasons for this. As we 

 have already noted in Chapter II, it is usually only the hard 

 parts of animals that are preserved as fossils, which therefore 

 consist mostly of shells, bones, teeth, and other skeletal struc- 

 tures ; there are occasionally, however, fossil remains of very 

 soft animals, such as jellyfishes, for example. As a rule the 

 soft parts are eaten by other animals or disintegrate and are 

 washed away, retaining no trace of their original form. We 

 noted further that fossils occur only in stratified rocks, which 

 are formed for the most part under water by the slow deposition 

 of material carried down by the rivers to the oceans, seas, and 

 lakes. Hence aquatic animals are much more likely to be pre- 

 served as fossils than land or air animals, for the chances that 

 the latter will become buried in such sedimentary deposits are 

 relatively small. Thus our knowledge of prehistoric marine 



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