THE HISTORY OF ANIMAL LIFE 



399 



disappear; it is only when such bodies are accidentally embedded 

 in mud under circumstances that leave their imprint in the result- 

 ing rock layers that any record is left. Violent upheavals of the 

 earth's surface have occurred, accompanied by the subjecting of 

 muds and slimes to terrific pressure and heat, which have de- 

 stroyed any record of animal or plant life that such deposits may 

 have contained. Others are buried by upheavals in inaccessible 

 places. It is safe to say that of the total number of forms many 

 have not been preserved, and of those preserved, only a small 

 number have been discovered up to the present. Discoveries of 

 fossil remains that are significant contain a large element of 

 chance; it is after all surprising that so many different types have 

 been found. To illustrate: the passenger pigeon less than a century 

 ago covered North America in untold millions. It is now extinct;, 

 some museums exhibit mounted specimens. Just where would one 

 go to look for preserved remains of this species whose habitat and 

 general characters are known? In view of this, it is quite under- 

 standable that the records of forms which existed millions of years 

 ago are incomplete, that here and there are gaps in the sequence 

 of animal groups. 



While the specialist regards the record as incomplete, a surprls-; 

 ing amount of information concerning the life of the past has been 

 collected and analyzed. It is extremely probable that the reconstruc- 

 tions of prehistoric scenes that form museum exhibits are highly 

 accurate, for even fragments of animals yield a great deal of infor- 

 mation to the trained observer who has a working knowledge of 

 biological principles. To illustrate: The presence of animal life in 

 any epoch presupposes the presence of plants, since animals are 

 utterly dependent on plants, as has been shown in previous pages. 

 A footprint indicates, in addition to other characters, whether or 

 not the animal was a swimmer, a wader, or a land form. Fossil 

 teeth indicate, in addition to the probable size and animal group 



