CLASSIFICATION AND DISTRIBUTION 113 



Fossil Insects. We have but a very imperfect picture of 

 the insect life of the past. The remains of any animal of a 

 past time are called a fossil. Contrary to general belief, a 

 fossil is not necessarily an animal which has turned to stone, 

 for many fossils are not petrified. However, most of the 

 records of insects from past ages are preserved for us in the 

 rocks of which the crust of the earth is built up. The agen- 

 cies whereby fossils have been preserved are commonly as 

 follows: Wherever areas of land are uplifted, the atmos- 

 pheric agencies of wind and water begin their work of wear- 

 ing them down again. The worn materials, in the form of 

 clay, sand, or mud, as may be seen today after a rain, find 

 their way in rivulets to lower ground, or into a river which 

 deposits them still lower, finally even to the bottom of the 

 sea. When there, or in a temporary resting place in some 

 lake or pond, the material forms a bed into which the re- 

 mains of animals may drop. Under favorable conditions 

 their hard parts are preserved in perfect form. The sub- 

 stance in them may be replaced by minerals, and the entire 

 mass consolidated into rock by heat and the pressure of 

 other materials upon it. Even footprints may be made in 

 the soft mud at the edge of ponds, and indelibly preserved 

 in the rocks of later times. 



The geologist has worked out in detail the order in which 

 the rock material of the world has been laid down. By study- 

 ing the fossil remains of living things the zoologist can pic- 

 ture something of the life of each of the great epochs in the 

 earth's history, though, owing to the conditions of preserva- 

 tion, by far the greater part of the record has been lost. It is 

 as though we should try to get a connected idea of the his- 

 tory of the United States from a book from which had been 

 torn the whole of the early voyages, much of the colonial 

 period, and many pages from the story of the Revolution, 

 the Civil War, and later history. Unfortunately the geo- 



