THE INSECTS: HEXAPODA 99 



generalized of the orders which we have mentioned are the 

 Thysanura, Plectoptera, Odonata, Orthoptera, and Hemiptera. 

 These are the orders, it will be remembered, in which meta- 

 morphosis is either entirely wanting, or where, if present, 

 there is usually no well-marked pupal resting-stage. 



In addition to morphology, another source of information 

 is the geological record of species, the fossil remains of 

 organisms preserved in the mud, clay, or sand at the bottom 

 of water. 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 to-day 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 remains of 

 animals may drop. Under favorable conditions their hard 

 parts are preserved in perfect form, but the substance in them 

 is replaced by minerals, and the entire mass is consolidated 

 into rock by heat and the pressure of other materials upon it. 

 Footprints may also 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 

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

 earth's history, thpugh 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 history 

 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 geological record 

 is especially incomplete with regard to the insects, so that it 



