114 GENERAL ZOOLOGY 



logical record is especially incomplete with regard to the 

 insects, so that it does not give us much help in this par- 

 ticular problem. 



Most fossil-bearing rocks are formed on the floor of the 

 oceans. Since insects very rarely live in salt water, it is not 

 surprising that fossil remains of these animals are relatively 

 rare. Coal is a form of fossilized plants of a past age. Some 

 of the best specimens of fossil insects are found in the coal 

 deposits. Especially around the Baltic Sea, another inter- 

 esting type of fossil insects has been discovered. These are 

 the perfectly natural-looking specimens entombed in fossil 

 amber. 



Of the remains of winged insects which have so far been 

 discovered, the earliest are those belonging to the orders 

 Orthoptera, Hemiptera, Ephemerida, and Odonata. 



Distribution of Insects. Insects occur in every part of the 

 earth which man has explored. Even in the Arctic and 

 Antarctic regions insects are not wanting. Although many 

 species are very widely distributed, being cosmopolitan (dis- 

 tributed over practically the whole earth), many others are 

 of very limited range. No one has been able to offer a satis- 

 factory explanation of why the monarch butterfly and many 

 species of beetles have spread over so much of the earth's 

 surface and some other species are found in only a limited 

 locality of but a few miles in extent. 



Any condition which tends to limit the distribution of a 

 species to a restricted area is called a barrier. To land 

 animals such a barrier may be a mountain range, a desert, 

 or a large body of water. To a desert-inhabiting species it 

 might be a forest. To aquatic animals, waterfalls or rapids 

 are often insurmountable. Temperature conditions are 

 very definite in their effects on the distribution of animal 

 life. Food supply is another important factor. The ocean is 

 a barrier to nearly all land species. 



