GENERAL ZOOLOGY 



Fig. 20.2. Fossil insects. A. 

 Closterocoris elegans, and B, Lygaeus 

 obsolescens: these are fossil hemi- 

 pterans found embedded in fine 

 shales marking the bed of an 

 ancient shallow lake, of Tertiary 

 age, near Florissant, Colorado 

 (<■/. Fig. 15.29, p. 463). C, 

 fossilized Pleistocene insects 

 from the McKittrick tar bed, 

 California; a locust and an 

 aquatic beetle can be recognized. 

 {A and B reproduced from 

 the original lithograph plates 

 in S. H. Scudder, 1890, Report 

 of the L ' .S. Geoloaical Survey, 

 vol. 13; C photograph by E. S. 

 Ross. ) 



regional distribution of modern forms, the subject matter of zoogeography. 

 Geologic distribution encompasses not only the temporal distribution of ex- 

 tinct organisms but also their geographic distribution in past times. There- 

 fore, these two aspects of distribution cannot be entirely separated, any more 

 logically than we can profitably study the anatomy of living animals without 

 reference to the anatomy of animals known only as fossils. It is convenient 

 to discuss the two aspects of distribution separately, however, before examin- 

 ing the correlations between them. 



Geologic Distribution. Fossils and the fossil record have been referred 

 to often in preceding chapters. Before we proceed to specific examples of 

 the information to be gained from them, it is desirable to discuss briefly the 

 nature of fossils and the manner of their formation. Fossils are the remains 



620 



