EVIDENCES— GEOLOGY 



141 



available fragments, and it is gratifying that it is so often adequate. 

 The mere fact that fossils prove the past existence of forms no 

 longer living although often related to existing forms is in itself an 

 evidence of the actual course of evolution, and the transition from 

 form to form through the ages is even more significant. 



In the phylum Chordata the record covers a briefer span and 

 is more complete. Both for these reasons and because we our- 



FiG. 80. — A male stylopid, Oplhalmochlus duryi. The front wings are short 

 club-shaped appendages while the hind wings are used for flight. Most 

 two-winged insects retain the front wings. (After Pierce, from Comstock's 

 Inlroduclion to Entomology, with the permission of the Comstock Publishing 

 Company.) 



selves are vertebrates it is of great interest and deserves special 

 treatment in succeeding chapters. 



Summary. The present state of the earth is the result of a 

 succession of physical phenomena. The ancient igneous rocks have 

 been supplemented by layers of sedimentary rocks formed from 

 the hard parts of minute organisms under water and from 

 materials washed down from land masses. These changes have 

 occupied an enormous span of time. Organisms of the various 

 periods have been Ijuried and preserved as fossils. From the com- 

 bined study of the rocks and fossils it is possible to learn of the 

 conditions prevailing at different times, as shown by the compari- 

 son of organic remains with existing organisms and the conditions 

 under which they live. The succession of organic forms is disclosed 

 in the same study. The result is a relative and incomplete record 

 of evolution, but it is sufficiently complete to show that a gradual 



