CHAPTEE II. 



RESULTS OF PAL^EONTOLOGICAL RESEARCH INTO THE 

 EVOLUTION OF ORGANIC LIFE. 



A. PALJEONTOLOGICAL RESULTS. 



1. Brief purview of the chronological succession of the 

 larger animal groups. 



FOSSILS are exclusively found in the so-called sedi- 

 mentary deposits which form the upper part of the 

 earth's crust. Sediments (sedimentary or secondary 

 formations) are such rocks, nearly always appearing as 

 layers, which by deposit of gravel, sand, or mud (or 

 by decomposition of dissolved minerals salts for 

 instance), are formed in the greater water-basins (in the 

 sea or fresh-water lakes). The foundation of investiga- 

 tion of the origin of organisms is therefore an exact 

 determination of the age of the sediments. It is only 

 when it is known which stratum or layer (a) is older or 

 younger than another (b) that we can also know which 

 organisms are older or younger than others accordingly. 

 This determination of the age of the earth's strata is, 

 however, a very difficult matter, and the course of evi- 

 dence which led to the generally recognized arrangement 

 of the four (or five) groups of formations, is not far 

 removed from a vicious circle, especially when we 



