THE GEOLOGIC FUNCTIONS OF LIFE. 647 



of rock contain fossil forms either identical with those now living, or 

 closely similar to them; that beds below these bear life relics that 

 depart somewhat more from the living forms, and are somewhat less 

 highly developed; that beds still lower bear fossils that depart still 

 more from the living types, and are more primitive in general, and so 

 on down as far as fossils are found. 



The general order of life succession determined by stratigraphy. — 

 Thus it appeared from the evidence of the strata that there was a general 

 order of life succession. It was also found that this was, in its main 

 features, the same for all the continents. By continued and close studies, 

 the particulars of the succession were worked out more and more fully, 

 and the work is still being pushed forward to greater and greater de- 

 grees of refinement. At the same time, it was found that there were 

 different faunas and floras in different parts of the world in past times, 

 much as there are now; that there were shif tings and migrations 

 as now; that given species were increasing in some regions and dying 

 out in others, and that innumerable variations and complications entered 

 into the evolution and distribution of the hfe forms. But under and 

 through all these there run a sufficient number of common features to 

 show beyond reasonable question the order of succession of life. 



Throughout all this study, the chief guide was the actual order in which 

 the fossils were found in the succession of strata, because there is no 

 evidence so conclusive of the order of events as the superposition of the 

 sedimentary beds when they are normal and undisturbed. By the 

 study of the fossils in the successive beds, it was foimd that there was 

 a more or less progressive evolution of plants and animals brought 

 about by modifications of their forms, and that these modifications 

 assisted in determining the order of succession when the evidence of the 

 strata was defective; and so the biological and stratigraphical factors 

 reacted helpfully on each other. 



Fossils as means of correlation. — While stratigraphy was thus, in the 

 earliest stages, the main rehance in determining the order of events, and 

 biology was the chief gainer, in the end stratigraphy received ample 

 compensation, if indeed it did not become the greater beneficiary; for 

 at no known and accessible place is there a complete succession of sedi- 

 mentary beds. There are great series here and there, but their con- 

 nections with one another are more or less concealed by surface forma- 

 tions or water-bodies. So also at many places the stratified series has 



