134 INTRODUCTION TO EVOLUTION 



found exposed for study. Younger strata are lacking because ( 1 ) none 

 were ever deposited in that particular region, or (2) they were formed but 

 were subsequently removed by erosion. Older strata may be buried from 

 view, if present at all. Because of the vagaries of the raising and lower- 

 ing of different portions of the earth's crust no one area has beneath it 

 strata representative of all past periods in geologic history. 



The question may arise as to how geologists determine the age of, for ex- 

 ample, a single stratum found exposed in a given region when the strata 

 immediately older and younger are not available for study. To solve the 

 problem requires the most painstaking comparison of the rock in this local- 

 ity with rock in other localities where the relationships among neighboring 

 strata are more evident. The comparison involves details of the rock struc- 

 ture itself, as well as distribution of "key fossils" which serve to distinguish 

 rocks formed at one time from those formed at other times and charac- 

 terized by other key fossils. Always when possible reference is made to a 

 region where the stratum in question can be found in undisturbed position 

 relative to strata deposited before and after it. 



Incompleteness of the Geologic Record 



In the foregoing discussion we have attempted to gain some insight into 

 the manner in which the relative ages of rocks are determined, as well as 

 into the difficulties attendant upon the making of such determinations. We 

 may now inquire into the reasons for the incompleteness of the record of 

 past Ufe on the earth. 



Of the animals living at any one time in the history of the earth how 

 many will be known to geologists a million or more years later? In the first 

 place we may subtract most of the animals having no "hard parts." This 

 will include most of the protozoa, though some of them, notably the fora- 

 miniferans, produce calcareous shells. These shells, though individually of 

 minute size, in the aggregate have produced thick deposits of limestone. 

 Most of the coelenterates and worms will also fail to leave fossil remains. 

 Among the coelenterates the corals are an exception to this statement since 

 their calcareous skeletal supports form another important source of lime- 

 stone. 



Of the animals possessing hard parts, how many will be known to geolo- 

 gists a million or more years later? The chances of fossilization vary 

 greatly. Evidently, from the foregoing account, animals living in the 

 ocean have the best chance of being preserved as fossils. Animals inhabit- 

 ing fresh water have, perhaps, the next best chance, while terrestrial ani- 



