4 THE SMALLEST LIVING THINGS 



came hardened into rock. Such remains are known as fossils.* 

 The oldest fossils are found in the lowest strata of rocks form- 

 ing the earth's crust. Higher strata, deposited later, are found 

 to contain types of organisms which lived at a later time. It is 



Courtesy of the American Museum of Natural History 



Fig. 1— A BRACHIOPOD 



Spirifer (Dcthysis) pcrlamcllosus, Hall 



thus possible to obtain something of a record, though often 

 rather indefinite and fragmentary, of the succession of types of 

 living things from the earliest formed rocks to the present day. 



Geologically, the history of the earth is divided into seven 

 eras; and some of these are further subdivided into periods as 

 shown in the accompanying table of Geologic Chronology. 

 Paleontology is a branch of the biological sciences dealing mainly 

 with fossil forms. Studies of these remains of past life led to 

 the interesting conclusion that there is an increasing complexity 



* See "The Earth" and "Fossils" both in this Series. 



