THE BRACHIOPODA 19 



principle over great distances; but how it can be done 

 will be explained later. At present we must consider 

 another point. 



At least 6,000 species of fossil brachiopods have been 

 described, and of these only 25 are identical with 

 living species, while 133 living forms are unknown as 

 fossils. These 6,000 species represent the members of a 

 great number of successive faunas that have succeeded 

 one another throughout geological time. If species 

 followed one another without any guiding principle of 

 succession, the human memory would be unable to 

 grapple with the geological sequence. Fortunately this 

 is not the case. While species are short-lived, geologic- 

 ally speaking, genera are longer lived, but yet most of 

 them are restricted to a limited portion of geological 

 time ; and the abundance of certain genera will char- 

 acterize large groups of strata, just as the abundance of 

 certain species marks a smaller group. The smallest 

 thickness of rock that can be distinguished by means of 

 its fossils over a wide geographical area is called a zone : 

 its actual thickness may vary in different cases from an 

 inch or even less to several hundred feet, according to 

 the net rate at which sediment was accumulating. A 

 zone can, if not very thin, be divided into a number of 

 beds. The time during which the strata constituting a 

 zone were deposited is called a hemera, and is defined as 

 the time during which some one selected species was at 

 the acme or height of its development. We are unable 

 at present to bring this unit of geological time into 

 relation with any of our astronomical time divisions. 



