( 437 ) 



PALAEONTOLOGY. 



CHAPTER XXIV. 

 ELEMENTARY IDEAS IN PALEONTOLOGY. 



PALEONTOLOGY is the history of the succession of life on the earth. 

 It begins with a remote past, when the great groups of organisms were 

 already characterised, and many surviving genera were in existence. 

 It passes over long ages of past time, distinguished by ordinal groups 

 of animals long since extinct. It follows the steps of geological his- 

 tory, and demonstrates how faunas and floras have been preserved by 

 becoming adapted to the circumstances which succeeding ages produced. 

 And it teaches how the existing distribution of life has been evolved 

 during the past history of the earth. We thus learn that species are 

 mutable ; that their distribution is limited in space and time ; that 

 they are thrown off from a genus, like leaves from a tree, and that the 

 genus survives by developing characters which repeatedly obliterate 

 its specific identity. The alterations which time elaborates fre- 

 quently affect the more vital structures, so that the genera in the 

 older rocks are replaced by allied genera which vary, partly by 

 increased growth of some parts with decreased growth in others. 

 Every type of life passes through a series of changes comparable to 

 those in the life of an individual, and species, genera, families, and 

 orders all grow old, lose their vitality, and disappear from the life that 

 survives. Other species and groups under new circumstances put on 

 an amazing vitality, and increase at the expense of surrounding life, 

 and become dominant in regions of the earth and periods of time. 

 These are the abundant demoid types which are termed characteristic 

 fossils, for their abundance is such that strata are easily recognised by 

 them. Every formation has its demoid types, which in the Primary 

 rocks are generally brachiopods, in secondary clays are oysters, and 

 in the limestones are frequently ammonites, or terebratuke. The 

 dwindling or asthenoid species, by failing with different successive 

 periods of time, have a value of another kind in associating rocks 

 together in groups. These two types of life are the most interesting 



