398 PALAEOZOIC TIME. 



there were often one or more new species with each new kind of 

 layer, and generally several with each change in the strata ; while 

 many appeared with the opening of an epoch, and a whole fauna, 

 nearly, with the commencement of a period. There is, then, this 

 grand principle : — 



Creations and extinctions of species were going on through the 

 whole course of the history, instead of being confined to particular 

 points of time ; but at the close of long periods and epochs there 

 were nearly universal extinctions, followed by abundant creations. 



The means by which exterminations may have been produced 

 have been often alluded to in the preceding pages, and need here 

 be recapitulated only in brief, — namely : 



For marine species, (1) a sinking of the sea-bottom, carrying the 

 species below the depth at which they flourish ; or (2) an elevation 

 of it so as to make it dry land, on which, of course, all the marine 

 life would become extinct ; (3) a change by variation of level or 

 raising of barriers sufficient to change clear and pure waters into 

 waters full of sediment, or expose them to an influx of sedimentary 

 material, and the reverse ; (4) a similar change, sufficient to turn the 

 salt-water sea into a fresh-water area. Besides these, there may be 

 mentioned submarine igneous action heating the waters. With 

 such causes for exterminations, it is intelligible that the extinction 

 of a fauna might be nearly complete in one area and not so in 

 another, and that where such extinctions were partial there might be 

 in some cases a partial repeopling of a district from that one in 

 which there were surviving species. 



3. Extinction of whole tribes, families, or genera of species. — The charac- 

 teristic tribes of land-plants in the Coal era, and those that became 

 extinct, have been mentioned on p. 333. 



The races of animals that were most prominent in giving a spe- 

 cial character to the Palaeozoic fauna were as follow : — 



Among Radiates, Crinoids and Cyathophylloid Corals; among 

 Mollusks, Brachiopods ; among Articulates, Trilobites ; among Verte- 

 brates, the vertebrate-tailed Ganoid fishes. Of these, the groups 

 of Cyathophylloid Corals and Trilobites become extinct with the 

 close of the Palaeozoic, and the vertebrate-tailed Ganoids very 

 nearly so ; and Crinoids and Brachiopods lose their pre-eminence 

 in numbers of species and individuals in their respective sub- 

 kingdoms. 



The following are a few other examples of the extinction of pro- 

 minent Palaeozoic groups : — 



Graptolites, which culminated in the Lower Silurian and ended in 

 the Clinton epoch of the Upper Silurian ; Cystideans, which culmi- 



