406 



PALAEOZOIC SYSTEM OF ROCKS. 



chlorides ; and with aqueous vapor representing all the tvater on the 

 earth. Of course, such a condition rendered life impossible. From 

 this primeval atmosphere, by cooling, the strong acids were first pre- 

 cipitated with the water ; and afterward more slowly the carbonic acid, 

 by the action of this acid upon the primeval silicates, with the forma- 

 tion of carbonates, especially limestone. All limestones, therefore, rep- 

 resent so much carbonic acid withdrawn from the air. This with- 

 drawal proceeded through the whole Archaean, Silurian, and Devonian. 

 During the Carboniferous, the purification of the air was accelerated 

 by the growth of vegetation and its preservation as coal, as already ex- 

 plained, pages 356 and 382. In this method of withdrawal the oxygen 

 of the carbonic acid is returned, and the air becomes more oxygenated. 

 Progressive Change in Organisms. — Corresponding with these 

 changes, physical and chemical, it is natural to expect changes in spe- 

 cies, genera, families, etc., of organisms : and such we find. The law 

 of continuance or geological range of species, genera, families, orders, is 



1. Paradox ides. 



2. Bathyurus, Agnos- 

 tus, etc 



3. Asaphus, Remo- 



pleurides, Tri- 

 nucleus, etc 



4. Calymene, Acidas- ) 



pis', etc j " 



I 



5. Homalonotus, Li- I 



chas, etc f - 



6. Phillipsia, Griffith- I 



ides f 



7. Distribution of ) 



species of Caly- v ~ 

 mene, etc ) 



Primordial. Lower Silurian. 



Upper 

 Silurian. 



Devonian. | <~" 



















^ 



*— * 

















*^^^~ 









1 









__ _.!_ 







- - 



r 



Fig. 583.— Diagram illustrating Distribution of Families, etc., in Time. 



very similar to that of extent or geographical range of the same groups ; 

 i. e., the laws of distribution in time are similar to those of distribution 

 in space. The period of continuance (range in time) of species is, of 

 course, less than that of genera (because the genus is continued in other 

 species of same the genus), and that of genera less than that of fami- 



