RECORD OF LIFE IX THE ROCKS. 



179 



full introduction of both being reserved for the age 

 which ushered in the advent of man. 



261. Divisions of the Earth's History. As the record 

 of the rocks is a life-record, it seems eminently proper 

 to base the division into ages upon the changes which 

 we find in the forms of life from age to age. There are 

 other modes of division ; but, as the developments of 

 the life-record have come out, the tendency has been 

 more and more to a division on this basis. This division 

 has been made differently by different authors. The one 

 which I shall follow is that which is brought out in that 

 grand American work, Dana's Geology. He divides the 

 geological history of the world into seven periods : the 

 Azoic age, in which there was no life (a privative, and 

 zoe, life) ; the age of Mollusks ; the age of Fishes ; the 

 age of Coal-plants, or the Carboniferous age ; the age of 

 Reptiles ; the age of Mammals ; and the age of Man. 



