278 CLARKE— THE AGE OF THE EARTH. 



forward from the sea to the land, an act which impHes time in im- 

 pressive measures and yet an act which we know has reversed itself 

 in later geological times, at least among the animals, with nostalgic 

 energy and must again and again have shown a like reversion in both 

 the animal and in the plant world. We see suggestions of these 

 reversionary movements among the Amphibia and the Mollusca and 

 many Mammalia, and it seems highly probable that a more exact 

 knowledge of extinct life will establish these suggestions and awaken 

 others. 



The Plankton epoch, says Church, gave rise to the first encysted 

 flagellate plants which, under conditions of the Benthon, developed 

 multicellular thallus, tissues and organs of special function and a 

 reproductive mechanism contrived so as to minimize waste. Then 

 followed the Epoch of the Land Flora brought on by the trans- 

 migration of highly developed algae which in fact "appear to have 

 been more highly organized than any single algal type at present 

 known to exist in the sea." " The algae of transmigration may be 

 . . . said to have combined the best features of the known great 

 conventional series of marine phytobenthon." " The origins of all 

 the main successful adaptations of the land are to be traced down to 

 the benthic phase of the sea." In this impressive statement we are 

 confronted by the quality of the plant life at its emergence from the 

 sea. 



Now as to the period of its emergence, of foremost importance 

 to our present consideration; Thomas C. Chamberlin in 191 3 directed 

 attention to the fact that the Precambrian rock complex is divided 

 into earlier and later stages on the basis of the degree of disintegra- 

 tion of the exposed rock surface. In the lower division there is an 

 immature disintegration which implies partial decomposition, but the 

 mature disintegration of the later division implies, he says, " some 

 restraining agency that held the rock in place while the slow weather- 

 ing completed its work." " This view favors the existence of a 

 vegetal covering of the land as far back as this period." 



Church, therefore, has a well-found argument when in the pres- 

 ence of this fact of precambrian weathering he intimates that it was 

 with the uplifting and exposure of the primary rock to the air that 



