3 2 ° 



University of California. 



[Vol, i. 



throughout Arctogaea, but also southward into all the southern 

 continents, except Australia. Madagascar, and the coast islands of 

 California seem also to have escaped.* These great changes, as 

 already explained, determined a more rapid evolution of organic 

 forms, especially among the higher animals. 



This, then, I believe ought to be regarded as a critical period, 

 and therefore a dividing line between two primary divisions of the 

 earth's history, viz.: the Cenozoic and the Present. The Present, 

 therefore, contrary to the view of most geologists, I regard, and 

 insist on it, as a primary division of time, and the glacial period 

 as the transition or critical or revolutionary period separating it 

 from another primary division, the Cenozoic. 



I so regard it because it bears all the marks of a primary divi- 

 sion of time. I. There is widespread unconformity of present sed- 

 iments on all previous strata. This is seen in old river beds, in the 

 deposits of great glacial lakes, and in all the sea margin deposits as 

 far as the submerged continental margin. In addition to this, if we 

 admit the great enlargement of the Antarctic continent already men- 

 tioned, the unconformity extends over the whole Antarctic Ocean 

 bottom. If to all this we add the unconformity of the drift itself on 

 its eroded bed rock, surely the wideness of this unconformity is 

 scarcely inferior to that of any other critical period. Very much of 

 this is now concealed beneath the ocean by subsequent subsidence 

 of land. 2. There are here also very great and rapid changes in 

 organic forms, especially among mammals. The change in the 

 mammalian species since the Tertiary, i. <?., during the Glacial Rev- 

 olution, is complete. In lower forms the change as yet is less com- 

 plete than in previous critical periods. I said, as yet, because it is 

 still going on rapidly. 3. There is here the introduction of a new 

 and higher dominant type, man. I must stop a moment to enlarge 

 on this. 



Effect of 'New Dominant Types. — New dominant types, as already 

 shown, are always an important factor, increasing, continuing, and 

 completing the changes commenced by other factors of evolution. 

 This is well illustrated by the rapid disappearance of the great Meso- 



'Ani. Jour., vol. 34, r- 457. 1887. 



