236 THE STORY OF THE EARTH AND MAN. 



reptiles, as the crocodiles and chelonians, liad already 

 appeared, and the first mammals. Henceforth the 

 progress of organic nature lies chiefly in the dropping 

 of many Mesozoic forms and in the introduction of the 

 higher tribes of mammals and of man. 



It is further to be observed that the new things 

 introduced in the later Mesozoic came in little by little 

 in the progress of the period, and anticipated the great 

 physical changes occurring at its close. On the other 

 hand, while many family and even generic types pass 

 over from the Mesozoic to the earlier Tertiary, very 

 few species do so. It would seem, therefore, as if 

 changes of species were more strictly subordinate to 

 physical revolutions than were changes of genera and 

 orders these last overriding under different specific 

 forms many minor vicissitudes, and only in part being 

 overwhelmed in the grander revolutions of the earth. 



Both in Europe and America there is evidence of 

 great changes of level at the beginning of the Ter- 

 tiary. In the west of Europe beds often of shallow- 

 water or even fresh-water origin fill the hollows in 

 the bent Cretaceous strata. This is manifestly the 

 case with the formations of the London and Paris 

 basins, contemporaneous but detached deposits of the 

 Tertiary age, lying in depressions of the chalk. Still 

 this does not imply much want of conformity, and 

 according to the best explorers of those Alpine regions 

 in which both the Mesozoic and Tertiary beds have 

 been thrown up to great elevations, they are in the 

 main conformable to one another. Something of the 



