482 CRITICAL PERIODS IN TEE HISTORY OF THE EARTH. 



continuity. There may bo some break, some unconformity, some lost re- 

 cord, but certainlj' it cannot be large. Yet the change, especially in the 

 higher animals, is immense. In America the break and the lost interval is 

 much greater between the Jurassic and Cretaceous than between the Cre- 

 taceous and Tertiary, still the organic change is far greater in the latter 

 case. The reason is that the changes of physical geography and climate in 

 the latter were more general. Although in America the break and the lost 

 interval is greater at the end of the Jurassic, yet, taking the strata all over 

 the earth, the break is far more general at the end of the Cretaceous; as^ 

 it is these general changes in physical geography which affect climate the 

 most, and which, therefore, produce the profoundcst changes in organic 

 forms. 



Now it is almost impossible to imagine a clearer proof of the fact of 

 rapid evolution-movement during critical periods than we find in the short- 

 ness of the lost interval and the greatness of the change in higher or- 

 ganisms just at this horizon in the rocky series. Nothing can be more 

 astonishing than the abundance, variety, and prodigious size of reptiles 

 in America up the very close of the Cretaceous, and the complete absence 

 of all the grander and more characteristic forms in the lower Tertiary, un- 

 less, indeed it be the correlative fact of complete absence of mammals in 

 the Cretaceous, and their appearance in great numbers and variety in the 

 lowest Tertiary. If Cretaceous mammals existed in America, surely their 

 remains would have been found in the wonderfully rich Cretaceous strata. 

 It seems certain that in America, or at least in that portion which has been 

 examined, mammals appeared somewhat suddenly and in great numbers on 

 the scene, and were a principal agent in the extermination of the large 

 reptiles. The wave of reptilian evolution had just risen to its crest, and 

 perhaps was ready to break, when it was met and overwhelmed by the ris- 

 ing wave of mammalian evolution. 



We have dwelt only on the great change in the higher classes, but the 

 change really extended to all classes. This was, therefore, a time of excep- 

 tionally general and rapid changes in all departments alike. In other words 

 it was a critical period in organic evolution. 



That it was also a time of very extensive changes in physical geogra- 

 phy here in America, as well as elsewhere, is well known. The Cretaceous 

 sea, which extended from the Gulf of Mexico to the Arctic Ocean, covering 

 the whole western plains and plateau region, and thus dividing the Ameri- 

 can continent into two — an eastern Appalachian continent and a western 

 or basin region continent — was abolished at the end of the Cretaceous, and 

 replaced by great fresh-water lakes in the same region, and the continents 

 became one. Moreover, it is probable that it was a period of widespread 

 oscillation, that is, of upheaval and again of subsidence to the condition of 

 things found at the beginning of the Tertiary. It is probable that the up- 

 heaval which destroyed the Cretaceous sea went much beyond the condi- 

 tion of things afterwards; that just at this interval the land was higher 



