1877.] Critical Periods in the History of the Earth. 545 
changes of physical geography and climate in the latter were 
more general. Although in America the break and the lost in- 
terval 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 ; and it is these general changes in physical geog- 
raphy which affect climate the most, and which, therefore, pro- 
duce the profoundest 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 shortness of the lost interval and the greatness 
of the change in higher organisms just at this horizon in the 
rocky series. Nothing can be more astonishing than the abun- 
dance, variety, and prodigious size of reptiles in America up to 
the very close of the Cretaceous, and the complete absence of all 
the grander and more characteristic forms in the lowest Tertiary, 
unless, indeed, it be the correlative fact of complete absence of 
mammals in the Cretaceous, and their appearance in great num- 
bers 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 rising 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, there- 
fore, a time of exceptionally general and rapid changes in all de- 
partments alike. In other words it was a critical period in organic 
evolution. 
That it was also a time of very extensive changes in physical 
geography 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 American continent into two, — an 
eastern Appalachian continent and a western or basin region 
continent, — was abolished at the end of the Cretaceous, and re- 
by great fresh-water lakes in the same region, and the 
Continents became one. Moreover, it is probable that it was a 
period of wide-spread oscillation, that is,-of upheaval and again 
of subsidence to the condition of things found at the beginning of 
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