544 Critical Periods in the History of the Earth. [September, 
dant life. It is the greatest and most general change in the 
forms of organisms which has ever occurred in the history of the 
earth. Jt took place, again, during a lost interval. A portion of 
the loss is recovered in the Permian, but the most critical time, 
the time of most rapid change, namely, that between the Per- 
mian and the Trias, is still missing. How we long to find the 
steps of this great change! What a flood of light would it shed 
on the process of evolution! But although the change in the 
organic kingdom was, just here, so enormously great, yet the lost 
interval does not seem very long, for in England the Trias and 
Permian seem to be conformable, though probably with change 
from marine to fresh-water conditions. It is hence impossible 
to resist the conclusion that the steps were just here fewer and 
longer and the progress more rapid than usual. As in human 
history, revolutions are the times of the birth of new social 
ideas, upon which, during the subsequent period of tranquillity, 
society is readjusted in prosperity and happiness on a higher 
plane, so also in geological history, critical periods are times of 
origin of new and higher organic forms, and the subsequent 
periods of tranquillity are times of readjustment of equilibrium 
and prosperous development of these forms. 
Like the previous lost interval, this was also a period of oscilla- 
tion, — a period of great increase of land, which was again partly 
submerged to inaugurate the Trias. It was, therefore, also 4 
continental period. The land-making commenced at the end of 
the Coal period, in this country with the formation of the Appa 
lachian Mountains, continued through the Permian, and culmi- 
nated in the lost interval, which is, in fact, for that very reason 
lost. 
Far less in length of time and perhaps in the sweeping character 
of the change of organisms, but far more important and interest- 
ing on account of the high position of the animals involved, 1s 
the lost interval between the Mesozoic and Cenozoic. The le 
of time lost here is comparatively small. In America, in many 
parts of the West, the uppermost Cretaceous seems to pass oe 
the lowermost Tertiary without the slightest break of continuity. 
There may be some break, some unconformity, some lost record, 
but certainly it cannot be large. Yet the change, especially 1m 
the higher animals, is immense. In America the break and the 
lost interval is much greater between the Jurassic and Creta- 
ceous than between the Cretaceous and Tertiary, still the organi’ 
change is far greater in the latter case. The reason is that 
