556 Critical Periods in the History of the Earth. [September, 
(4.) During critical periods, oscillations of the crust, with 
rapid changes of physical geography and climate, determine a 
more rapid rate of change in all forms: first, by greater pressure 
of physical conditions; and, second, by migrations partly enforced 
by the changes of climate and partly permitted by removal of 
barriers, and the consequent invasion of one fauna and flora by 
another and severe struggle for mastery. This would tend to 
equalize again the extreme diversity caused by the second law; 
but the effect would be more marked in the case of animals than 
plants, because voluntary migrations are possible only in this 
kingdom. Hence it follows that a geological horizon is far better 
determined by the fauna than by the flora. 
Ill. Historie Value of the Present Time. Most geologists re- 
gard the Present as one of the minor subdivisions of the Cæno- 
zoic era, or even of the Quaternary period. More commonly the 
Quaternary and Present are united as one age —the age of man 
- — of the Cenozoic era. The Cenozoic is thus divided into two 
ages: the age of mammals commencing with the Tertiary, and 
the age of man commencing with the Quaternary; and the 
Quaternary subdivided into several epochs, the last of which is 
the Present or Recent. But if the views above expressed in re- 
gard to critical periods be correct, then the Present ought not to 
be connected with the Quaternary as one age, nor even with the 
Ceenozoic as one era, but is itself justly entitled to rank as one of 
the primary divisions of time, as one of the great eras separated 
like all the other eras by a critical period ; less distinct it may 
be, at least as yet, in species than the others, the inaugurating 
change less profound, the interval less long, but dignified by the 
appearance of man as the dominant agent of change, and there- 
fore well entitled to the name Psychozoic sometimes given it. 
The geological importance of the appearance of man is not due 
only or chiefly to his transcendent dignity, but to his importance 
as an agent which has already very greatly, and must hereafter 
still more profoundly, modify the whole fauna and flora of the 
earth. It is true that man first appeared in the Quaternary; 
but he had not yet established his supremacy ; he was still fight- 
ing for mastery. With the establishment of his supremacy er 
reign of man commenced. An age is properly characterized by 
the culmination, not the first appearance, of a dominant class. 
As fishes existed before the age of fishes, reptiles before the ag° 
of reptiles, and mammals before the age of mammals, so man 
appeared before the age of man. 
