114 J. LeConte—Critical Periods in the History of the Earth. 
the changes of climate and partly permitted by removal of 
barriers, and the consequent znvasion 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 Siac horizon is far 
better determined by the fauna than by the 
Ill. Historic value of the Present time. Most geologists regard 
the Present as one of the minor subdivisions of the Cenozoic 
ra, or even of “ Quaternary period. More commonly the 
Quaternary and Present are united as one age—the age of 
—of the eee Era. The Cenozoic is thus divided into 
fet ages; the age of mammals coupe agp with the Tertiary, 
and the age of man commencing with the Quaternary ; an 
the Sheeree subdivided into several ae the last of 
which is the Present or Recent. But if the views above ex- 
pressed in regard to critical periods, be eg ee: then the Pres- 
ent ought not to be connected with the Quaternary as one 
age, nor even with the Cenozoic as one era, but i 1s itself ee 
the hee the inaugurating change less a. the interval 
less long, but dignified by the appearance of man as the 
dominant agent of change, and therefore rel entitled to the 
name Psychozoic sometimes given it. The geological import- 
ance of the appearance of man is not due only or chiefly to 
his transcendent dignity, but to his importance as an agent 
had not yet established his supremacy; he was still fighting 
for mastery. With the establishment of his supremacy the 
mals, so man peared before the age of man 
We, therefore ,rezard the Cenozoic and Psychozoic as two 
consecutive era e Quaternary as the critical, revolu- 
Berkeley, California, March a 187 i: 
