530 CRITICAL PERIODS IN THE HISTORY OF THE EARTH. 



hy another and severe struggle for mastery. This would tend to equalhe 

 again the extreme diversity caused by the second law ; but the eifect would 

 "bo more marked in the case of the animals than plants, because voluntary 

 migrations are possible only in this kingdom. Hence it follows that a geo- 

 logical horizon is far better determined by the fauna than by the flora. 



III. Historic value of the Present Time. Most geologists regard the 

 Present as one of the minor subdivisions of the C»nozoic era, or even of the 

 'Quaternary period. More commonly the Quaternary and Present are uni- 

 ted as one age — the age of man — of the Ca^nozoic era. The Ca^nozoic is thus 

 divided into two ages : the age of mammals commencing with the Tertiarj^, 

 and the age of man commencing with the Quaternary ; and the Quaternary 

 subdivided into several epochs, the last of which is the Present or Eecent. 

 But if the views above expressed in regard to critical periods be correct, 

 then the present ought not to be connected with the Quaternary as one age, 

 nor even with the Ctconozic 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 domi- 

 nant agent of change, and therefore well entitled to the name Psychozoic 

 sometimes given it. The geological importance of the appearance of man is 

 not due only or cliiefly 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 jai established his 

 supremacy ; he was still fighting for mastery. With the establishment of his 

 supremacy the reign of man commenced. An age is properly characterized 

 by the culmination, not the first apperance, of a dominant class. As fishes 

 existed before the age of fishes, reptiles before the age of reptiles, and mam- 

 mals before the age of mammals, so man also appeared before the age of 

 man . 



We therefore regard the Canozoic and Psychozoic as two consecutive 

 eras, and the Quaternary as the critical, the revolutionary, or transitional 

 period between. But since the record of this last critical period is not 

 lost, and we must place it somewhere, it seems best to place it with the 

 Cffinozoic era and the mammalian age, and to commence the Psychozoic era 

 and age of man with the completed supremac}' of man, that is, with the 

 Present epoch. 



Berkeley, Califorma, IMarch 15, 1877. 



