542 VOSEPTGEE GON Ls 
to this question is plain. Critical periods, as I have shown in 
previous papers, are the great landmarks separating the primary 
divisions of geological time—the eras. They are the compara- 
tively fixed points between which we correlate periods as best 
we can by comparison. Now, if the Quaternary be indeed one 
of these and the last, then it is evident that the Ozarkian, being 
a time of great elevation and enlargement of continents, and 
therefore a period of lost record, belongs par excellence to that 
period. It is the most important and characteristic part, and 
the part which determined the whole succession of changes 
which inaugurated a new order of things, viz., Present; a new 
era, viz., the Psychozoic. The Tertiary was a period of compara- 
tive quiet, of gradual changes, and abundant life. The Ozarkian 
commenced the series of evolutionary changes which inaugu- 
rated a new era and is the most characteristic and important 
epoch in the series. 
That the Ozarkian belongs to the Quaternary, therefore, is 
certain. But the question still remains: to what era should we 
attach the Quaternary ? Upham thinks that to be consistent I 
ought to put it in the Psychozoic, because man was introduced 
in the Quaternary.* On the contrary, I believe it should be 
allied with the Cenozoic. The Permian, too, is a_ transition. 
revolutionary, critical period between the Paleozoic and Mesozoic, 
But after much discussion it has been put with the Paleozoic. 
The Laramie is the transition, revolutionary, critical period 
between the Mesozoic and Cenozoic. Again, after long dis- 
cussion, it has been put in the Mesozoic. So, also, the Quater- 
nary is the transitional, revolutionary, critical period between 
the Cenozoic and Psychozoic. After some discussion it will 
undoubtedly be put in the Cenozoic with the Tertiary. It is 
true that man, the characteristic dominant type of the Psychozoic, 
was introduced in the Quaternary. But here, also, the analogy 
with other critical periods holds good. A new era begins when 
the readjustment and the new order is established. Reptiles 
were introduced in the Permian, but the reign of reptiles did not 
*Am. Nat., Vol. XXVIII, p. 980, 1894. 
