114 THE AMERICAN NATURALIST: [VoL. XXXIII. 
Miocene in age, but in many of its elements it is unique, and 
is distinct from that of any other American Tertiary locality. 
The collections of Eocene and Miocene plants which have been 
made in the west contain different species, and those from 
Bridgeton are rare or else entirely wanting in them. As a 
whole, however, the flora seems to be more nearly comparable 
with that of certain European Miocene localities, and this idea 
is also in accordance with the well-recognized fact that plant 
development was more advanced in Europe than in America. 
Thus European Eocene plants are in part represented by 
Miocene plants in America. European Miocene by American 
Pliocene, and European Pliocene by our present living flora. 
From its general character I am inclined to consider it as more 
recent in age than that of any other recognized Tertiary horizon 
in America. 
Towards the close of the Tertiary period an era of elevation 
began which raised the northern part of the North American 
continent many hundreds of feet above its former level and 
extended the shore line out far beyond its former or present 
position, so that the edge of the continent was about where we 
now find the one hundred fathom contour to be. 
Up to this time in the world’s history we have every reason 
to believe that there were no extremes of climate between the 
poles and the equator such as prevail to-day. The temperature 
of the entire earth’s surface was more or less uniform during 
each of the several periods, up to and including the Tertiary, 
although a constant change had been in progress from tropical 
to temperate conditions. 
The elevation which began in the Tertiary period, however, 
caused, or at least was coincident with, the greatest changes, 
climatic and biologic, which are anywhere recorded in geologic 
history. The climate gradually became more and more severe, 
and finally culminated in what we call the Glacial epoch of the 
Quaternary period. 
That the changes wrought were gradual, extending over a 
long period of time, we are justified in concluding, for the 
reason that the vegetation which was in existence at the time 
when it was finally overwhelmed by the accumulations of ice 
