1886. ] some of its Apparent Causes. 31 
series of them, for example in Arctic America, another in North- 
eastern United States, and in the Central States of the Upper Mis- 
sissippi valley. 
Other crises, extending over comparatively brief periods of 
geological time, however long when measured by centuries, were 
the elevation of the Rocky mountains, of the Wasatch, the Uin- 
tah ranges, the Sierra Nevada and Cascade ranges; and, mean- 
while, the union of the Atlantic moiety and the Pacific moiety of 
our continent into a continuous land-mass. These periods of ac- 
tivity, signalized by extensive volcanic outpourings and great 
changes in the relative distribution of land and sea, must have 
been, as palzontology shows, periods of rapid extinction as well 
as of reparation or recreation. Progress in continent-making was 
accompanied by progress and an onward sweep in the tide of life, 
not only in animals with jointed bodies and limbs, but more espe- 
cially in those with back bones and brains to correspond with their 
vertebrate rank. 
Until the end of the Tertiary period the earth’s climate was still 
nearly uniform. There was through the Miocene a general, in- 
deed most remarkable resemblance between the flora of Europe 
and the United States, with that of Greenland and Spitzbergen, or 
the regions now lying in the frigid zone ; this flora being in some 
respects like that of Louisiana. 
It was not until the Glacial epoch that the earth’s climate be- 
came differentiated into tropical and frigid and temperate zones: 
That great geological crisis, whether due to astronomical or geo- 
logical causes, or both combined, by which over enormous tracts 
of land in Northern and Central Europe, and Northeastern Amer- 
ica a frigid climate, with continental glaciers, was spread—that 
crisis produced results on the life of the glaciated region which we 
can easily appreciate. The extinction of life over the stated areas 
became widespread. The incoming of the Ice age also must 
have induced extensive migrations to the southward. As the 
glaciers melted, and the climate ameliorated, fresh migrations 
from the south set in, and thus in the early Quaternary period, 
when species were exterminated on a vast scale by causes readily 
appreciable, we have set before us, in a language which every one 
can translate, some of the geological causes of extinction, modi- 
fication and consequent evolution of new forms. 
We will begin, then, with a reference to the changes in the life 
