1877.] Critical Periods in the History of the Earth. 553 
States in Glacial times ; and while some of them afterward went 
northward to their present home, some in each country sought 
arctic conditions in Alpine isolation. This explanation, which 
has been long recognized for plants, has been recently applied 
by Mr. Grote to arctic insects found on the top of Mt. Wash- 
ington and the mountains of Colorado.} 
Undoubtedly changes of climate during this time enforced 
similar migrations among mammals also. But it is evident that 
while plants and invertebrates might endure such modifications 
of climate and such enforced migrations with little alteration of 
form, the more highly organized and sensitive mammalian spe- 
cies must be either destroyed or else must undergo more profound ' 
changes. Moreover, the opening of land connections between 
regions previously isolated by barriers would be far more quickly 
taken advantage of by mammals than by invertebrates and 
plants. The migrations of plants are of necessity very slow, that 
is, from generation to generation. The migrations of mammals, 
too, so far as they are enforced by changing climate, are of a sim- 
ilar kind ; but the voluntary migrations of mammals, permitted 
by removal of barriers, may take place much more rapidly, even 
in a few generations. This introduces another element of very 
rapid local change, namely, the invasion of one fauna by an- 
other equally well adapted to the environment, and the strug- 
gle for life between the invaders and the autochthones. 
. For example: in America during the Glacial epoch, coinci- 
dently with the rigorous climate, there was an elevation of the 
continent, greatest in regions of high latitude, but also probably 
great along the line of the Mississippi River; for in this re- 
gion it extended southward even to and beyond the shores of the 
Gulf. Professor Hilgard has shown that the elevation at the 
mouth of the Mississippi River was at least four hundred and 
fifty to five hundred feet above the present condition. Until the 
Glacial times the two Americas were certainly separated by sea 
in the region of the Isthmus, as shown by the Tertiary deposits 
there. This barrier was removed by upheaval during the Glacial 
epoch, and a far broader connection existed then than now. 
Through this open gate-way came the fauna of South America, 
especially the great Edentates, into North America. Similarly a 
broad connection then existed between America and Asia in the 
_ | This application, with reference to Mt. Washington and other arctic insects 
in America, was previously made by Prof. A. S. Packard, Jr., in the Memoirs of the 
Boston Soc. Nat. Hist., i. p. 256, 1867. — Eps. 
