40 DISTRIBUTION OF ANIMALS. [PART I. 
nature to affect the larger features of the earth’s surface or to 
determine the boundaries of great zoological regions. 
The only other other recent change of great importance which 
can be adduced to illustrate our present subject, is that which 
has taken place between North and South America. The living 
marine shells of the opposite coasts of the isthmus of Panama, 
as well as the corals and fishes, are generally of distinct species, 
but some are identical and many are closely allied; the West 
Indian fossil shells and corals of the Miocene period, however, 
are found to be largely identical with those of the Pacific coast. 
The fishes of the Atlantic and Pacific shores of America are 
as a rule very distinct; but Dr. Giinther has recently shown 
that a considerable number of species inhabiting the seas on 
opposite sides of the isthmus are absolutely identical. These 
facts certainly indicate, that during the Miocene epoch a broad 
channel separated North and South America; and it seems pro- 
bable that a series of elevations and subsidences have taken 
place uniting and separating them at different epochs ; the most 
recent submersion having lasted but a short time, and thus, 
while allowing the passage of abundance of locomotive fishes, 
not admitting of much change in the comparatively stationary 
mollusca. ; 
The Glacial Epoch as affecting the Distribution of Animals — 
The remarkable refrigeration of climate in the northern hemi- 
sphere within the epoch, of existing species, to which the term 
Glacial epoch is applied, together with the changes of level that 
accompanied and perhaps assisted to produce it, has been one of 
the chief agents in determining many of the details of the exist- 
ing distribution of animals in temperate zones. A comparison 
of the effects produced by existing glaciers with certain super- 
ficial phenomena in the temperate parts of Europe and North 
America, renders it certain that between the Newer Pliocene and 
the Recent epochs, a large portion of the northern hemisphere 
must have been covered with a sheet of ice several thousand 
feet thick, like that which now cnvelopes the interior of Green- 
land. Much further south the mountains were covered with 
perpetual snow, and sent glaciers down every valley ; and all the 
