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 Eecent 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 envelopes the interior of Green- 

 land. Much further south the mountains were covered with 

 perpetual snow, and sent glaciers down every valley ; and all the 



