Glacial Epoch. 41 



ORIGIN OF TYPKS AND FAC.XAS GEOLOGIC EVIDENCE. 



In speaking of the Boreal and Sonoran origin of species and 

 groups in the present paper, the term l origin ' is used exclusively 

 in a sense intended to indicate present centers of distribution 

 not real or ancient centers of origin for it must be borne in 

 mind that the history of the inhabitants of the earth is not only 

 a history of the successive appearance and disappearance of 

 types now extinct, but a history of great movements of vast 

 migrations to and fro over the surface of the globe and little is 

 known of the real points of origin of our Boreal and Tropical 

 faunas and floras. The geologic evidence demonstrates that in 

 the past large land areas have been many times joined together 

 and many times rent asunder. The establishment of land con- 

 tinuity between areas previously disconnected has made it pos- 

 sible for new forms of animals and plants to obtain a footing 

 and spread over regions previously uninhabited by them often, 

 doubtless, at the expense of the indigenous fauna and flora. 

 Even great continents, as North and South America, have been 

 more than once united and separated ; and the last union of 

 these continents it so recent we can distinctly trace at the pres- 

 ent day the course and distribution of the intrusive forms. 



On the other hand, in comparatively recent times, multitudes 

 of species and genera, and even families and higher groups, 

 have suddenly disappeared from large areas where they were 

 formerly abundant, and some of them from the face of the 

 earth, so that the fauna of the recent past compared with that 

 of today presents some strange contrasts. North America in 

 Pleistocene times was inhabited by associations of mammals 

 not now living on this continent but found in as far distant 

 parts of the earth as Asia and South America; for horses, 

 camels, and elephants then lived here with llamas, tapirs, and 

 capybaras. With them were others now altogether extinct, as 

 huge tigers, wolves, cave bears, the great Mastodon, the Megathe- 

 rium, Megalonyx, Mylodon, and other gigantic sloths. 



GLACIAL EPOCH. 



The cause of this sudden extermination of dominant types is 

 believed to have been the Glacial epoch, which is known to have 

 driven species of animals and plants from the poles to the 



(I RIOT,. Sor., WASH., VOL. VII, Isii-j. 



