(ilacial Kpoch. 41 



()i!i<;i\ OF T^-^>Ks AM) Faixas — ({kologic Evidence. 



Ill .^])('aking' of the Jk)rc:il and .Sonunin origin of species and 

 groups in the present pajxn-, the term ' oricihi ' is used exclusively 

 in a sense intended to indicate jin'sciil anitera of (listrihufioii — 

 not I'eal or ancient centers of origin — for it must l)e l)orne in 

 mind that the history of tlie inhahitants of the earth is not. only 

 a liistory of the successive apjiearance and disappearance of 

 tyj)es now extinct, hut a history of great movements — of vast 

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

 known of the real })oints of origin of our Boreal and Tropical 

 faunas and tioras. The geologic evidence demonstrates that in 

 the past large land areas have Ijecn many times joined together 

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

 tinuity between areas })revioiisly disconnected has made it pos- 

 sil)le for new forms of animals and plants to obtain a footing 

 and s})read over regions previously uninhabited by them — often, 

 doubtless, at the expense of the indigenous itiuna 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 iVom 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 l)y associations of mammals 

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

 jiarts of the earth as Asia and .South America; for liorses, 

 camels, and elephants then lived here with llamas, tai)irs, and 

 capybaras. With them were others now altogether extinct, as 

 huge tigers, wolves, cave bears, the great Mastodon, tl)c Megathe- 

 rium, ^legalonyx, ^lylodon, and otlier gigantic slotlis. 



Glacial Epoch. 



Tlic cause of this sudden extermination of dominant ty])es is 

 believed to have l)een the (ilacial epoch, which is known to have 

 driven s])ccies of animals ami jilants from tlic jioles to tlie 



(1— r.idi.. Sdc. Wash.. ^^lI.. \'II, Isiii'. 



