80 THE AGE OF MAMMALS 



VII. Pal^ogeography 



The present and past geographic distribution of the mammals, their migra- 

 tions, their sudden invasions, afford most valuable data for the science of 

 'palceogeography. Througii this science we attempt to restore the former out- 

 lines of the continents and seas. While, as shown by Wallace, small mam- 

 mals may be carried over sea considerable distances on driftwood, and while 

 many of the larger mammals are capable of swimming great distances, it is 

 certain that the distribution and colonization of the large terrestrial mammals 

 have only taken place through land connections between the various conti- 

 nents and islands. We are, in fact, forced again and again to assume the 

 existence of such land connections, but this fact does not enable us to plot the 

 former geographic outlines. To do this we require evidence furnished by 

 geology, which with the aid of that furnished by fossil shells makes it possible 

 to trace the former invasions of the sea over what is now land, and even to 

 establish closely certain ancient shorelines. The conclusions drawn from 

 existing and fossil mammals must also be compared with those drawn from 

 the distribution of birds, reptiles, fresh and sea water fishes, insects, and other 

 invertebrates, and in a very important degree of plants. 



Palseogeography is still an embryonic or nascent science with great possi- 

 bilities of future growth and usefulness. The hypothetical outlines of the 

 continents during the Age of Mammals as presented in this volume are repro- 

 ductions with slight modifications of those published by Matthew.' These 

 outlines represent a synthesis of the work of De Lapparent, Dall, Hill, 

 Hatcher, and others. 



They are to be interpreted, as originally stated by their author, as 

 both hypothetical and temporary. The author himself has now (1910) 

 abandoned as a matter of imperfect record the theory of an Antarctic land 

 connection between South America and Australia, as represented in his 

 map of the world in post-Cretaceous time, on p. 64 of this volume. He now 

 believes that the greater part of the animals and plants of the southern con- 

 tinents are of northern origin, and that the evidence advanced for Antarctic 

 connections is probably explainable through distribution from the north. 



Asiatic-American land bridge. — The theory of a North Pacific land 

 bridge over what is now Behring Straits still depends largely on the evidence 

 afforded by the distribution of the mammals, because the Csenozoic geology 

 of Alaska and of northeast Siberia is so imperfectly known. The most inter- 

 esting fact concerning this land bridge is that both from zoological and palae- 

 ontological data it appears to have been intermittent, to have emerged from 

 the sea and then to have become submerged. American geologists have con- 

 cluded that the northwestern coasts of Alaska and British Columbia were 

 above water during the Eocene to an extent as great or greater than at present. 



' Hypothetical Outlines of the Continents in Tertiary Times. Bull. Amer. Mas. Nat. Hist., 

 Vol. XXn, Art. xxi, 1906. 



