59 2 THE POPULAR SCIENCE MONTHLY. 



cudweed ( Gnaphtdium siqnnwn), the bog bilberry ( ~Vaccinium 

 uliginosum), the Alpine bearberry (Arctostcqyhylos Alpind), and the 

 Lapland phlox (Diapensia Lapponicd). Some of these, and others 

 like them, were old friends already familiar to me on European mount- 

 ains ; but some were fresh American acquaintances, whose faces I was 

 glad indeed to see on these bald summits. Both types, however, were 

 alike in their thoroughly northern and almost arctic aspect ; they were 

 the plants of Greenland, of Finland, of the North Cape in Norway, of 

 frozen Rupert's Land, of equally frozen Siberia. Some of them were 

 common to both hemispheres, some were peculiar to the New World, 

 but all, indiscriminately, were members of that same old circumpolar 

 flora which came into being at the extreme end of the Pliocene period, 

 when the world was just beginning to cool down at its extremities for 

 the long secular winter of the Glacial Epoch. 



Traces of that gradual cooling down are by no means wanting in 

 the geological deposits of either hemisphere. The Pliocene period, as 

 a whole, both in Europe and America, was an age of warm and genial 

 climates, of large and vigorous animal types, of rich, sub-tropicablook- 

 ing vegetation. In the Red and Norwich crags of England, for exam- 

 ple, we find the remains of mastodons and elephants, of hipparions and 

 hyenas, of the hippopotamus and the rhinoceros, of the tapir and the 

 horse. In the rich leaf-beds of the nearly contemporary Vienna basin, 

 we meet with a correspondingly warm sub-tropical flora — a vegetation 

 abounding in sequoias, liquidambars, and chestnuts, fragrant with cin- 

 namon, laurel, and tamarind. In the Loup River beds of the upper 

 Missouri region, again (Professor Marsh's " Niobrara group "), America 

 possesses similar mammalian remains of tropical and almost Oriental 

 character — a tiger larger than the Bengal beast, an elephant, a masto- 

 don, several rhinoceroses, and the earlier sketchy prototypes of the 

 camels and the horses. The period when such warm-weather creatures 

 flourished in such northern latitudes must surely have been one of very 

 genial climatic conditions. But, toward the close of the Pliocene age, 

 mutterings and forewarnings of the great glaciation begin to show 

 themselves, and to herald the advent of that vast ice-sheet which grad- 

 ually swallowed up in its devouring bosom the better portion of either 

 continent. 



Already in the Norwich crag of England the evolution of such 

 northern molluscan species as Scalaria Grcenlandica, Panopxza Nor- 

 wegica, and Astarte borealls (whose very names attest their arctic 

 habits in our own day), gave evidence of a slow but certain lowering 

 of the world's temperature. Nature only produces these cold-weather 

 types where the surrounding conditions have rendered the change 

 absolutely necessary. In the somewhat later Chillesford beds, the 

 great invasion of arctic kinds begins in earnest ; about two thirds of 

 the shells whose fossil remains form the fauna of the period still sur- 

 vive in high northern waters. Slowly, as the period of greatest ecccn- 



