908 THE EXTINCTION OF SPECIES. 



another were continuous or an intermingling and exchange of species were rendered 

 possible by floral displacements occasioned by vicissitudes of climate. Before the 

 influx of the first Miocene sea through Servia into Hungary and Austria, the 

 Bakonyer Wald were joined to the Southern Limestone Alps; peaks of the height 

 of the Grossglockner lifted their heads where now only low crests surmount the 

 deposits of the Miocene sea, and those lofty peaks were no doubt clothed with 

 an alpine flora. Similarly there was then no lack of high mountains covered with 

 alpine vegetation between the Alps and the Carpathians. Geological information of 

 this character is certainly of great value when it is a question of explaining the close 

 relationships existing between the alpine flora of the Eastern Alps and that of the 

 Carpathians; but the presence of such mountains before the Miocene Period does not 

 suffice to explain the uniformity of the alpine species, the affinity existing between 

 the natural groups to which they belong, and the curious overlapping and interlacing 

 of the boundaries of their areas of distribution on the high mountain ranges which 

 run from west to east and from north to south. There must also have been at that 

 time some impelling cause to account for the intermingling of the floras in question, 

 and for the displacement of their boundaries. The only phenomena which can be 

 presumed to meet the case are alterations of climate of so drastic a nature as to 

 cause a simultaneous descent — and subsequently again a simultaneous return — of 

 the alpine species belonging to the two mountain-chains. These climatic changes 

 must have been the same as those which culminated in the successive formation and 

 advance and subsequent retreat of glaciers in those of the mountains which were 

 lofty enough and of suitable conformation. 



In the most widely different strata of our earth's crust, deposits occur which are 

 to all appearance moraine-debris, and are looked upon as glacial deposits by every 

 unbiassed geologist. There is, therefore, good ground for the hypothesis that an 

 alternate advance and retrogression of glaciers has taken place not only in the 

 Diluvial period, but also in the Tertiary period, and generally in all the periods dis- 

 tinguished by geologists. In my opinion the periodical return of a cold, wet climate, 

 manifested in suitable localities by an increase of glaciers, has everywhere and in 

 every age been the cause of migrations, and indirectly of inter-crossing, the formation 

 of new species and the extinction of old ones; and I think that, so far as it goes, it 

 accounts for the displacements, modifications of type, and other changes undergone 

 by the various floras in successive geological periods. Mountains have played an 

 important part in this history. They are able to produce an inexhaustible supply 

 of plants ever ready to colonize less elevated regions down to the plains below, for 

 their slopes are the camping-ground of plants adapted to every kind of climate. 

 When a slight diminution of temperature occurs, the denizens of the lower forest 

 region spread over the plains; a more considerable access of cold impels the plants 

 of the upper forest-region to become the invaders, and so on until it comes to the 

 turn of the vegetation which subsists close to the limit of perpetual snow, where the 

 snow vanishes for only about 50 days each year. And, just as on occasion of a fall 

 in temperature, the plants gradually descend the mountain sides and disperse them- 



