THE EXTINCTION OF SPECIES. 903 



species wliich in one district form a considerable part of the flora in possession at 

 the present day, whilst in another floral area they are only found in the fossil state 

 and under conditions which leave no doubt that they formerly lived there, but have 

 long died out. Rhododendron Ponticum, for example, is an important constituent 

 of the flora which now flourishes in the neighbourhood of the Black Sea, and is also 

 found far to the west in an unrestricted area in the South of Spain at a great 

 distance from the main area of distribution. It is encountered in the fossil state on 

 the southern slopes of the Solstein chain in Tyrol in the upper strata of the so-caUed 

 Hettinger Breccia. Thus this plant must have ranged formerly through Southern 

 and Central Europe to 47° N. Lat. In the South of Spain it has survived on an 

 isolated spot, whilst in the Northern Dolomites it has died out. A similar instance 

 is ofiered by the case of several Juglandacese which form part of the woods of North 

 America at the present day, and are only found as fossils in Europe. 



The results of researches into the history of the separate species constitute the 

 foundation for a history of the entire plant- world. Formerly the discovery of fossil- 

 remains was looked upon as the most important means of eliciting that history, but 

 now the distribution of living plants is taken into account, and the significance of 

 such circumstances as the presence of endemic species and of species isolated in the 

 midst of a foreign flora is fully recognized. A study of endemic species and of 

 outlying sub-areas of distribution yields in particular most valuable information 

 concerning the conditions which prevailed in the earliest periods of the earth's 

 history. The most noteworthy inference made in this connection is that over a 

 great part of central Europe since the last ice-age a flora was evolved which was 

 only capable of existing under the influence of a continental climate of far greater 

 warmth than now prevails. For instance, plants whose main areas of distribution 

 at the present day are in the steppes of Southern Russia, in the Crimea, and in the 

 vaUeys of the Caucasus are found growing, sometimes alone sometimes in com- 

 munities, in Central Europe, in the region of the Baltic Flora, on warm, sunny 

 mountain-slopes, and in sequestered glens far from the modern lines of traffic, and 

 under circumstances which exclude all possibility of an immigration having taken 

 place in recent times. Such exceptional habitats of the plants in question occur on 

 hot, sombre rocks of serpentine in Lower Austria, on terraces of loess and mountains 

 of schist, situated on the eastern border of the Bohmer Wald and the Mahrische 

 Gebirge, in the interior of Bohemia and westwards on scattered spots as far as the 

 Harz Mountains, and, again, in the region of the Northern and Central Alps, from 

 the Wiener Becken to the Lake of Constance, as, for example, far away in the highest 

 parts of the valleys of the Adige and the Inn. These plants may be for the most part 

 described as Steppe-plants, and if, as can no longer be doubted, they are the remains 

 of a flora which once ranged in Central Europe as far as the Harz Mountains, we 

 may conclude that just before the establishment of the present climatic conditions 

 which suit the Baltic Flora, a Steppe-climate prevailed over the area referred to, 

 and the summer was hot and dry. There is good ground for supposing that the 

 various animals belonging to the Steppe-fauna (Steppe -antelope, Steppe -marmot. 



