904 THE EXTINCTION OF SPECIES. 



&c., cf. p. 462) which have been discovered in Central Germany are relics of this 

 period, that they lived with the Steppe-plants and withdrew eastward at the same 

 time in consequence of the change of climate. It is difficult to say when these 

 changes took place in Central Europe, but this m.uch is certain, that the Steppe- 

 climate prevailed for an exceedingly long time, that the alteration of that climate 

 into the kind which now prevails took place quite gradually, and that accordingly 

 the migration of the Steppe-flora and fauna into the region now occupied by them 

 was performed very slowly. 



As the very thing which is injurious to the members of one flora is usually 

 beneficial to those of a neighbouring flora the migrations of plants really take 

 the form of displacements of the boundaries of distribution. No sooner do the 

 species of one flora withdraw to escape a climate that has become unsuitable to them 

 than their place is taken by those members of the neighbouring flora which arc 

 adapted to the new climate. In the case above referred to, an immigration of such 

 members of the Baltic flora as prefer a comparatively cool, moist summer would 

 inevitably take place synchronously with the retreat of the Steppe-plants. As 

 regards the situation of the previous home of these Baltic plants there can be no 

 doubt. They came from adjacent regions where the climatic conditions congenial to 

 them already existed, that is to say, from parts then forming the coast and from 

 those mountains which had not been ascended by the Steppe-flora. In advancing 

 inland from the coast and descending from the mountains these plants were only 

 in a measure retracing their steps to places where they formerly occupied the 

 ground, and from which they had been ousted by the Steppe-plants. In other 

 words, before the reign of the Steppe-flora of the Black Sea was established in the 

 valleys and lowlands of Central Europe another flora lived there which closely 

 resembled that which we now call the Baltic flora. No approximate estimate can 

 be given of the length of time, previous to the immigration of Steppe-plants, during 

 which the Baltic flora was in possession of the tract of country thus destined to fall 

 a second time under its dominion; but it has been established beyond question that 

 it was not as yet upon the scene at the period of the greatest prevalence of glaciers 

 in Central Europe, and that its first immigration cannot have taken place until after 

 the retreat of the large glaciers. 



At the epoch when glaciers attained their maximum dimensions the places now 

 covered by the forests of Pines and Firs, which are so characteristic of the Baltic 

 flora, and by vast scrubs of heaths and broom, were occupied by low Alpine plants 

 which may for the sake of brevity be spoken of collectively as an Alpine Flora. 

 Formerly botanists were of opinion that this wonderful flora spread southwards like 

 a flowing stream from the Arctic Regions at the epoch in question. This view is not, 

 however, in harmony with more recent discoveries. It was based on the erroneous 

 assumption that the flora of the Arctic Regions was the same as that of the alpine 

 regions of Central and Southern Europe. When we compare the Arctic and the 

 Alpine floras merely by means of their records in books and herbaria, it does indeed 

 look as if the closest relationship existed between the two; for a not inconsiderable 



