162 THE OHIO IN OF SPECIES 



more es])ecially allied to the plants of northern Scandi- 

 navia; those of the United States to Labrador; those of 

 the mountains of Siberia to the arctic regions of that 

 country. These views, grounded as they are on the 

 perfectly well-ascertained occurrence of a former Glacial 

 period, seem to me to explain in so satisfactory a manner 

 the present distribution of the Alpine and Arctic produc- 

 tions of Europe and America, that when in other regions 

 we find the same species on distant mountain-summits we 

 may almost conclude, without other evidence, that a 

 colder climate formerly permitted their migration across 

 the intervening lowlands, now become too warm for 

 their existence. 



As the arctic forms moved first southward and after- 

 ward backward to the north, in unison with the changing 

 climate, they will not have been exposed during their 

 long migrations to any great diversity of temperature; 

 and as they all migrated in a body together, their mutual 

 relations will not have been much disturbed. Hence, in 

 accordance with the principles inculcated in this volume, 

 these forms will not have been liable to much modifica- 

 tion. But with the Alpine productions, left isolated from 

 the moment of the returning warmth, first at the bases 

 and ultimately on the summits of the mountains, the case 

 will have been somewhat different; for it is not likely 

 that all the same arctic species will have been left on 

 mountain-ranges far distant from each other, and have 

 survived there ever since; they will also, in all proba- 

 bility, have become mingled with ancient Alpine species, 

 which must have existed on the mountains before the 

 commencement of the Glacial epoch, and which during 

 the coldest period will have been temporarily driven down 



