CHAP. XII. J THE GLACIAL PERIOD. 811 



inhabitants would descend to the plains. By the time that the 

 cold had reached its maximum, we should have an arctic fauna 

 and flora, covering the central parts of Europe, as far south as the 

 Alps and Pyrenees, and even stretching into Spain. The now 

 temperate regions of the United States would likewise be 

 covered by arctic plants and animals and these would be nearly 

 the same with those of Europe; for the present circumpolar 

 inhabitants, which we suppose to have everywhere travelled 

 southward, are remarkably uniform round the world. 



As the warmth returned, the arctic forms would retreat north- 

 ward, closely followed up in their retreat by the productions of 

 the more temperate regions. And as the snow melted from the 

 bases of the mountains, the arctic forms would seize on the 

 cleared and thawed ground, always ascending, as the warmth 

 increased and the snow still further disappeared, higher and 

 higher, whilst their brethren were pursuing their northern journey. 

 Hence, when the warmth had fully returned, the same species, 

 which had lately lived together on the European and North 

 American lowlands, would again be found in the arctic regions of 

 the Old and New Worlds, and on many isolated mountain-sum- 

 mits far distant from each other. 



Thus we can understand the identity of many plants at points 

 so immensely remote as the mountains of the United States and 

 those of Europe. We can thus also understand the fact that the 

 Alpine plants of each mountain-range are more especially related 

 to the arctic forms living due north or nearly due north of them : 

 for the first migration when the cold came on, and the re-migra- 

 tion on the returning warmth, would generally have been due 

 south and north. The Alpine plants, for example, of Scotland, 

 as remarked by Mr. H. C. Watson, and those of the Pyrenees, as 

 remarked by Ramond, are more especially allied to the plants of 

 northern Scandinavia ; 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 productions of Europe and America, that 

 when in other regions we find the same species on distant moun- 

 tain-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 afterwards back- 

 wards 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. 



