CiiAi'. XI. DUKING THE GLACIAL PERIOD. 339 



United States to Labrador ; those of the mountains of Siberia 

 to the arctic regions of that country. These views, f^rounded 

 as they are 011 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 pro- 

 ductions of Europe and .Vmcrica, that, when in other regions 

 we lind the same species on distant mountain-summits, Ave may 

 almost conclude, without other evidence, that a colder climate 

 lV)rnierly permitted their migration across the intervening low- 

 lands, now become too warm for their existence. 



As the arctic forms moved first southward and afterward 

 backward to the north, in unison with the changing climate, 

 they w ill not have been exposed during their long migrations 

 to any great diversity of temperature ; and, as they all migrated 

 in a body tog-ether, their mutual relations will not have been 

 much disturlicd. Hence, in accordance with the principles in- 

 culcated in tliis volume, these forms will not have been liabli> 

 to much modification. But with the Alpine productions, left 

 isolated fi-om 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, antl have survived there ever since; 

 they will, also, in all probability, have become mingled Avitli 

 ancient Alpine species, which must have existed on the moun- 

 tains before tlie commencement of the Glacial epoch, and which 

 during its coldest ])eriod will have been temporarily driven 

 down to the plains ; they will, also, have been exposed to some- 

 what different climatal influences. Their mutual relations will 

 thus have been in some degree disturbed ; consequiMitlv they 

 will have been liable to modification ; and this we find has been 

 the case : for if we compare the present Alpine j)lants and ani- 

 mals of the several great European mountain-ranges one with 

 another, though many of the species remain idmitically the same, 

 some exist as varieties, some as doubtful forms or sub-species, 

 and some as certainly distinct yet closely-allied species repre- 

 senting each other on the several ranges. 



In the foregoing illustration T have assumed that at the 

 commencement of our imagin.ary Glacial period, the arctic pro- 

 ductions were as uniform round the polar regions as they arc 

 at the present day. But it is necessary also to include many 

 sulvarctic and some few temperate forms, for some of these are 

 the same on the lower mountain-slopes and on the plains of 



