312 DISPERSAL DURING [CHAP. XII 



Hence, in accordance with the principles inculcated in this volume, 

 these forms will not have been liable to much modification. 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 probability, 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 to the plains ; they will, also, have been subsequently 

 exposed to somewhat different climatal influences. Their mutual 

 relations will thus have been in some degree disturbed; conse- 

 quently they will have been liable to modification ; and they have 

 been modified ; for if we compare the present Alpine plants and 

 animals of the several great European mountain-ranges one with 

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

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

 some as distinct yet closely allied species representing each other 

 on the several ranges. 



In the foregoing illustration I have assumed that at the com- 

 mencement of our imaginary Glacial period, the arctic productions 

 Avere as uniform round the polar regions as they are at the present 

 day. But it is also necessary to assume that many sub-arctic and 

 some few temperate forms were the same round the world, for 

 some of the species which now exist on the lower mountain-slopes 

 and on the plains of North America and Europe are the same ; 

 and it may be asked how I account for this degree of uniformity 

 in the sub-arctic and temperate forms round the world, at the 

 commencement of the real Glacial period. At the present day, 

 the sub-arctic and northern temperate productions of the Old and 

 New Worlds are separated from each other by the whole Atlantic 

 Ocean and by the northern part of the Pacific. During the 

 Glacial period, when the inhabitants of the Old and New Worlds 

 lived farther southwards than they do at present, they must have 

 been still more completely separated from each other by wider 

 spaces of ocean ; so that it may well be asked, how the same 

 species could then or previously have entered the two continents. 

 The explanation, I believe, lies in the nature of the climate 

 before the commencement of the Glacial period. At this, the 

 newer Pliocene period, the majority of the inhabitants of the 

 world were specifically the same as now, and we have good reason 

 to believe that the climate was warmer than at the present day. 

 Hence we may suppose that the organisms which now live under 

 latitude 60, lived during tLe Pliocene period farther north under 





