340 THE ORIGIN OF SPECIES 



fication; 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 of 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 

 were 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 temporate 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 further southward 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 con- 

 tinents. 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 degrees, lived during the Pliocene period further north, 

 under the Polar Circle, in latitude 66-67 degrees; and that the 

 present arctic productions then lived on the broken land still 

 nearer to the pole. Now, if we look at a terrestrial globe, we see 

 under the Polar Circle that there is almost continuous land from 

 Western Europe through Siberia, to Eastern America. And this 

 continuity of the circumpolar land, with the consequent freedom 

 under a more favorable climate for intermigration, will account for 

 the supposed uniformity of the sub-arctic and temperate produc- 

 tions of the Old and New Worlds, at a period anterior to the 

 Glacial epoch. 



Believing, from reasons before alluded to, that our continents 

 have long remained in nearly the same relative position, though 



