390 DISPERSAL DURING 



of their having migrated from one point to the other. It 

 is indeed a remarkable fact to see so many plants of the 

 same species living on the snowy regions of the Alps or 

 Pyrenees, and in the extreme northern parts of Europe; 

 but it is far more remarkable, that the plants on the White 

 Mountains, in the United States of America, are all the 

 same witli tliose of Labrador, and nearly all the same, as 

 we hear from Asa Gray, with those on the loftiest mount- 

 ains of Europe. Even as long ago as 1747, such facts led 

 Gmelin to conclude that the.sarae species must have been 

 independently created at many distinct points; and we 

 might have remained in this same belief, had not Agassiz 

 and others called vivid attention to the Glacial period, 

 which, as we shall immediately see, affords a simple expla- 

 nation of these. facts. We have evidence of almost every 

 conceivable kind, organic and inorganic, that, within a 

 very recent geological period, central Europe and North 

 America suffered under an arctic climate. The ruins of a 

 house burned by fire do not tell their tale more plainly than 

 do the mountains of Scotland and Wales, with their scored 

 flanks, polished surfaces, and perched bowlders, of the icy 

 streams with which their valleys were lately filled. So 

 greatly has the climate of Europe changed, that in North- 

 ern Italy, gigantic moraines, left by old glaciers, are now 

 clothed by the vine and maize. Throughout a large part 

 of the United States, erratic bowlders and scored rocks 

 plainly reveal a former cold period. 



The former influence of the glacial climate on the dis- 

 tribution of the inhabitants of Europe, as explained by 

 Edward Forbes, is substantially as follows. But we shall 

 follow the changes more readily, by supposing a new glacial 

 period slowly to come on, and then pass away, as formerly 

 occurred. As the cold came on, and as each more' south- 

 ern zone became fitted for the inhabitants of the north, 

 these would take the places of the former inhabitants of 

 the temperate regions. The latter, at the same time, 

 would travel further and further southward, unless they 

 were stopped by barriers, in which case they would perish. 

 The mountains would become covered with snow and ice, 

 and their former Alpine inhabitants would descend to the 

 plains. By the time that the cold had reached its maxi- 

 mum, we should have an arctic fauna and flora, covering 



