330 DISPERSAL DURLNG THE GLACIAL PERIOD. [Chap. Xli. 



lapse of geological time, whilst the island was being upheaved, and 

 before it had become fully stocked with inhabitants. On almost 

 bare land, with few or no destructive insects or birds living there, 

 nearly every seed which chanced to arrive, if fitted for the climate, 

 would germinate and survive. 



Dispersal during the Glacial Period. 



The identity of many plants and animals, on mountain-summits, 

 separated from each other by hundreds of miles of lowlands, where 

 Alpine species could not possibly exist, is one of the most striking 

 cases known of the same species living at distant points, without 

 the apparent possibility 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 with those of 

 Labrador, and nearly all the same, as we hear from Asa Gray, with 

 those on the loftiest mountains of Europe. Even as long ago as 

 1747, such facts led Gmelin to conclude that the same 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 explanation 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 burnt by fire do not tell their tale more plainly 

 than do the mountains of Scotland and Wales, with their scored 

 Hanks, polished surfaces, and perched boulders, of the icy streams 

 with which their valleys were lately filled. So greatly has the 

 climate of Europe changed, that in Northern Italy, gigantic moraines, 

 left by old glaciers, are now clothed by the vine and maize. Through- 

 out a large part of the United States, erratic boulders and scored 

 rocks plainly reveal a former cold period. 



The former influence of the glacial climate on the distribution of 

 the inhabitants of Europe, as explained by Edward Forbes, is sub- 

 stantially 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 southern zone became fitted for the inhabitants of the 

 north, these would take the places ef the former inhabitants of 

 the temnerate regions. The latter, at the same time, would travel 



