310 DISPERSAL DURING [CHAP. Xfl, 



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 flanks, 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. Throughout 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 

 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 southern 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 



