Chap. XIL] THE GLACIAL PERIOD. 151 



as to become naturalised. But this is no valid argu- 

 ment against what would be effected by occasional 

 means of transport, during the long lapse of geologi- 

 cal 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 in- 

 sects or birds living there, nearly every seed which 

 chanced to arrive, if fitted for the climate, would ger- 

 minate and survive. 



Dispersal during the Glacial Period. 



The identity of many plants and animals, on moun- 

 tain-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 ex- 

 treme northern parts of Europe; but it is far more re- 

 markable, 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 



