GEOGRAPHICAL DISTRIBUTION 159 



the mainland, would not receive colonists by similar 

 means. Out of a hundred kinds of seeds or animals 

 transported to an island, even if far less well-stocked 

 than Britain, perhaps not more than one would be so 

 well fitted to its new home as to become naturalized. 

 But this is no valid argument against what would be 

 effected by occasional means of transport, during the long 

 lapse of geological time, while the island was being 

 upheaved, and before it had become fully stocked with 

 inhabitants. On almost bare land, with few or no de- 

 structive 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 ex- 

 ist, 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 



—Science — 24 



