GEOGRAPHICAL DISTRIBUTION 337 



from the West Indies to our western shores, where, if not killed 

 by their very long immersion in salt water, they could not en- 

 dure our climate. Almost every year, one or two land-birds are 

 blown across the whole Atlantic Ocean, from North America to 

 the western shores of Ireland and England; but seeds could be 

 transported by these rare wanderers only by one means, namely, 

 by dirt adhering to their feet or beaks, which is in itself a rare 

 accident. Even in this case, how small would be the chance of a 

 seed falling on favorable soil, and coming to maturity! But it 

 would be a great error to argue that because a well-stocked island, 

 like Great Britain, has not, as far as is known (and it would be 

 very difficult to prove this), received within the last few cen- 

 turies, through occasional means of transport, immigrants from 

 Europe or any other continent, that a poorly stocked island, though 

 standing more remote from the mainland, would not receive col- 

 onists 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 argu- 

 ment against what would be effected by occasional means of 

 transport, during the long lapse of geological time, while the is- 

 land 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 neaily 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 



