MBA]}IS OF DISPERSAL. 389 



island, but not from one distant continent to another. 

 The floras of distant continents would not bv sucli means 

 become mingled; but would remain as distinct as they now 

 are. The currents, from their course, would never \v\uit 

 seeds from North America to Britain, though they might 

 and do bring seeds from the West Indies to our western 

 shores, where, if not killed by their very long immersion 

 in salt water, they could not endure our climate. Almost 

 every year, one or two land-birds are blown across thy 

 whole Atlantic Ocean, from Kortli America to the western 

 shores of Ireland and England; but_ seeds could be tnins- 

 ported by these rare wanderers only by one means, luimelv, 

 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 tliat 

 because a well-stocked island, like Great Britain, has not, 

 as far as is known (and it would be very dithcult to prove 

 this), received within the last few centuries, through occa- 

 sional 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 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 transpoit, 

 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 

 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 i)ossibly exist, is 

 one of the most striking cases known of the same species 

 living at distant points, without the apparent possibility 



