MEANS OP DISPERSAL. 865 



but would remain as distinct as they now are. The currents, 

 from their course, would never bring seeds from North 

 America to Britain, though they might and do bring seeds 

 irom 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 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 

 centuries, through occasional means of transport, immigrants 

 from Europe of 5iiy 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 transport^ 

 during the long i^pse of geological time, while the island 

 was being upheaved, and before i>. 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. 



t 



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 Whit© 



