CHAP. XII.] MEANS OF DISPERSAL. 309 



answered that he had found large fragments of granite and other 

 rocks, which do not occur in the archipelago. Hence we may 

 safely infer that icebergs formerly landed their rocky burthens on 

 the shores of these mid-ocean islands, and it is at least possible 

 that they may have brought thither some few seeds of northern 

 plants. 



Considering that these several means of transport, and that 

 other means, which without doubt remain to be discovered, have 

 been in action year after year for tens of thousands of years, it 

 would, I think, be a marvellous fact if many plants had not thus 

 become widely transported. These means of transport are some- 

 times called accidental, but this is not strictly correct : the 

 currents of the sea are not accidental, nor is the direction of 

 prevalent gales of wind. It should be observed that scarcely any 

 means of transport would carry seeds for very great distances : 

 for seeds do not retain their vitality when exposed for a great 

 length of time to the action of sea-water ; nor could they be long 

 carried in the crops or intestines of birds. These means, however, 

 would suffice for occasional transport across tracts of sea some 

 hundred miles in breadth, or from island to island, or from a 

 continent to a neighbouring island, but not from one distant 

 continent to another. The floras of distant continents would not 

 by such means become mingled ; 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 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 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 favourable 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 

 or any other continent, that a poorly-stocked island, though stand- 

 ing 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 naturalised. But this is no valid argument 

 against what would be effected by occasional means of transport, 

 during the long lapse of geological time, whilst the island was 



