22 BKITISH SEA-WEEDS. 



does not love hard boundary-lines, and among Sea-weeds, 

 as in her other works, one form fades into another by 

 slow and almost imperceptible degrees. The marine 

 flora of our own shores offers a happy illustration of 

 this law, for while it embraces all the hardy, cold-loving 

 species, it also includes a very large proportion of deli- 

 cate southern forms, which reach their northern limit 

 among the sheltered bays of Devon, Cornwall, the South 

 and West of Ireland, and the Channel Islands. 



Oceanic currents have a very powerful influence on the 

 distribution of Sea-weeds. They carry masses of water 

 above or below the mean temperature far beyond the 

 limits where it would otherwise exist, and thus extend 

 the range of habitats suitable for particular genera and 

 species. They also convey the plants themselves, or their 

 spores, to new localities along the shores round which 

 they flow, and even from one coast to another, when they 

 cross a channel, a gulf, or any larger tract of sea. 

 The great Gulf Stream is the most notable example of 

 this agency, and its effect is very visible around these 

 Islands. The coasts of Devon and Cornwall and the 

 Western shores of Ireland are, with the exception of the 

 Channel Islands, the chief seats of our Algological 

 wealth ; and it is a curious fact that the same tender spe- 

 cies which appear to maintain with difiBculty a precari- 

 ous footing on the extreme southern corner of England, 

 grow more boldly in Ireland in a considerably higher la- 

 titude. The explanation of this apparent inconsistency 

 is doubtless to be found in the presence of the warmer 

 waters of the Gulf Stream, which flows in that direction. 



I have hitherto spoken of the effects of average climate 

 or temperature. I must not omit to mention that of ex- 

 ceptional intensity either of heat or cold. A long cold 



