JV. INTRODUCTION. 23 



a great resemblance is observed between the ordinary seaweeds that clothe the rocks 

 on the eastern and western sides ; with this difference, that the species do not reach 

 so high a latitude on the American shore as on the European. The reason of this 

 will be readily understood by inspecting a physical map of the Atlantic, on which 

 Humboldt's Isothermal lines, or lines of mean annual temperature, are laid down. 

 I^or then it will at once be seen that there is a very considerable bending of the 

 Isothermal lines in favour of the continent of Europe. Thus the same line that 

 runs through New York, in lat. 41°, strikes the shores of Europe in the North of 

 Ireland, lat. 54°. And though there is less difference in mean temperature in the 

 southern parts of the continents than in the northern, still there is a marked 

 difference throughout. 



"With respect to vegetation, Laminaria longicriins is common on the American 

 shore — at least as far south as Cape Cod (lat. 42°) ; while on the European it has 

 not been found south of Norway, save some stray, waterworn stems occasionally 

 cast on the north of Ireland or Scotland. 



Khody7nenia cristata, so very abundant in Boston harbour, (42° 30'), where it 

 enters largely into the composition of seaweed pictures, is rarely found in Europe 

 south of Iceland and the northern parts of Norway ; its most southern limit being 

 in the Frith of Forth, (56°), where it has been found but once or twice. 



Deksseria hypoglossum has not been observed in America north of Charleston, 

 (lat. 33°), while in Europe it occurs in Orkney, (lat. 59°), and is in great profusion 

 and luxuriance on the north coast of Ireland in lat. 55°. The distribution of this 

 species on the American shore is very anomalous if Charleston be its northern 

 limit, for it certainly extends southward at least to Anastasia Island, (lat. 29° 50'). 

 In the British seas it is most luxuriant on the Antrim shore, (55°), where its fronds 

 are sometimes three feet in length ; southern specimens are generally much 

 smaller, and in Devonshire it rarely measures more than three or four inches, 

 which is the average size of specimens from the south of Europe, as well as of those 

 found in Charleston harbour. If we are correct in limiting the American distribu- 

 tion of this species northward by Charleston, we have the remarkable fact that the 

 greatest latitude attained by Del. hypoglossum in the north-western Atlantic is less 

 by about 5° or 6° than the southern limit of the same species on the north-eastern, 

 and by about 27° than the northern boundary of its distribution. This indicates 

 a range which the isothermal lines can scarcely explain ; for the line which runs 

 through Charleston strikes the coast of Spain. It is the more remarkable in this 

 species, because the genus Deksseria is most numerous in the colder parts of the 

 sea, its finest species being natives of Northern Europe and of Cape Horn and the 

 Falkland Islands ; and, as we have seen, this very D. hypoglossum is no where of 

 greater size or in greater plenty than in latitude 55° on the Irish coast. 



It is different with Padina Pavonia, itself a tropical form, and belonging to a 

 group peculiarly lovers of the sun. We are not surpi'ised that in America this 

 plant should not grow further north than the Keys of Florida, although, under 

 some peculiarly favourable circumstances, it attains a limit 27° further north, 

 on the south coast of England ; for in the land-vegetation of the Uvo coasts there 

 is something like an approach to similar circumstances, oranges and citrons being 



