IV. 
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. 
For 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 longicruris 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, watenvorn stems occasionally 
cast on the north of Ireland or Scotland. 
Rhodymenia 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. 
Delesseria 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 Delesseria 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 surprised 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 two coasts there 
is something like an approach to similar circumstances, oranges and citrons being 
