76 NOTES ON THE MARINE ALG 
Though not reckoned among marine alge, the occurrence of this 
species in brackish water seems rightly to give it a place in our 
list. I am not aware that it has been previously noticed in other 
than fresh water. Sir W. J. Hooker, conjectures that it may 
be merely Ulva Lactuca (a marine species) altered by difference 
of habitat; and its occurrence in the intermediate position, I 
think, materially strengthens his view. In point of structure 
there is no material difference between the two plants. 
Note on Callithamnion Rothit. 
Tue influence of external circumstances in introducing changes 
of species, and the range of altered conditions which individual 
species are qualified to bear without change or death, is a matter 
which seems as yet to be very imperfectly understood, notwith- 
standing that it lies near the root of much that has recently been 
written, respecting the succession of life on the earth. Viewed 
in this light, the following remarks on one of our native alge 
may not be without interest. 
In February of last, and June of the present year, I observed 
that certain stones near the mouth of a cave, a little north of 
Hawthorne Dene, were covered with a red velvetty fleece, which 
on examination proved to be Callithamnion Rothit, one of the 
smallest of the Rhodosperms, its filaments seldom much exceeding 
half an inch in length. The stones thus coated with the Callitham- 
nion were exposed to a heavy dripping of fresh water from the roof of 
the cave, (not merely a scanty fall ofdrops, few and far between, but 
a copious pelting, sufficient to wet one very unpleasantly while 
gathering specimens) and beyond the area of this dripping the 
plant did not extend. The mouth of the cave is not very far 
below high water mark, and I should judge that in this position 
the plant must be exposed to alternations of about four hours 
washing with sea water, and eight hours with fresh. I have met 
with no other instance of a Callithamnion growing under exposure 
to fresh water, and when C. Rothii is found (as is often the case) 
on rocks near high water mark, it is almost always so stunted as 
to amount to little more than a red film or crust, in which state 
