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MEMORANDA. 65 
creased if a little space were afforded in it for queries and 
answers from your numerous subscribers.—F rep. H. Lane, 
President, Reading Microscopical Society. 
[We know of but few published diagrams available for 
microscopical lectures; and we think our correspondent’s 
suggestion a good one. The best diagram of microscopic 
objects that we know for a class-room is one published for 
the Government, by Messrs. Chapman and Hall, and exe- 
cuted by Mr. Marshall. The subject is the “ Microscopie 
Structure of the Tissues of the Human Body.” Our pages 
are always open to questions and answers on subjects of this 
kind.—Ebs. | 
Value of Habitat as a Distinctive Species.—At the time my 
short account of Homeeocladia in fresh water, contained in the 
July number of this Journal, was written, I was not aware that 
the subject I wished to draw attention to, viz., the occurrence 
of so-called marine diatoms in fresh water, and vice versd, 
had before been treated in that publication, or, in fact, any- 
where else. Shortly after I read a very able and valuable 
memoir by Professor Walker Arnott, on “ What are marine 
Diatoms,” which fully sustains my own views, and brings a 
large amount of evidence, well and forcibly displayed, to bear 
on the subject, the author arriving at the conclusion that, 
although Mr. Smith’s statements, ascribing a limited sphere 
of habitat to each species, may on the whole be correct, they 
are of very little practical value, and may even tend to mis- 
lead when applied to the distinction of species. Now, a 
theory disproved by the facts from which it is said to be 
deduced, should certainly be altogether dismissed, and those 
species which, directly or indirectly, derive their existence 
from the supposition of its correctness should be submitted 
to a thorough revision. It is not impossible that the ques- 
tion, whether certain species will survive a transfer from one 
medium into another and continue to thrive there, though 
perhaps becoming in time modified in appearance, may be 
decided by actual experiment. It is likely, then, that species 
hitherto considered distinct from presenting slight differences 
of aspect and from occurring in different media, will be 
united, when, instead of adding these grounds to form evidence, 
the one is found naturally to account for the other. Such 
experiments, if carefully conducted, would prove to be of 
more than practical utility, though this would be something ; 
and also of general interest, as throwing hght upon an im- 
VOL. V.— NEW SER. E 
