ivill thus assist in the resolution of our doubts, and enable us to
assign to a form a systematic position and a specific name.
Ihese remarks, if they do not enable the inquirer to arrive at
certainty in the determination of specific characters, will at all
events, it is to be hoped, guard him from the rash conclusions
and hasty generalizations of the amateur microscopist, who is
disposed to rely upon the obvious characteristics of size and
form, and to regard such features as important distinctions.
Among organisms of such simplicity, these latter characters
are far from sufficient to establish specific distinctions, and it
requires a careful examination of specimens collected in various
localities, and m every condition of growth, to enable the observer
to fix upon the size that may be taken as the average, or the
outline which ought to be regarded as the type. Striation is
the best guide; but it sometimes happens that this feature
is so obscure, or so alike in allied species of the simpler forms, as
Cocconema, Cymbella, or Navícula, that our determinations must
be influenced by other considerations, and the arrangement of
the endochrome, or the habitat of the living frustule, or even less
important considerations, must be taken into account. A neglect
of these precautions will lead to the multiplication of synonyms,
to vagueness of description, and to a cumbrous and unscientific
nomenclature.
S e c t i o n XL
On t h e D is t r ib u t io n a n d U s e s oe t h e D ia t o m a c eæ .
The geographical range of the Diatomaceæ is so much more
general and uniform than that of the higher orders of plants,
that it would seem to be an established fact that many of thé
commoner species are universally distributed throughout the
waters of the globe. There is, however, some difficulty in coming
at very certain conclusions on this point in reference to any large
number of species, both from the little attention that has hitherto
been paid to the subject by the generality of naturalists, and
from the imperfect representations given of the forms observed.
Professor Ehrenberg has indeed discussed the subject on a
wide and comprehensive plan, and given to the world, in his late
great work, the ‘ Microgeologie,’ the results of researches to
which he has devoted an amazing amount of laborious patience.
Many of the gatherings described by this writer belong to formations
of a date sufficiently recent to illustrate the distribution
of living forms, but unhappily the want of specific descriptions,
and of minute accuracy in the figures, are serious drawbacks to
the value of the ‘ Microgeologie ’ in the hands of other ob-
seTvers not in possession of the specimens themselves, and leave
much to be desired by those who would seek to adopt its statements
as the basis of their generalizations. Without the confirmatory
evidence of personal observation, I therefore hesitate
to bring my own experience into parallelism with the statements
of this great authority, and content myself with the humbler
task of mentioning a few casual facts that have fallen under my
own notice ; many of them will be found recorded in the body
of this work ; but a notice here of some of the more curious or
important may interest the general reader.
Of freshwater species frequent in the British Islands, the
Mowing seem almost cosmopolitan, viz. Synedra radians, Pin-
nnlaria viridis, Pinnularia borealis, and Cocconema lanceolatum.
Gatherings from many localities in Europe, from Smyrna, and
Ceylon, from the Sandwich Islands, New Zealand, and New
York, from the loftiest accessible points of the Himalaya in Asia,
and the Andes in America, have supplied specimens of these
forms.
Navicula serians abounds in all our mountain bogs, and is
equally common in the marshes of Lapland and America.
Epithemia yibba is an inhabitant of the Geysers of Iceland
and the lakes of Switzerland.
The South Sea Islands supply Stauroneis acuta, and Ceylon
Synedra Ulna ; while Stauroneis Phoenicenteron is equally abundant
in Britain, Sicily, and Nova Scotia.
These notes of localities will give some idea of the wide
distribution of our fluviatile Diatomaceæ ; more numerous
gatherings would no doubt greatly extend the list, and the
following ch’cumstance will show how generally our commoner
British forms are diffused throughout European localities that