tlie animal kingdom, for the secretion of siliceous spicnla, as
an internai skeleton, in some of tlie Spongideæ, cannot be regarded
as an analogous phænomenon ; whereas the vegetable
kingdom fmmishes us with cases, not merely of the secretion of
silex as a vegetable product in the Bamboo, hut with frequent
instances of its intimate union with cellulose in the membrane
which forms the epiderm of the cell, as in the Natural Orders
ah-eady mentioned, in the Palmaceæ and others.
The rapid evolution of oxygen from the frustules of the Diatomaceæ
Wilde in the active discharge of their nutritive function,
under the influence of the sun’s light and heat, is another circumstance
which confirms the view we have taken of their vegetable
nature : this may be noticed in any mass of Diatomaceæ
during the warmer months of the year, or in gatherings freely
exposed to the sun, in the elevated temperature of a confined
apartment during the winter or spring. Under these conditions
the water in the vessel becomes covered with minute
bubbles of oxygen, and portions of the Diatomaceous stratum
are floated up by the buoyancy of the globules of this gas adhering
to their frustules. Such phænomena can only be accounted
for by supposing that the Diatomaceæ are plants, and
' that they exhale, like all plants in a state of active vegetation,
oxygen from their tissues ; but this process is irreconcileable
with the hypothesis of their animal nature.
Another view of the nature of the Diatomaceæ has been
advocated by some writers, who, admitting that the arguments
for the animality of these organisms are devoid of weight, maintain
that the evidence of their vegetable nature is equally inconclusive,
and contend that they occupy a neutral position between
the two kingdoms, and cannot in the present state of our knowledge
be assigned to either. It does not appear to me that any
benefit would accrue to physiology or science by thus constituting
a new domain of being, occupied by organisms of indefinable
and mysterious attributes, and for whose description, if
description in their case were possible, it would be necessary to
invent a new and probably unintelligible nomenclature. This
would be merely to disguise our ignorance under the veil of a
metaphysical sophistry. It may be admitted, that in the case of
entities so simple and minute, it is difficult to detect any very
manifest specialties of function, or to assign to them the attributes
that are obviously characteristic of higher organisms ;
but if we can show that their organization ministers to functions
that are purely of a vegetative kind, and that none of the
circumstances which attend their growth, development, and reproduction,
are inconsistent with the known phænomena of
plant-life, we are warranted, and indeed constrained, to associate
them in a systematic arrangement with that department of
organized being to which they are thus functionally and structurally
allied, or from which at all events they arc not in these
respects excluded by any well-ascertained or notable diversities.
We conclude then that the Diatomaceæ are plants belonging
to the Sub-Class Algæ ; or, following the more recent system-
atists, to the Class Protophyta, all of whose forms, unless when
united into filaments, or aggregated into masses by the mecha^
nical aid of the mucus they so frequently secrete, are microscopic
and unicellular,—homologues of the component parts of
the tissue which forms the entirety of the Thallogen, and enters
largely into the composition of the higlier and more organized
forms of vegetable life. The Diatomaceæ, with specialties of
their own, have also intimate alliances with the other orders of
the Protophyta, resembling the Zygnemaceæ and Besmidiaceoe in
the reproductive process,—the Nostocaceæ in the tendency shown
by several genera to surround their frustules with frondose
masses of mucus, within which linear series of cells are subsequently
developed,—the Oscillatoriaceoe in their movements,—
the Palmellaceæ and all the orders I have named, in the self-
dividing act by which the individuals of the species are multiplied,
or the aggregate of specific life maintained and increased.
S e c t i o n X.
On t h e D e t e rm in a t io n oe S p e c ie s in t h e D ia t o m a c eæ .
In an order prolific of forms, so minute in size, and simple in
organization, it is by no means an easy matter to fix upon any
certain elements of specific arrangement.