I
and fragile nature of the DesmidiaceEe is taken into consideration, this is hardly
to be wondered a t ; but as regards the Polycystina and Diatomacese, it is far from
improbable that we only fail to detect them owing to the presence, in both fresh
and salt water, of substances which unite with and thus obliterate all trace of
their siliceous structures. Should this surmise be correct, fossil representatives
of these organisms may yet be found whensoever an ancient sea, lake, or river-bed
shall present itself which has been so suddenly upheaved by subterranean force
as to have been denuded of its waters before chemical combination could take
place*. But, even without this additional fact, we have ample proof that the
were detected by me in the material obtained from the stomachs of Salpce in the Indian Ocean and mid-
Atlantic ; and although a t the time regarded by me as true pelagic representatives of the Desmidiacese
and Diatomacese, more recent observation has convinced me that, in the case of the foi-mer, the resemblance
is one merely of form and not of structure, the Xanthidium-like bodies being distinctly of
animal origin.
* Since the above observation was put in type, a short notice has appeared in the ‘ Geologist ’ for
June 1862, which, in a great measure, confirms this surmise. I t is there stated that, in the “ horn-
stone nodules of the Devonian and S ilw ia n rocks of New York, organisms similar to, and probably
identical with, those which occur in the Cretaceous flints and the Chalk itself, have been detected by
Professor Dana and Dr. M. C. 'White.” As the importance of this fact is in reality far greater than
a t first sight appears, I subjoin an extract from a paper, detailing my reasons for doubting the vegetable
origin of the organisms in question, published by me iu the ‘ Quarterly Journal of Microscopical Science ’
for October 1860, “ On the Siliceous Organisms found in the Digestive Cavities of the Salpm ” :—
“ The history of the fossil Xantliidia of the Cretaceous flint nodules has hitherto been beset with a
most perplexing difficulty. Apart from the much-vexed question as to the manner in which the
siliceous element of the nodules became aggregated into masses, so as to enshroud these structures, the
occurrence of organisms, held to be strictly inhabitants of fresh water, in deposits of unquestionable
marine formation, presented an ample field for discussion and conjecture. The Desmidiacete, of
which family the Xanthidia constitute the sporangia, are thus described by Mr. R a lfs: ‘ All tbe
family are inhabitants of fresh water. Mr. Thwaites, indeed, has gathered two or three species in
water slightly brackish; but the same species axe also found in localities remote from the sea. Certain
marine objects th at have been classed with the Desmidieae have the internal matter of a brown colour,
but these belong to the Diatomaceje.’—Ralfs, ‘ British Desmidiece,’ Introd. p. 2.
“ In order, therefore, to account for their occurrence amongst purely marine deposits, it was deemed
necessary to assume that they had been washed down by rivers or floods, or transported hy winds from
the land to the sea, where, after gradually subsiding, they became incorporated with the siliceous
material in which they were found imbedded. The tough, semi-horny texture of their outer covering,
and weR-known power of withstanding extreme climatic changes, seemed to favour this idea of their
indestructibility, even under conditions so opposed to those in which they originally lived. But the
explanation was, at best, unsatisfactory.
lower the organism in the scale of being, the greater is its numerical intensity,
the more universal its distribution, and the longer its duration in time,—one and
all of these characteristics being dependent, not on increase of power to withstand
destructive influences, but on decrease of sensibility to them.
I t is well known that the geographical distribution of terrestrial life is mainly
dependent on climate, that is to say, on the several agencies which modify the
condition of the atmosphere and the earth’s surface in different latitudes. These
agencies are temperature, elevation, amount of moisture, carbonic acid and
accidental gases in the atmosphere, configuration of surface, and light. I f we
turn to the ocean, we find that similar or at least equivalent agencies determine
the distribution of the creatures that inhabit it. The following Table will serve
to illustrate the relation of these agencies in the two cases, their aggregate values
being empirically estimated at 1000 :—
Land.
Temperature of a ir ........................................ 40O
Elevation above se a-lev e l........................... 1
Rarefaction of a i r .........................................J
Amount of moisture in d itto ........................... 100
Carbonic-acid gas in d itto ........................... 1
Ammoniacal and accidental gases in ditto J
Configuration and composition of surface .. 150
L ig h t.................................................................... 100
100
Sea.
Temperature of w a te r ........................................ 350
Aeration of d i tto .................................................. 300
Depth below surface .........................
Amoimt of pressure..............................
Carbonic acid in water.................................... j
Saline constituents in d i tto ...........................J
Configuration and composition of bottom .. 50
Amount of light ................................................... 50
Total.. 1000 1 Total.. 1000
“ The discovery of two well-marked varieties of Xanthidia, in a recent state, amongst the alimentary
contonts of the Salpcv, was therefore fraught with no slight interest. From the condition of their
cndochrome it is certain they had but recently been inclosed in the cavities of these creatures; and
from the situation in which they were discovered, it is equally certain that they could not have been
derived from freshwater sources by any of the agencies above referred to. But whether or not these
bodies are the sporangia of genuine pelagic Desmidiacece, they certainly exhibit an identity of appearance
and structure with the fossil forms, too striking to admit of question. The probability, however,
of their being trac pelagic representatives of this family is greatly enhanced by tho occurrence, in the
same material, of bodies closely resembling tho Closteria. These have only been met with sparingly,
and it has been impossible, therefore, to determine their nature with certainty. But the strongest
evidence in favour of thcii- identity with the fossil Xanthidia is derived from their association, in the
recent state, with other organisms, such as Diatomacece and Foraminifera, whose fossil remains, in
like manner, occur associated with them in the flints. Tho question as to them being true sporangia
of Desmidiacece becomes, therefore, of secondary importance.”