[125] The name Diatomaceæ is restricted by M. Brébisson to those species which have a siliceous envelope, or cuticle; and that of Desmidiæ to those which are not siliceous, but reducible by heat to carbon.
[126] The reader interested in this subject should consult the beautiful work of Mr. Hassall on the Desmidiaceæ, published by Messrs. Reeve & Benham.
The extent of this infinitesimal flora throughout regions where no other forms of vegetation are known, is strikingly demonstrated by the observations of the eminent botanist and traveller. Dr. Hooker, in his account of the Antarctic regions.
"Everywhere," he states, "the waters and the ice alike abound in these microscopic vegetables. Though too small to be visible to the unassisted eye, their aggregated masses stained the iceberg and pack-ice wherever they were washed by the sea, and imparted a pale ochreous colour to the ice. From the south of the belt of ice which encircles the globe, to the highest latitudes reached by man, this vegetation is everywhere conspicuous, from the contrast between its colour and that of the white snow and ice in which it is imbedded. In the eightieth degree of south latitude all the surface ice carried along by currents, and the sides of every berg, and the base of the great Victoria barrier itself—a perpendicular wall of ice, from one to two hundred feet above the sea-level—were tinged brown from this cause, as if the waters were charged with oxide of iron. The majority of these plants consist of simple vegetable cells, enclosed in indestructible silex (as other Algæ are in carbonate of lime); and it is obvious that the death of such multitudes must form sedimentary deposits of immense extent.
"The universal existence of such an invisible vegetation as that of the Antarctic ocean, is u truly wonderful fact, and the more so, from its being unaccompanied by plants of a high order. This ocean swarms with mollusca, and entomostracous crustaceans, small whales, and porpoises; and the sea with penguins and seals, and the air with birds: the animal kingdom is everywhere present, the larger creatures preying on the smaller, and these again on those more minute; all living nature seems to be carnivorous. This microscopic vegetation is the sole nutrition of the herbivorous animals; and it may likewise serve to purify the atmosphere, and thus execute in the antarctic latitudes the office of the trees and grasses of the temperate regions, and the broad foliage of the palms of the tropics."[127]
[127] From Dr. Hooker's account of the botany of the South Polar regions in Sir J. Ross's Voyages of Discovery.
Dr. Hooker also observes, that the siliceous cases of the same kind of Diatomaceæ now living in the waters of the South Polar Ocean, have contributed in past ages to the formation of European strata; for the tripoli and the phonolite stones of the Rhine contain the siliceous shields of identical species. Such are the comments of one of our most eminent botanists on the phenomena under review. The reader will probably ask,—What, then, are the essential characters which separate the animal from the vegetable kingdom? To this question it is impossible to give a satisfactory reply: perhaps the only distinction that will be generally admitted by zoologists and botanists is the following:—animals require organic substances for their support; vegetables derive their sustenance from inorganic matter.
The facts thus cursorily reviewed throw much doubt on many of M. Ehrenberg's statements as to the identity of species of animalcules now living, with those whose remains occur in the eocene, and in the secondary strata. The so-called Xanthidia of the chalk, are certainly altogether distinct from the recent diatomæ to which the name was first applied; the chalk organisms are probably the gemmules of sponges or other zoophytes.[128]
[128] It would be convenient to distinguish these fossils by another name, and thus avoid the perpetuation of the error; I would propose that of Spiniferites, in allusion to the numerous spines with which all the species are beset.
Infusorial earths may therefore be composed either of microscopic vegetable or animal remains, or of both. The brackish and fresh-water deposits I have examined are siliceous and almost wholly diatomaceous: the marine calcareous strata composed of microscopic organisms, consist chiefly of various kinds of foraminifera, a large proportion belonging to the polythalamia, or chambered shells. I am not certain as to the animal or vegetable nature of some of the beautiful siliceous disks (Coscinodisci, Arachnoidisci, Actinocyclus, &c.) so abundant in the Richmond, Barbadoes, and Bermuda infusorial earths, and which occur in so splendid a state in the Guano deposits of Ichaboe.