"The universal existence of such an invisible vegetation as that of the Antarctic Ocean is a 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." Dr. Hooker also remarks that the siliceous envelopes of the same kinds 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 envelopes of identical species. Such are the comments of one of our most distinguished botanists, on the phenomena under review. The reader will perhaps 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_. [Sidenote: RECENT DIATOMACEÆ.] Recent Diatomaceæ. _Plate IV._--To familiarize the reader with the nature of these vegetable organisms, a few recent species are represented in Plate IV., coloured as they appear when alive, under the microscope; the figures are magnified as expressed by the fractions. _Xanthidium._ _Plate IV. figs. 1, 2, 3, 4, 5._--The case or frustule of this genus consists of a hollow, siliceous globe, beset with spines. The increase of the Xanthidia by self-division, produces the double appearance in the figures, all of which are in the progress of separation.[63] [63] The organisms so abundant in the flint and chalk, and which were referred by M. Ehrenberg to this genus, and consequently described under the name of Xanthidia by myself and others, are certainly in nowise related to the recent forms: they are flexible envelopes, and probably belong to zoophytes; as will be shown in the sequel. _Pyxidiculum._ _Plate IV. fig. 2._--The case is a little saucer-shaped box, and is invested by a membrane.