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] [Footnote 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. With the corrections which the above remarks will enable the reader to make, I would refer to the account of Fossil Infusoria in the Medals of Creation, and Wonders of Geology. [129] [Footnote 129: See also "Thoughts on Animalcules."]