In regarding geological phenomena, the absence of any fossil remains has often been supposed to indicate a period previous to any organic formations. The inorganic constituents of matter are probably of prior origin to the organic combinations; the vessel was constructed, upon which the organic creation was to float in space before any vital organisms were created. The supposed evidences in favour of the assumption that there was no organic life during the formation of the oldest rocks we know, are in some respects doubtful; and we can well understand that changes may have been induced in the earlier rock formations, by heat or by other powers, quite sufficient to destroy all traces of organized forms. It was long thought that phosphoric acid was not to be detected in rocks which are regarded as of igneous origin; and since this acid is peculiarly a constituent of organic bodies, this has been adduced as a proof that the plutonic rocks must have existed previously to the appearance of vegetable or animal life upon the surface of the globe. The researches of modern chemists have, however, shown that phosphoric acid is to be found in formations of granitic origin, in porphyry, basalt, and hornblende rocks.[233] If, therefore, we are to regard this substance as of organic origin, the rational inference is against the speculation; but there is no more necessity for supposing phosphorus to be formed in the animal economy than in the mineral kingdom, from which it will probably be found the animal obtained it. Without attempting to enter into any account of the apparent progress of life over the earth, it appears desirable that some description should be given of the kinds of plants and animals which we know to have existed at different epochs. We shall thus learn, at least, some of the prevailing characteristics of the earth during its transitions, and be in a better condition for applying our knowledge of physical power to the explanation of the various geological phenomena. Among the earliest races we have those remarkable forms, the trilobites, inhabiting the ancient ocean. These crustacea bear some resemblance, although a very remote one, to the common wood-louse, and, like that animal, they had the power of rolling themselves into a ball when attacked by an enemy. The eye of the trilobite is a most remarkable organ; and in that of one species, _Phacops caudatus_, not less than two hundred and fifty lenses have been discovered. This remarkable optical instrument indicates that these creatures lived under similar conditions to those which surround the crustacea of the present day. At the period of the trilobites of the Silurian rocks, all the animals contemporaneous with them had the organs necessary for the preservation of life in the waters.