Palaeontology, then, is the science which treats of the living beings, whether animal or vegetable, which have inhabited the earth during past periods of its history.  Its object is to elucidate, as far as may be, the structure, mode of existence, and habits of all such ancient forms of life; to determine their position in the scale of organised beings; to lay down the geographical limits within which they flourished; and to fix the period of their advent and disappearance.  It is the ancient life-history of the earth; and were its record complete, it would furnish us with a detailed knowledge of the form and relations of all the animals and plants which have at any period flourished upon the land-surfaces of the globe or inhabited its waters; it would enable us to determine precisely their succession in time; and it would place in our hands an unfailing key to the problems of evolution.  Unfortunately, from causes which will be subsequently discussed, the palaeontological record is extremely imperfect, and our knowledge is interrupted by gaps, which not only bear a large proportion to our solid information, but which in many cases are of such a nature that we can never hope to fill them up.

Fossils.—­The remains of animals or vegetables which we now find entombed in the solid rock, and which constitute the working material of the palaeontologist, are termed “fossils,"[3] or “petrifactions.”  In most cases, as can be readily understood, fossils are the actual hard parts of animals and plants which were in existence when the rock in which they are now found was being deposited.  Most fossils, therefore, are of the nature of the shells of shell-fish, the skeletons of coral-zoophytes, the bones of vertebrate animals, or the wood, bark, or leaves of plants.  All such bodies are more or less of a hard consistence to begin with, and are capable of resisting decay for a longer or shorter time—­hence the frequency with which they occur in the fossil condition.  Strictly speaking, however, by the term “fossil” must be understood “any body, or the traces of the existence of any body, whether animal or vegetable, which has been buried in the earth by natural causes” (Lyell).  We shall find, in fact, that many of the objects which we have to study as “fossils” have never themselves actually formed parts of any animal or vegetable, though they are due to the former existence of such organisms, and indicate what was the nature of these.  Thus the footprints left by birds, or reptiles, or quadrupeds upon sand or mud, are just as much proofs of the former existence of these animals as would be bones, feathers, or scales, though in themselves they are inorganic.  Under the head of fossils, therefore, come the footprints of air-breathing vertebrate animals; the tracks, trails, and burrows of sea-worms, crustaceans, or molluscs; the impressions left on the sand by stranded jelly-fishes; the burrows in stone or wood of certain shell-fish; the “moulds” or “casts” of shells, corals, and other organic remains; and various other bodies of a more or less similar nature.