CHAPTER VII. THE CONTINUITY OF LIFE. Explanation of the term-The domain of paleontology-The records of geological time-Nature and character of fossils-Cases of remarkable preservation-Fossils represent extinct species-The laws of succession and descent-Inferences from fossils as to the distribu. tion of sea and land—Imperfections of the record-Proportion of terrestrial and aquatic forms preserved-Continuity of the zoological scale-The doctrine of evolution of species, in opposition to catastrophism. THE term "continuity of life," placed at the head of this chapter, is employed to express the result of the examination and comparison of all the plant and animal remains discovered in the various formations; whence it has been concluded that organisms of different degrees of perfection have succeeded each other in a long line of descent, broken at several points, but preserving in the main indisputable evidences of progress from lower to higher forms of structure. On the accuracy with which the line of succession is determined, the relation in time of one formation to another depends. In itself, a sedimentary rock gives no true indication whether it was formed at the very earliest or very latest period. Thus, a sandstone or limestone of Jurassic age could not be distinguished by mere mineral composition from one of Devonian, or even Meiocene age; and were we dependent upon lithological characters, the rocks could only be roughly classed in accordance with the nature and proportions of mineral matters contained in them, and we should have before us an accumulation of almost meaningless facts, which would tell us no more than that some rocks had been formed from minute particles of carbonate of lime, others from small grains of silica, etc., and that water had probably been concerned in the special arrangement of their constituents. Further than this it would be impossible to go, and the true history of the earth must for ever have remained a sealed book. But we are not thus left in ignorance, for each rock of the sedimentary class contains clear records of its position in the series, and from a careful comparison of these we reach trustworthy conclusions as to the relations of rocks to one another in the great scale of existence. Fossils are the records of life on the earth-the remnants of its past botany and zoology-the foundation of paleontology or the "science of ancient living beings." No indications of the presence of life can be found in the igneous or fire-formed rocks, as will be obvious from their origin; but they abound in those of stratified character, and, within certain limits, serve as sure guides to the determination of the relative ages of the formations in which they are present. * A fossil may be described as the trace of the existence of any once animated being, preserved in the rocks, and * Fossils may, however, be accidentally included in lava. the most untutored mind would generally have no hesitation in recognizing it as such from its resemblance to living forms. It may be a mere fragment, or it may be perfect so far as its hard parts are concerned-the shell of a mollusc, the backbone of a fish, the skeleton of an animal, or the leaf or stem of a plant. Further, there may be internal or external casts of such objects as shells, which have fallen upon an originally soft deposit. In the first case, the deposit, hardening, will have given an impression of the inside of the shell; and in the second, one of the outside; while the calcareous shell itself may have been dissolved by chemical action, its place being occupied by other material. Impressions of the footprints of birds and animals, of raindrops, and even the ripple-marks of water, may also be classed as fossils, and they are valuable indications of the conditions which prevailed at the time when they were made. As a general rule, fossils may be fairly assumed to be contemporaneous with the deposits in which they occur; though, in later times especially, it has frequently happened that richly fossiliferous rocks have been broken up and become partially recemented, when the older fossils are found associated with the new formation as intruders among those properly belonging to the latter. This, however, will not vitiate the general conclusion that any fossil imbedded in a rock belongs to the life-period of the deposit containing it. If a fossil were presented to us for examination without any previous knowledge of its history, we should observe that the shell, bone, or wood had undergone an extraordinary change. It is no longer in the form of lime or wood, comparatively light and soft, but exceedingly heavy and hard. Decomposition has removed all the animal or vegetable matter the fibres, tissues, and muscles and nothing but the skeleton remains; and even this is unfamiliar to us as the substance of any shell we may pick up upon the sea-shore, or of a bone bleached by the sun. Tested chemically, it is silica in some cases, in others iron pyrites, or some mineral not proper to the original organism. In fact, these minerals have penetrated it entirely, entering into the minutest recesses, and usurping frequently the whole of the structure; so that it may be said no part of the animal or plant remains, only an exact representation of it in another material. Apparently these minerals must have been in solution in water, and have thus infiltrated the organism, whose present aspect is that of a mass of flint or stone, combined, perhaps, with bisulphide of iron. Between this extreme case of fossilization and a recent shell, bone, etc., there are various degrees, depending on the greater or less quantity of mineral matter with which the organism has come into contact; and, as a general rule, the remains from the older rocks are more completely fossilized than those of, for example, the Cretaceous period, though some few of the oldest have not been so greatly changed. On the other hand, among deposits of the TERTIARY era, fossils are often only slightly penetrated by foreign matter, and, under exceptional circumstances, the gelatine has not altogether disappeared from the bones, and traces remain of the actual tissues |