As to the first of these questions, it is certain that, up to this time, the origination of the living being from the non-living is an inscrutable mystery. No one has witnessed this change, or has been able to effect it experimentally. Nor have we any direct evidence of the origination of one specific type from another. Such reasonings as assume the possibility of these things, or on analogical grounds assert their probability, belong rather to the domain of philosophical speculation than to science. As to the laws of the succession of life, however, it is possible to learn something from the sequence of facts as already ascertained; and though much remains to be discovered, there are a few leading statements on this subject which can already be made with safety. Unity and uniformity, within the limits imposed by progress and increasing complexity, can be affirmed of the whole process. From the dawn of life to the present time the great laws of physical nature which operate on animals and plants have been uniform. These stable laws have regulated the action of the outer world on organisms. The plans of structure of these organisms laid down at the first have been followed throughout. Thus the succession of life presents nothing fortuitous or arbitrary, but a continuous plan carried out uniformly in time and space, with certain materials of fixed properties, and with certain structures predetermined from the first. There is, for example, a great sameness of plan throughout the whole history of the marine invertebrate life of the Palæozoic. If we turn over the pages of an illustrated text-book of geology, or examine the cases or drawers of a collection of fossils, we shall find extending through every succeeding formation, representative forms of Crustaceans, Mollusks, and Corals, in such a manner as to indicate that in each successive period there has been a reproduction of the same type with modifications; and if the series is not continuous, this appears to be due to lack of specimens, or to abrupt physical changes; since sometimes, where two formations pass into each other, we find a gradual change in the fossils by the dropping out and introduction of species one by one. Thus in the whole of the great Palæozoic period, both in its fauna and flora, we have a continuity and similarity of a most marked character. There is, indeed, nothing to preclude the supposition that many forms reckoned as species are really only race modifications. My own provisional conclusion, based on the study of Palæozoic plants, published many years ago,[93] is that the general law will be found to be the existence of distinct specific types independent of each other, but liable in geological time to minor modifications, which have often been regarded as distinct species.