Summary.--The chief steps up to this time in the growth of the science of fossil life may now be set forth in categories, though we must remember that the advances proceeded concurrently and were much intermingled, so that, whatever arrangement we may adopt, it does not represent a strict chronological order of events: I. The determination of the nature of fossils. Owing to the labors of Da Vinci, Steno, and Cuvier, the truth was established that fossils are the remains of former generations of animals and plants. II. The comparison of organic fossils with living forms that was instituted on a broad scale by Cuvier resulted in the conclusion that some of the fossils belong to extinct races. The belief of Cuvier that entire populations became extinct simultaneously, led him to the theory of catastrophism. The observations of Lamarck, that, while some species disappear, others are continued and pass through transmutations, were contrary to that theory. III. The recognition that the stratified rocks in which fossils are distributed are sedimentary deposits of gradual formation. This observation and the following took the ground from under the theory that fossils had been deposited during the Mosaic deluge. IV. The discovery by William Smith that the arrangement of fossils within rocks is always the same, and the relative age of rocks may be determined by an examination of their fossil contents. Upon the basis of the foregoing, we come to the next advance, _viz._: V. The application of this knowledge to the determination of the history of the earth. Fossil Remains as an Index to the Past History of the Earth.--The most advanced and enlightened position that had been taken in reference to the fossil series during the first third of the nineteenth century was that taken by Lamarck, he being the first to read in the series the history of life upon the globe, weaving it into a connected story, and establishing thereon a doctrine of organic evolution. It was not until after 1859, however, that the truth of this conclusion was generally admitted, and when it was accepted it was not through the earlier publications of Lamarck, but through the arguments of later observers, founded primarily upon the hypothesis set forth by Darwin. There were several gradations of scientific opinion in the period, short as it was, between the time of Cuvier and of Darwin; and this intermediate period was one of contention and warfare between the theologians and the geologists. Cuvier had championed the theory of a succession of catastrophes, and since this hypothesis did not come into such marked conflict with the prevailing theological opinion as did the views of Lamarck, the theologians were ready to accept the notion of Cuvier, and to point with considerable satisfaction to his unique position as an authority.