In the few years that have elapsed since the publication of the first edition of this work, great additions have been made to our knowledge of fossil animals and plants. Many new species have been described, and many new facts have been discovered, respecting species previously known. This rapid progress of discovery has, however, invalidated few of the statements made in the first edition, and has certainly established nothing against the general laws of the succession of life as stated in this work. Perhaps the most interesting phase of recent discovery is the tracing back of certain forms of life to earlier periods of the earth’s geological history. Some of the most recent facts of this kind are the finding, by M. Charles Brongniart, of a fossil insect, allied to the _Blattae_ or cockroaches, in the Silurian of Spain, that of true Scorpions in the Upper Silurian of Sweden by Lindström, and in the Upper Silurian of Scotland by Peach, who has also described fossil Millipedes from the Lower Devonian. The tendency of such discoveries is to carry farther back the origin of highly specialised forms of life, and thus to render less probable their origin by any process of gradual derivation. Other discoveries serve to fill up blanks in our knowledge, and thus to render the geological record less imperfect. Of this kind is the close approximation now worked out in Western America between the end of the reign of the great Mesozoïc reptiles and the beginning of that of the mammals of the Tertiary—a great and abrupt revolution, effected apparently by a _coup de main_. I have myself had opportunity to show that a similarly sharp line separates that quaint old Mesozoïc flora of pines, cycads and ferns, which extends upward into the Lower Cretaceous, from the rich and luxuriant assemblage of broad-leaved trees of modern aspect, which takes its place in the middle part of the same formation. It is not too much to say that these and similar discoveries, while they serve to bridge over gaps in the succession of organic beings, do not favour the theory of slow modification of types. They rather point to a law of rapid development of new forms under special conditions as yet unknown to science, and this accompanied with the extinction of older species. Recent discoveries also present many remarkable instances of the early introduction of highly specialised types, of higher forms preceding those that are lower in the same class, and of the persistence of certain types throughout geological time without any important change. J. W. D. MCGILL COLLEGE. CONTENTS. CHAP. PAGE I. PRELIMINARY CONSIDERATIONS AS TO THE EXTENT AND SOURCES OF OUR KNOWLEDGE 1 II. THE BEGINNING OF LIFE ON THE EARTH 21 III. THE AGE OF INVERTEBRATES OF THE SEA 45