It is from these two courses--which resembled each other in comprising between thirty and forty lectures, but differed largely in other respects--that the present treatise has grown. Seeing, however, that it has grown much beyond the bulk of the original lectures, I have thought it desirable to publish the whole in the form of three separate works. Of these the first--or that which deals with the purely historical side of biological science--may be allowed to stand over for an indefinite time. The second is the one which is now brought out and which, as its sub-title signifies, is devoted to the general theory of organic evolution as this was left by the stupendous labours of Darwin. As soon as the translations shall have been completed, the third portion will follow (probably in the Autumn season), under the sub-title, "Post-Darwinian Questions." As the present volume is thus intended to be merely a systematic exposition of what may be termed the Darwinism of Darwin, and as on this account it is likely to prove of more service to general readers than to professed naturalists, I have been everywhere careful to avoid assuming even the most elementary knowledge of natural science on the part of those to whom the exposition is addressed. The case, however, will be different as regards the next volume, where I shall have to deal with the important questions touching Heredity, Utility, Isolation, &c., which have been raised since the death of Mr. Darwin, and which are now being debated with such salutary vehemence by the best naturalists of our time. My obligations to the Senatus of the University of Edinburgh, and to the Board of Management of the Royal Institution, have already been virtually expressed; but I should like to take this opportunity of also expressing my obligations to the students who attended the lectures in the University of Edinburgh. For alike in respect of their large numbers, their keen intelligence, and their generous sympathy, the members of that voluntary class yielded a degree of stimulating encouragement, without which the labour of preparing the original lectures could not have been attended with the interest and the satisfaction that I found in it. My thanks are also due to Mr. R. E. Holding for the painstaking manner in which he has assisted me in executing most of the original drawings with which this volume is illustrated; and likewise to Messrs. Macmillan and Co. for kindly allowing me to reprint--without special acknowledgment in every case--certain passages from an essay which they published for me many years ago, under the title "Scientific Evidences of Organic Evolution." Lastly, I must mention that I am indebted to the same firm for permission to reproduce an excellent portrait of Mr. Darwin, which constitutes the frontispiece. G. J. R. CHRIST CHURCH, OXFORD, _April 19th, 1892._ CONTENTS PAGE