I make this attempt to sum up and present as a harmonious whole the theories which for forty years I have been gradually building up on the basis of the legacy of the great workers of the past, and on the results of my own investigations and those of many fellow workers, not because I regard the picture as complete or incapable of improvement, but because I believe its essential features to be correct, and because an eye-trouble which has hindered my work for many years makes it uncertain whether I shall have much more time and strength granted to me for its further elaboration. We are standing in the midst of a flood-tide of investigation, which is ceaselessly heaping up new facts bearing upon the problem of evolution. Every theory formulated at this time must be prepared shortly to find itself face to face with a mass of new facts which may necessitate its more or less complete reconstruction. How much or how little of it may remain, in face of the facts of the future, it is impossible to predict. But this will be so for a long time, and it seems to me we must not on that account refrain from following out our convictions to the best of our ability and presenting them sharply and definitely, for it is only well-defined arguments which can be satisfactorily criticized, and can be improved if they are imperfect, or rejected if they are erroneous. In both these processes progress lies. This book consists of 'Lectures' which were given publicly at the university here. In my introductory lecture in 1867 I championed the Theory of Descent, which was then the subject of lively controversy, but it was not till seven years later that I gave, by way of experiment, a short summer course with a view to aiding in the dissemination of Darwin's views. Then very gradually my own studies and researches and those of others led me to add to the Darwinian edifice, and to attempt a further elaboration of it, and accordingly these 'Lectures,' which were delivered almost regularly every year from 1880 onwards, were gradually modified in accordance with the state of my knowledge at the time, so that they have been, I may say, a mirror of the course of my own intellectual evolution.