I presented all the essential and distinctive elements of my monistic and genetic philosophy thirty-three years ago, in my _General Morphology of Organisms_, a large and laborious work, which has had but a limited circulation. It was the first attempt to apply in detail the newly established theory of evolution to the whole science of organic forms. In order to secure the acceptance of at least one part of the new thought which it contained, and to kindle a wider interest in the greatest advancement of knowledge that our century has witnessed, I published my _Natural History of Creation_ two years afterwards. As this less complicated work, in spite of its great defects, ran into nine large editions and twelve different translations, it has contributed not a little to the spread of monistic views. The same may be said of the less known _Anthropogeny_[1] (1874), in which I set myself the difficult task of rendering the most important facts of the theory of man's descent accessible and intelligible to the general reader; the fourth, enlarged, edition of that work appeared in 1891. In the paper which I read at the fourth International Congress of Zoology at Cambridge, in 1898, on "Our Present Knowledge of the Descent of Man"[2] (a seventh edition of which appeared in 1899), I treated certain significant and particularly valuable advances which this important branch of anthropology has recently made. Other isolated questions of our modern natural philosophy, which are peculiarly interesting, have been dealt with in my _Collected Popular Lectures on the Subject of Evolution_ (1878). Finally, I have briefly presented the broad principles of my monistic philosophy and its relation to the dominant faith in my _Confession of Faith of a Man of Science: Monism as a Connecting Link between Religion and Science_[3] (1892, eighth edition, 1899). The present work on _The Riddle of the Universe_ is the continuation, confirmation, and integration of the views which I have urged for a generation in the aforesaid volumes. It marks the close of my studies on the monistic conception of the universe. The earlier plan, which I projected many years ago, of constructing a complete "System of Monistic Philosophy" on the basis of evolution will never be carried into effect now. My strength is no longer equal to the task, and many warnings of approaching age urge me to desist. Indeed, I am wholly a child of the nineteenth century, and with its close I draw the line under my life's work.