I had said, in the preface to _The Riddle of the Universe_ in 1899, that I proposed to close my study of the monistic system with that work, and that "I am wholly a child of the nineteenth century, and with its close I draw the line under my life's work." If I now seem to run counter to this observation, I beg the reader to consider that this work on the wonders of life is a _necessary_ supplement to the widely circulated _Riddle of the Universe_, and that I felt bound to write it in response to the inquiries of so many of my readers. In this second work, as in the earlier one, I make no pretension to give the reader a comprehensive statement of my monistic philosophy in the full maturity it has reached--for me personally, at least--at the close of the nineteenth century. A subjective theory of the world such as this can, naturally, never hope to have a complete objective validity. My knowledge is incomplete, like that of all other men. Hence, even in this "biological sketch-book," I can only offer studies of unequal value and incomplete workmanship. There still remains the great design of embracing all the exuberant phenomena of organic life in one general scheme and explaining all the wonders of life from the monistic point of view, as forms of one great harmoniously working universe--whether you call this Nature or Cosmos, World or God. The twenty chapters of _The Wonders of Life_ were written uninterruptedly in the course of four months which I spent at Rapallo, on the shore of the blue Mediterranean. The quiet life in this tiny coast-town of the Italian Riviera gave me leisure to weigh again all the views on organic life which I had formed by many-sided experience of life and learning since the beginning of my academic studies (1852) and my teaching at Jena (1861). To this I was stimulated by the constant sight of the blue Mediterranean, the countless inhabitants of which had, for fifty years, afforded such ample material for my biological studies; and my solitary walks in the wild gorges of the Ligurian Apennines, and the moving spectacle of its forest-crowned mountain altars, inspired me with a feeling of the unity of living nature--a feeling that only too easily fades away in the study of detail in the laboratory. On the other hand, such a situation did not allow a comprehensive survey of the boundless literature which has been evoked by the immense advances in every branch of biology. However, the present work is not intended to be a systematic manual of general biology. In the revision of the text, on which I was engaged during the summer at Jena, I had to restrict myself to occasional additions and improvements. In this I had the assistance of my worthy pupil, Dr. Heinrich Schmidt, to whom also I am indebted for the careful revision of the proofs.