Hence the present work on the wonders of life is, as the title indicates, a supplementary volume to _The Riddle of the Universe_. While the latter undertook to make a comprehensive survey of the general questions of science--as cosmological problems--in the light of the monistic philosophy, the present volume is confined to the realm of organic science, or the science of life. It seeks to deal connectedly with the general problems of biology, in strict accord with the monistic and mechanical principles which I laid down in 1866 in my _General Morphology_. In this I laid special stress on the universality of the law of substance and the substantial unity of nature, which I have further treated in the second and fourteenth chapters of _The Riddle of the Universe_. The arrangement of the vast material for this study of the wonders of life has been modelled on that of the _Riddle_. I have retained the division into larger and smaller sections and the synopses of the various chapters. Thus the whole biological content falls into four sections and twenty chapters. I should much have liked to add illustrations in many parts of the text to make the subject plainer, especially as regards chapters vii., viii., xi., and xvi.; but this would have led to a considerable increase in the size and price of the book. Moreover, there are now many illustrated works which will help the reader to go more fully into the various sections of the study. Among others, my _History of Creation_ (English translation) and _Evolution of Man_ (English translation now in course of preparation) will be found helpful in this way. The German reader will also find many illustrations to elucidate the text of this book in my recently completed work, _Kunstformen der Natur_ (10 parts, with 100 tables, 1899-1904).