PREFACE 



which I spent at Rapallo, on the shore of the blue 

 Mediterranean. The quiet Ufe 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 Hfe 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 ad- 

 vances 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. 



When I completed my seventieth year at Rapallo, on 

 February i6th, I was overwhelmed with a mass of con- 

 gratulations, letters, telegrams, flowers, and other gifts, 

 most of which came from unknown readers of The 

 Riddle of the Universe in all parts of the world. If my 

 thanks have not yet reached any of them, I beg to tender 

 them in these lines. But I should be especially gratified 

 if they would regard this work on the wonders of life as 

 an expression of my thanks, and as a literary gift in 

 return. May my readers be moved by it to penetrate 



