AUTHOR'S PREFACE. 



THE essays which now appear for the first time in the form 

 of a single volume were not written upon any prearranged plan, 

 but have been published separately at various intervals during 

 the course of the last seven years. Although when writing the 

 earlier essays I was not aware that the others would follow, the 

 whole series is, nevertheless, closely connected together. The 

 questions which each essay seeks to explain have all arisen 

 gradually out of the subjects treated in the first. Reflecting upon 

 the causes which regulate the duration of life in various forms, 

 I was drawn on to the consideration of fresh questions which 

 demanded further research. These considerations and the results 

 of such research form the subject-matter of all the subsequent 

 essays. 



I a.m here making use of the word ' research ' in a sense 

 somewhat different from that in which it is generally employed 

 in natural science ; for it is commonly supposed to imply the 

 making of new observations. Some of these essays, especially 

 Nos. IV, V, and VI, essentially depend upon new discoveries. 

 But in most of the remaining essays the researches are of a 

 more abstract nature, and consist in bringing forward new 

 points of view, founded upon a variety of well-known facts. 

 I believe, however, that the history of science proves that 

 advance is not only due to the discovery of new facts, but 

 also to their correct interpretation : a true conception of natural 

 processes can only be arrived at in this way. It is chiefly in 



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