AUTHORS PREFACE TO FIRST 



EDITION. 



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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 w^as not aware that the others 

 would follow, the whole series is, nevertheless, closely con- 

 nected 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 con- 

 sideration 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 am here making use of the word ' research ' in a sense 

 somewhat different from that in which it is generally em- 

 ployed 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 re- 

 searches are of a more abstract nature, and consist in bring- 

 ing 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 wa}^ It is chiefly in this sense that the contents of 

 these essays are to be looked upon as research. 



