PREFACE TO THE THIRD EDITION 



LITTLE over half a year has passed since the 

 publication of this work. In so short a space of 

 time there could not be much advance made in 

 science, so that it was unnecessary to make extensive 

 alterations in the text for the present edition. In a 

 few places there have been additions, relating to new 

 investigations ; the third chapter, especially, has been 

 considerably modified. No change was needed as 

 regards the view that runs through the entire work, 

 and is summarily presented in the final chapter. I 

 have carefully examined the objections raised on 

 several points, but have found them invalid. However, 

 I thought it proper to put the chief of them before 

 the reader, because I am anxious that he should use 

 his own judgment on the facts independently. I have, 

 therefore, given the objections in the notes, with a 

 few explanatory remarks. Where the objections were 

 due to misunderstanding, I have taken care to make 

 the corresponding passage in the text clearer, or to 

 add further explanations. The purely philosophical 

 questions that are dealt with in the later chapters have 

 not been enlarged on. A more thorough discussion of 



