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 



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