vi PREFACE. 



by a critical review of the facts of biology in the 

 light of the great conclusions derived from the allied 

 sciences of physics and psychology, we may obtain a 

 view of the great phenomena of life which shall bring 

 into harmony a more extensive range of facts, and 

 explain intelligibly relations which have hitherto 

 been hidden. In the following pages I have at- 

 tempted to bring together a number of facts and 

 conclusions which have not generally been supposed 

 to be related. The success which has attended the 

 effort to find in this way explanations for phenomena 

 which were before inexplicable, has convinced me 

 that the theory which has forced itself upon me is 

 a real advance in the right direction. Rather than 

 attempt to perfect the theory before publishing it, I 

 desire to put it in a form that it may be discussed, 

 so that what is good in the theory may become of 

 general use ; and, in adapting anything for general 

 use, many heads are better than one. Then, too, the 

 experimental tests of the theory can be carried out 

 by many better than by one, — if the theory shall 

 fortunately arouse so much interest. 



