PREFACE. 



^HTHE pressing duties of an ordinarily 

 busy life, it might be s^cpposed, would 

 be quite stifficient to keep one not specially 

 trai^ted to scientific work front meddling 

 with such matters as are presented in this 

 brief monograph. Indeed, I have no good 

 excuse for finding myself eitgaged in this 

 business. It might, however, be said in ex- 

 tenuation that the topics I have been deal- 

 ing with in these pages have occupied rny 

 attention from time to time, and the con- 

 clusions have been lightly held by me as mere 

 probabilities for a long period. With later 

 investigations and discoveries they began to 

 crystallize into opinions, and have at length 

 assumed the firmness of convictions. Having 

 in an unguarded hour disclosed them to a 

 party of friends well fitted to jtidge of their 



