6 PREFACE. 



Finally, the author presents this volume, with the 

 simple statement that he has made an honest attempt 

 to compass the great subject to which it is devoted, the 

 magnitude and importance of which he never appreci- 

 ated so fully as at the present moment. In the prepara- 

 tion of this volume, it was expected to include in it the 

 special senses, and chapters upon touch, smell, and sight, 

 were written, so that at least one-fifth of the last volume 

 of the original series is already completed. The fifth vol- 

 ume is therefore so far advanced, that it is hoped that the 

 entire work will be finished within a year. The last part 

 will be devoted to the Special Senses and Generation. 



NEW YORK, May, 1872. 



