PREFACE. 



THE contents of this volume appeared for the first time 

 as an introduction to a larger work now in course of 

 publication, under the title of Contributions to the Natu- 

 ral History of the United States. 1 Friends in whose 

 opinion I have great confidence, having expressed the 

 desire that a separate edition of that part of my book 

 which relates to the general principles of Zoology should 

 be published in England, I gladly complied with the 

 request of Messrs. Triibner and Co., that I should autho- 

 rize such a reprint, and have availed myself of the oppor- 

 tunity to make such corrections as seemed necessary, and 

 also to complete the references up to the latest possible 

 date. Besides this, I have also added a new chapter 

 upon the Categories of Analogy, a subject which, until 



1 Thus far, two volumes quarto, ton by Messrs. Little, Brown, and Co., 

 with thirty-four plates, have ap- in October 1857. The third volume 

 peared. They were published in Bos- is now in the press. 



