ESSAY ON CLASSIFICATION. 



CIIATTER FIRST. 



THE FUNDAMENTAL RELATIONS OF ANIMALS TO ONE ANOTHER AND TO THE 

 WORLD IN WHICH THEY LIVE, AS THE BASIS OF THE NATURAL SYSTEM OF 

 ANI5IALS. 



SECTION I. 



THE LEADING FEATUKES OF A NATURAL ZOOLOGICAL SYSTEM ARE ALL FOUNDED 



IN NATURE. 



Modern classifications of animals and j^lants are based upon the peculiarities of 

 their structure; and this is generally considered as the most important, if not the 

 only safe, guide in our attempts to determine the natural relations which exist 

 between animals. This view of the subject seems to me, however, to circumscribe 

 the foundation of a natural system of Zoology and Botany within too narrow limits, 

 to exclude from our consideration some of the most striking characteristics of the 

 two organic kingdoms of nature, and to leave it doubtful how far the arrangement 

 thus obtained is founded in reality, and how far it is merely the expression of our 

 estimate of these structural diiferences. It has appeared to me appropriate, therefore, 

 to present here a short exposition of the leading features of the animal kingdom, as 

 an introduction to the embryology of the Chelonians, — one of the most extraordinar\- 

 types among Vertebrata, — as it would afford a desirable opportunity of establishing 

 a standard of comparison between the changes animals undergo dming their growth, 

 and the permanent characters of full-grown individuals of other types, and, perhaps, 

 of sho-vving also what other points beside structure might with advantage be consid- 



