244 ESSAY ON CLASSIFICATION. 



upon us by their form, give us at once a very correct idea 

 of their nearest relationship'? "We perceive, before ex- 

 amining any structural character, whether a Beetle is a 

 Carabicine, a Longicorn, an Elaterid, a Curculionid, a 

 Chrysomeline ; whether a Moth is a Noctuelite, a Geo- 

 metrid, a Pyralid, etc. ; whether a bird is a Dove, a Swal- 

 low, a Humming-bird, a Woodpecker, a Snipe, a Heron, 

 etc., etc. But, before we can ascertain its genus, we have 

 to study the structure of some characteristic parts ; before 

 we can combine families into natural groups, we have to 

 make a thorough investigation of their whole structure, 

 and compare it with that of other families. So form is 

 characteristic of families; and I can add, from a careful 

 investigation of the subject for several years past, during 

 which I have reviewed the whole animal kingdom with 

 reference to this and other topics connected with classifi- 

 cation, that form is the essential characteristic of families. 1 

 I do not mean the mere outline, but form as determined 

 by structure; that is to say, that families cannot be well 

 defined, nor circumscribed within their natural limits, 

 without a thorough investigation of all those features of 

 the internal structure which combine to determine the 

 form. 



The characteristic of the North American Chelonians, 

 which I have published in my Contributions to the 

 Natural History of the United States, 2 may serve as an 

 example how this subject is to be treated. I will only 

 add here, that, however easy it is at first, from the gene- 

 ral impression made upon us by the form of animals, to 



1 These investigations, which have which I would not allow to appear 



led to most interesting results, have before I could revise the whole ani- 



delayed thus far the publication of mal kingdom in this new light, in 



the systematic part of the Principles order to introduce as much precision 



of Zoology, undertaken in common as possible into its classification, 



with my friend, Dr. A. A.'Gould, and 2 Seevol.i 3 pp.317-366of thatwork. 



