LEADING GROUPS OF EXISTING SYSTEMS 167 



by structure; that is to say, that families cannot be well defined nor 

 circumscribed within their natural limits without a thorough in- 

 vestigation of all those features of the internal structure which com- 

 bine to determine the form. 



The characteristic of the North American Chelonians^^ 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 general impression made 

 upon us by the form of animals, to obtain a glimpse of what may 

 fairly be called families, few investigations require more patient 

 comparisons than those by which we ascertain the natural range of 

 modifications of any typical form and the structural features upon 

 which it is based. Comparative anatomy has so completely discarded 

 every thing that relates to Morphology; the investigations of anato- 

 mists lean so uniformly towards a general appreciation of the con- 

 nections and homologies of the organic systems which go to build 

 up the body of animals, that for the purpose of understanding the 

 value of forms and their true foundation they hardly [if] ever afford 

 any information, unless it be here and there a consideration re- 

 specting teleological relations. 



Taking for granted that orders are natural groups characterized 

 by the complication of their structure and that the different orders 

 of a class express the different degrees of that complication; taking 

 now further for granted that families are natural groups characterized 

 by their form as determined by structural peculiarities, it follows that 

 orders are the superior kind of division, as we have seen that the 

 several natural divisions which are generally considered as orders 

 contain each several natural groups, characterized by different forms, 

 that is to say, constituting as many distinct families. 



After this discussion it is hardly necessary to add that families can- 

 not by any means be considered as modifications of the orders to 

 which they belong, if orders are to be characterized by the degrees 

 of complication of their structure and families by their forms. I 

 would also further remark that there is one question relating to the 

 form of animals which I have not touched here and which it is still 

 more important to consider in the study of plants, namely, the mode 



in order to introduce as much precision as possible in its classification. [This work 

 was never completed.] 

 2° See my Contributions to the Natural History of the United States, I, 317-366. 



