170 ESSAY ON CLASSIFICATION. Part I. 



ascertain what were the species the authors of such condensed descriptions had 

 before them. But for the tradition which has ti'ansmitted, generation after gener- 

 ation, the knowledge of these species among the cultivators of science in Europe, 

 this confusion would be still greater; but for the preservation of most original 

 collections it would be inextricable. In countries, which, like America, do not enjoy 

 these advantages, it is often hopeless to attempt critical investigations upon doubtful 

 cases of this kind. One of our ablest and most critical investigators, the lamented 

 Dr. Harris, has very forcibly set forth the difficulties under which American 

 naturalists labor in this respect, in the Preflice to his Report upon the Insects 

 Injurious to Vegetation. 



SECTION VII. 



OTHER NATURAL DIVISIONS AMONG ANIMALS. 



Thus far I have considered only those kinds of divisions which are introduced 

 in almost all our modern classifications, and attempted to show that these groups 

 are founded in nature and ought not to be considered as artificial devices, invented 

 by man to facilitate his studies. Upon the closest scrutiny of the subject, I find 

 that these divisions cover all the categories of relationship which exist among 

 animals, as far as their structure is concerned. 



Branches or ii/pes are characterized by the plan of their structure, 



Classes, by the manner in which that plan is executed, as far as ways and means 

 are concerned, 



Orders, by the degrees of complication of that structure, 



Families, by their form, as far as determined by structure. 



Genera, by the details of the execution in special parts, and 



Species, by the relations of individuals to one another and to the world in 

 which they live, as well as by the proportions of their parts, their ornamentar 

 tion, etc. 



And yet there are other natural divisions which must be acknowledged in a 

 natural zoological system ; but these are not to be traced so uniformly in all 

 classes as the former, — they are in reality only limitations of the other kinds of 

 divisions. 



A class in which one system of organs may present a peculiar development, 

 while all the other systems coincide, may be subdivided into sub-classes ; for instance, 

 the Marsupialia when contrasted with the Placental Mammalia. The characters 



