180 ESSAY ON CLASSIFICATION 



And yet there are other natural divisions which must be acknowl- 

 edged 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 de- 

 velopment while all the other systems coincide may be subdivided 

 into sub-classes; for instance, the Marsupialia when contrasted with 

 the Placental Mammalia. The characters upon which such a subdivi- 

 sion is founded are of the kind upon which the class itself is based, 

 but do not extend to the whole class. An order may embrace natural 

 groups, of a higher value than families, founded upon ordinal char- 

 acters, which may yet not determine absolute superiority or inferior- 

 ity, and therefore not constitute for themselves distinct orders; as the 

 characters upon which they are founded, though of the kind which 

 determines orders, may be so blended as to determine superiority in 

 one respect, while with reference to some other features they may 

 indicate inferiority. Such groups are called sub-orders. The order of 

 Testudinata, may best illustrate this point, as it contains two natural 

 sub-orders.-'^ A natural family may exhibit such modifications of its 

 characteristic form that upon these modifications subdivisions may 

 be distinguished which have been called sub-families by some au- 

 thors, tribes or legions by others. In a natural genus a number of spe- 

 cies may agree more closely than others in the particulars which con- 

 stitute the genus and lead to the distinction of sub-genera. The 

 individuals of a species, occupying distinct fields of its natural geo- 

 graphical area, may differ somewhat from one another, and consti- 

 tute varieties, etc. 



These distinctions have long ago been introduced into our systems, 

 and every practical naturalist who has made a special study of any 

 class of the animal kingdom must have been impressed with the pro- 

 priety of acknowledging a large number of subdivisions to express 

 all the various degrees of affinity of the different members of any 

 higher natural group. Now while I maintain that the branches, the 

 classes, the orders, the families, the genera, and the species are groups 

 established in nature respectively upon different categories, and 

 while I feel prepared to trace the natural limits of these groups by 

 the characteristic features upon which they are founded, I must con- 



^ See my Contributions, I, 308. 



