Chap. II. OTHER NATURAL DIVISIONS. 171 



upon which such a subdivision 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 A-alue than families, founded upon ordinal characters, 

 which may yet not determine absolute superiority or inferiority, 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, which I shall consider more in detail in the second part of this volume, 

 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 authors, tribes or legions by others. In a natural genus, a number of 

 species may agree more closely than others in the particulars which constitute 

 the genus and lead to the distinction of sub-genera. The individuals of a species, 

 occupying distinct fields of its natural geographical area, may differ somewhat from 

 one another, and constitute 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 kino-- 

 dom, must have been impressed with the propriety of acknowledging a large number 

 of subdivisions, to express all the various degrees of affinity of the diflferent 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 confess at the same time that I have not yet been able to discover the 

 principle which obtains in the limitation of their respective subdivisions. All I 

 can say is, that all the different categories considered above, upon which branches, 

 classes, orders, families, genera, and species are founded, have their degrees, and upon 

 these degrees sub-classes, sub-orders, sub-fliraiUes, and sub-genera have been established. 

 For the present, these subdivisions must be left to arljitrary estimations, and we 

 shall have to deal with them as well as we can, as long as the principles which 

 regulate these degrees in the difl'erent kinds of groups are not ascertained. I 

 hope, nevertheless, that such arbitrary estimations are for ever removed from our 

 science, as far as the categories themselves are concerned. 



Thus far, inec|uality of weight seems to be the standard of the internal valua- 

 tion of each kind of group; and this inequality extends to all group.s, for even 

 within the branches there are classes more closely related among themselves 

 than others: Polypi and Acalephs, for instance, stand nearer to one another than 



