LEADING GROUPS OF EXISTING SYSTEMS 161 



orders are, and I would insist upon considering the complication or 

 gradation of structure as the feature which should regulate their 

 limitation, if under order we are to imderstand natural groups ex- 

 pressing the rank, the relative standing, the superiority or inferiority 

 of animals in their respective classes. Of course groups thus charac- 

 terized cannot be considered as mere modifications of the classes, be- 

 ing founded upon a special category of features. 



SECTION IV 

 FAMILIES 



Nothing is more indefinite than the idea of form, as applied by 

 systematic writers, in characterizing animals. Here, it means a system 

 of the most different figures having a common character, as, for 

 instance, when it is said of Zoophytes that they have a radiated form; 

 there, it indicates any outline which circumscribes the body of ani- 

 mals, when, for instance, animal forms are alltided to in general, in- 

 stead of designating them simply as animals; here, again, it means 

 the special figure of some individual species. There is in fact no 

 group of the animal kingdom, however extensive or however 

 limited, from the branches down to the species, in which the form is 

 not occasionally alluded to as characteristic. Speaking of Articulates, 

 C. E. V. Baer characterizes them as the type with elongated forms; 

 Mollusks are to him the type with massive forms; Radiates that with 

 peripheric symmetry; Vertebrates that with double symmetry, evi- 

 dently taking their form in its widest sense as expressing the most 

 general relations of the different dimensions of the body to one an- 

 other. Cuvier speaks of form in general with reference to these four 

 great types as a sort of mould, as it were, in which the different 

 types \vould seem to have been cast. Again, form is alluded to in 

 characterizino- orders; for instance, in the distinction between the 

 Brachyourans and the Macrourans among Crustacea, or between 

 the Saurians, the Ophidians, and the Chelonians. It is mentioned 

 as a distinguishing feature in many families, e.g. the Cetacea, the 

 Bats, etc. Some genera are separated from others in the same family 

 on the gTOiuid of differences of form; and in almost every descrip- 



