THE CLASSIFICATION OF ANIMALS. 125 



Moreover, the discovery of new kinds of animal, or of new facts 

 about old ones, brings into notice features of resemblance or 

 difference among the various groups of animals which have 

 not hitherto been regarded, and thus constantly calls for the 

 alteration of existing systems of classification. Hence, in 

 studying classification, only the newest text-books are of use. 

 The student should carefully consider the reasons which 

 determine the placing of ■ different' kinds of animal in one 

 class. Let us suppose that most of the kinds of animal placed 

 in a given group have half a dozen different characteristics 

 in common : then very likely it will be found that there are 

 a few other kinds which do not possess these six character- 

 istics, but possess five of them ; a few others which possess 

 four, or three, or two, or one. If these are included, then the 

 definition of the group must be accordingly narrowed, so that 

 perhaps some of the most striking features of the major por- 

 tion of the group have to be excluded. The best way to define 

 such a group is, therefore, to take some one animal which has 

 a number of characters in common with a large number of the 

 members of the group, and, considering this as a type, to es- 

 tablish our group not by a definition, but by comparing its 

 members with the type. Thus, the earthworm may be taken 

 as a type of the higher worms (annelids), and the lobster as a 

 typo of the Crustacea. In the words of Whewell, " Natural 

 groups are best described, not by any Definition which marks 

 their boundaries, but by a Ti/pe which marks their centre. The 

 Type of any natural group is an example which possesses in- a 

 marked degree all the leading characters of the class." A 

 natural group is therefore " determined, not by a boundary 

 without, but by a central point within — not by what it strictly 

 excludes, but by what it eminently includes." "A Natural 

 System. " of classification is thus one " which attempts to 

 make aZZ the divisions natural, the widest as well as the 

 narrowest," and " applies no characters peremptorily." ' 



' The student may with advantage read the chapter on Methods of 

 Natural Classification in Whewell's " Novum Organon Benovatum " (Lon- 



