XXX.] CLASSIFICATION. 723 



possess all these properties, naturalists select a typical 

 specimen, and they group around it all other specimens 

 which resemble this type more than any other selected 

 type. " The type of each genus," we are told,^ " should be 

 that species in which the characters of its group are 

 best exhibited and most evenly balanced." It would 

 usually consist of those descendants of a form which bad 

 undergone little alteration, while other descendants had 

 suffered slight differentiation in various directions. 



It would be a great mistake to suppose that this classi- 

 fication by types is a logically distinct method. It is 

 either not a real method of classification at all, or it is 

 merely an abbreviated mode of representing a complicated 

 system of arrangement. A class must be defined by the 

 invariable presence of certain common properties. If, 

 then, we include an individual in which one of these 

 properties does not appear, we either fall into logical con- 

 tradiction, or else we form a new class with a new 

 definition. Even a single exception constitutes a new 

 class by itself, and by calling it an exception we merely 

 imply that this new class closely resembles that from 

 which it diverges in one or two points only. Thus in the 

 definition of the natural order of Eosaceae, we find that 

 the seeds are one or two in each carpel, but that in the 

 genus Spiraea there are three or four ; this must mean 

 either that the number of seeds is not a part of the fixed 

 definition of tlie class, or else that Spiraea does not belong 

 to that class, though it may clos<'ly approximate to it. 

 Naturalists continually find themselves between two horns 

 of a dilemma ; if they restrict the number of marks 

 specified in a definition so that every form intended to 

 come within the class shall possess all those marks, it will 

 then be usually found to include too many forms ; if the 

 definition be made more particular, the result is to ])r()duce 

 so-called anomalous genera, which, wliile they are held to 

 belong to the class, do not in all respects confoi'ui to its 

 definition. The practice has hence arisen of iiUowing con- 

 siderabht latitude in the definition of natural orders. The 

 family of Crucifenc, for instance, forms an exceedingly well- 

 marked natural order, and among its characters we find it 



* Waterhou.He, fiuoted by Woodward in his Rudimei.tary Treatisa 

 of Recent and Fossil IShells, p. 6i. 



3 A 2 



