412 THE PRINCIPLES OF SCIENCE. 



do not possess all these properties, naturalists select a 

 typical form or specimen, and they group around it all 

 oilier forms or specimens which resemble this type more 

 than any other selected type. * The type of each genus/ 

 we are told 1 , should be that species in which the charac 

 ters of its group are best exhibited and most evenly 

 balanced. It would usually consist of those descendants 

 of a form which had 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 

 a merely abbreviated mode of representing a very com 

 plicated system of arrangement. A class must be defined 

 by the invariable presence of certain common properties. 

 If, then, we venture to include an individual in which 

 one of these properties does not appear, we either fall 

 into logical contradiction, or else we form a new class 

 with a new definition. Even a single exception consti 

 tutes a new class by itself, and by calling it an excep 

 tion we merely imply that this new class closely resembles 

 that from which it diverges in one or two points only. 

 Thus if in the definition of the natural order of Rosacere, 

 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 the class, or else that Spiraea does 

 not belong to that class, though it may be closely ap 

 proximated 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 



1 Water-house, quoted by Woodward in his Rudimentary Treatise of 

 Recent and Fossil Shells, p. 61. 



