XXX.] CLASSIFICATION. 711 



gator must have means of recording and systematising hia 

 observations of any large groups of objects like the vege- 

 table and animal kingdoms. 



Now vvlienever a class has been properly formed, a 

 definition must have been laid down, stating the qualities 

 and circumstances possessed by all the objects which are 

 intended to be included in the class, and not possessed 

 completely by any other objects. Diagnosis, therefore, 

 consists in comparing the qualities of a certain object 

 with the definitions of a series of classes ; the absence 

 in the object of any one quality stated in the definition 

 excludes it from the class thus defined; wliereas, if we 

 find every point of a definition exactly fulfilled in the 

 specimen, we may at once assign it to the class in 

 question. It is of course by no means certain that every- 

 thing which has been affirmed of a class is true of all 

 objects afterwards referred to the class ; for this would 

 be a case of imperfect inference, which is never more 

 than matter of probability. A definition can only make 

 known a finite nuuibev of the qualities of an object, and 

 it always remains possible that objects agreeing in those 

 assigned qualities will differ in others. An incHvidiud 

 caniwt he defined, and can only be nuide known by the 

 exhibition of the individual itself, or by a material speci- 

 men exactly representing it. But this and other questions' 

 relating to definition must be treated when 1 am able to 

 take up the subject of language in another work. 



Diagnostic systems of classification should, as a general 

 rule, be arranged on the bifurcate method explicitly. Any 

 quality may be chosen wliich divides the whole group of 

 objects into two distinct parts, and each part may be sub- 

 divided successively by any prominent and M-ell-nuirked 

 circumstance which is present in a large part of the genus 

 and not in the other. To refer an object to its projx-r 

 place in sucli an arrangement we have only to note whether 

 it does or does not possess the successive critical differentisp. 

 Dana devised a classification of this kind^ by which to refer 

 a crystal to its place in the series of six or seven classes 

 already described. If a crystal has all its edges modified 

 alike or the angles replaced by three or six similar planes, 



' Dana's Mineralogy, vol. i. p. 123 ; iiuoted in Watts' Dictionarij 

 of (Jhemistry, vol. ii. p. 166. 



