CLASSIFICATION. 711 



gator must have means of recording and systematising his 

 observations of any large groups of objects like the vege- 

 table and animal kingdoms. 



Now whenever 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; whereas, 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 number of the qualities of an object, and 

 it always remains possible that objects agreeing in those 

 assigned qualities will differ in others. An individual 

 cannot be defined, and can only be made 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 I 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 which divides the whole group of 

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

 divided successively by any prominent and well-marked 

 circumstance which is present in a large part of the genus 

 and not in the other. To refer an object to its proper 

 place in such an arrangement we have only to note whether 

 it does or does not possess the successive critical differentia?. 

 Dana devised a classification of this kind 1 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, 



1 Dana's Mineralogy, vol. i. p. 123 ; quoted in Watts' Dictionary 

 tf Chemistry, vol. ii. p. 1 66. 



