174 CLASSIFICATION. 



Positive characters offering means of comparison, shew the 

 resemblances and differences which exist between individuals ; 

 those plants in which these characters present but slight differ- 

 ences, should be collected in groups; those in which these 

 characters differ more sensibly, should be separated ; here we 

 follow strictly the laws of the mind. But negative characters, 

 as they allow no comparison, can only be employed to separate 

 individuals, and never to bring them together. 



When we say that plants have seeds with one or two cotyle- 

 dons ; that they have monopetalous or polypetalous flowers, 

 and are provided with stamens and pistils, we point out particu- 

 lars, where visible and striking resemblances may be observ- 

 ed ; these characters, then, are positive, since they are founded 

 on something real. 



When we say that some plants are destitute of cotyledon, 

 corolla, stamens or pistils, we do not establish any real basis 

 for the foundation of a comparison. If we wish to separate 

 plants with monopetalous corollas, from such as have polypeta- 

 lous corollas, this single character establishes at once, the 

 difference which exists between the two groups, and the resem- 

 blance, which exists between the individuals of each group. 

 Thus positive characters possess a great advantage over nega- 

 tive ones ; the latter should never be employed when the former 

 can be used ; and in proportion, as positive characters can be 

 substituted for negative, the science of botany will be per- 

 fected. 



Positive characters can only be founded upon evident facts, 

 and never upon a presumption of the existence of facts, derived 

 from analogy. For it is contrary to true philosophy, to suffer 

 hypothetical reasoning to usurp the place of direct observation 

 of facts. 



5th. Positive characters are constant or inconstant. All 

 seeds produced by plants of the same species have the same 

 structure ; all plants which grow from these seeds, produce 

 other seeds, similar to those from which they have had their 

 origin : of course the characters derived from the structure 

 of these seeds are constant. But among these plants some are 

 large and others small ; some may have white corollas, some, 

 red, or blue ; some are more fragrant than others ; of course, 

 size, colour and odour offer inconstant characters. 



6th. All real science in botany must rest upon constant char- 

 acters; therefore, these characters are much more important 

 than the others. 



Positive and negative characters Give illustrations of these characters, with 

 their uses Advantage of positive characters over negative Founded only 

 upon evident facts What is the fifth rule ? The sixth F 



