418 THE PRINCIPLES OF SCIENCE. 



specially applicable to these brandies of science. What 

 more I may be able to say upon the subject will be better 

 said, if ever, when I am able to take up the closely- 

 connected subject of Scientific Nomenclature, Terminology, 

 and Descriptive Representation. In the meantime, 1 have 

 wished to show, in a negative point of view, that natural 

 classification in the animal and vegetable kingdoms is a 

 special problem, and that the special methods and diffi- 

 culties to which it gives rise are not those common to all 

 cases of classification, as so many physicists have sup- 

 posed. Genealogical resemblances are only a special case 

 of resemblances in general. 



Unique or Exceptional Objects. 



In framing a system of classification in almost any 

 branch of science, we must expect to meet with unique 

 or peculiar objects, which are so called because they seem 

 to stand alone, having few analogies with other objects. 

 They may also be said to be sui generis, each unique ob- 

 ject forming, as it were, a class by itself; or they are 

 called nondescript, because in thus standing apart it is 

 difficult to find terms in which to explain their properties. 

 The rings of Saturn, for instance, form a unique object 

 among the celestial bodies. We have indeed considered 

 this and many other instances of unique objects in the 

 preceding chapter, on Exceptional Phenomena. Apparent, 

 Singular, and Divergent Exceptions especially, are analo- 

 gous in nature to unique objects. 



In the classification of the elements, Carbon stands 

 apart as a substance entirely unique in its powers of 

 producing compounds. It is considered to be a quadri- 

 valent element, and it obeys all the ordinary laws of 

 chemical combination. Yet it manifests powers of affinity 

 in such an exalted degree that the substances in which it 



