728 THE PRINCIPLES OF SCIENCE. 



difficulties to which it gives rise are not those common 

 to all cases of classification, as so many physicists have 

 supposed. 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 stand alone, having comparatively 

 few analogies with other objects. They may also be said 

 to be sui generis, each unique object forming, as it were, a 

 genus by itself ; or they are called nondescript, because from 

 thus standing apart it is difficult to find terms in which to 

 describe 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 Excep- 

 tions especially, are analogous 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 

 appears are more numerous than all the other compounds 

 known to chemists. Almost the whole of the substances 

 which have been called organic contain carbon, and are 

 probably held together by the carbon atoms, so that many 

 chemists are now inclined to abandon the name Organic 

 Chemistry, and substitute the name Chemistry of the 

 Carbon Compounds. It used to be believed that the 

 production of organic compounds could be effected only 

 by the action of vital force, or of some inexplicable cause 

 involved in the phenomena of life ; but it is now found 

 that chemists are able to commence with the elementary 

 materials, pure carbon, hydrogen, and oxygen, and by 

 strictly chemical operations to combine these so as to form 

 complicated organic compounds. So many substances have 

 already been formed that we might be inclined to genera- 

 lise and infer that all organic compounds might ultimately 



