CLASSIFICATION. 391 



of Corollas/ were of equal extent. Subsequently a number 

 of plants plainly belonging to the same class were found 

 in tropical countries, and they possessed bright coloured 

 corollas. Naturalists believe with the utmost confidence 

 that 'Ruminants' and 'Ruminants with cleft feet' are 

 identical terms, because no ruminant has yet been dis- 

 covered without cleft feet. But we can see no impossibility 

 in the conjunction of rumination with uncleft feet, and it 

 would be too great an assumption to say that we are 

 certain that an example of it will never be met with. 

 Instances can be quoted, without end, of objects being ulti- 

 mately discovered which combined properties or forms 

 which had never before been seen together. In the animal 

 kingdom the Black Swan, the Ornithorhyncus Paradoxus, 

 and more recently the singular fish called Ceratodus For- 

 steri, all discovered in Australia, have united characters 

 never previously known to co-exist. At the present time 

 deep-sea dredging is bringing to light many animals of a 

 new and unprecedented nature. Singular exceptional dis- 

 coveries may certainly occur in other branches of science. 

 When Davy first succeeded in eliminating metallic potas- 

 sium, it was a well established empirical law that all 

 metallic substances possessed a high specific gravity, the 

 least dense of all metals then known being zinc, of which 

 the specific gravity is 7*1. Yet, to the surprise of chemists, 

 potassium was found to be an undoubted metal of less 

 density than water, its specific gravity being 0*865. 



It is hardly requisite to prove by further examples that 

 our knowledge of nature is incomplete, so that we cannot 

 safely assume the non-existence of new combinations. 

 Logically speaking, we ought to leave a place open for 

 animals which ruminate but are without cleft feet, and 

 for every other possible intermediate form of animal, plant, 

 or mineral. A purely logical classification must take 

 account not only of what certainly does exist, but of what 

 may in after ages be found to exist. 



