710 THE PRINCIPLES OF SCIENCE. [CHAP. 



economy of the animal, but because such variations are 

 proved by empirical observation to coincide with most im- 

 portant differences in the general affinities. It is found 

 that the minor classes and genera of mammals can be 

 discriminated accurately by their teeth, especially by the 

 foremost molars and the hindmost pre-molars. Some teeth, 

 indeed, are occasionally missing, so that zoologists prefer to 

 trust to those characteristic teeth which are most constant, 1 

 and to infer from them not only the arrangement of the 

 other teeth, but the whole conformation of the animal. 



It is a very difficult matter to mark out a boundary -line 

 between the animal and vegetable kingdoms, and it may 

 even be doubted whether a rigorous boundary can be estab- 

 lished. The most fundamental and important difference of 

 a vegetable as compared with an animal substance probably 

 consists in the absence of nitrogen from the constituent 

 membranes. Supposing this to be the case, the difficulty 

 arises that in examining minute organisms we cannot ascer- 

 tain directly whether they contain nitrogen or not. Some 

 minor but easily detected circumstance is therefore needed 

 to discriminate between animals and vegetables, and this is 

 furnished to some extent by the fact that the production 

 of starch granules is restricted to the vegetable kingdom. 

 Thus the Desmidiaceae may be safely assigned to the vege- 

 table kingdom, because they contain starch. But we 

 must not employ this characteristic negatively ; the Diato- 

 maceas are probably vegetables, though they do not pro- 

 duce starch. 



Diagnostic Systems of Classification. 



We have seen that diagnosis is the process of discover- 

 ing the place in any system of classes, to which an object 

 has been referred by some previous investigation, the 

 object being to avail ourselves of the information relating 

 to such an object which has been accumulated and re- 

 corded. It is obvious that this is a matter of great impor- 

 tance, for, unless we can recognise, from time to time, 

 objects or Substances which have been investigated, recorded 

 discoveries would lose their value. Even a single investi- 



1 Owen, Essay on the Classification and Geographical Distribution 

 of the Mammalia, p. 20. 



