396 THE PRINCIPLES OF SCIENCE. 



registered and discriminated accurately by their teeth, 

 especially by the foremost molars and the hindmost pre- 

 molars. Some of the teeth, indeed, are occasionally missing, 

 so that zoologists prefer to trust to those characteristic 

 teeth which are most constant ^, 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 any boundary- 

 line between the animal and vegetable kingdoms, and it 

 may even be doubted whether any rigorous division can 

 be established. The most fundamental and important 

 character of a vegetable structure 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 ascertain directly 

 whether they contain nitrogen or not. Some minor but 

 easily detected circumstance is therefore needed to dis- 

 criminate 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- 

 maceae are probably vegetables, though they do not 

 produce starch. 



Diagnostic Systems of Classification. 



We have seen that diagnosis is the process of dis- 

 covering the place in any system of classes, to which an 

 object has already been referred by some previous investi- 

 gation, the object being to avail ourselves of the informa- 

 tion concerning such an object which has been already 

 accumulated and recorded. It is obvious that this is a 



y Owen, ' Essay on the Classification and Geographical Distribution of 

 the Mammalia/ p. 20. 



