350 THE PRINCIPLES OF SCIENCE. 



sition. The identically same substance, again, is occa- 

 sionally found crystallized in two essentially different 

 crystallographic forms ; calcium carbonate, for instance, 

 appearing as calc-spar and arragonite. Now the simple 

 truth is that if we are unable to discover any correspond- 

 ence, or, as we shall call it, any correlation between the 

 several properties of a mineral, we cannot make any one 

 arrangement which will enable us to treat at any one 

 time all these properties. We must really classify mine- 

 rals in as many different methods as there are different 

 unrelated properties of sufficient importance. Even if, 

 for the purpose of describing minerals successively in 

 some one order in a treatise, we select one system, that, 

 for instance, having regard to chemical composition, we 

 ought mentally at least to regard the same minerals as 

 classified in all other possible modes. 



Exactly the same may be said of the classification of 

 plants. An immense number of different modes of classi- 

 fying plants have been proposed at one time or other, 

 an exhaustive account of which will be found in Rees' 

 ' Cyclopaedia/ article ' Classification/ or in the Introduc- 

 tion to Lindley's ' Vegetable Kingdom/ There have been 

 the Fructistae, such as Csesalpinus, Morison, Hermann, 

 Boerhaave or Gaertner, who arranged plants according 

 to the form of the fruit. The Corollistas, Eivinus, Lud- 

 wig, and Tournefort, paid attention chiefly to the number 

 or arrangement of the parts of the corolla. Magnol se- 

 lected the calyx as the critical part, while Sauvage 

 arranged plants according to their leaves ; nor are these 

 instances more than a small selection from the actual 

 variety of modes of classification which have been tried. 

 Of such attempts it may be said that every proposed sys- 

 tem will probably yield some information concerning the 

 relations of plants, and it is only after trying many modes 

 that it is possible to approximate to the best. 



