G78 THE PRINCIPLES OF SCIENCE. [CHAP. 



useful or useless, abundant or scarce, ferro- magnetic or 

 diamagnetic, and so on. 



Mineralogists have spent a great deal of labour in trying 

 to discover the supposed natural system of classification for 

 minerals. They have constantly encountered the difficulty 

 that the chemical composition does not run together with 

 the crystallographic form, and the various physical pro- 

 perties of the mineral. Substances identical in the forms 

 of their crystals, especially those belonging to the first or 

 cubical system of crystals, are often found to have no 

 resemblance in chemical composition. The same sub- 

 stance, again, 'is occasionally found crystallised in two 

 essentially different crystallographic forms ; calcium car- 

 bonate, for instance, appearing as calc-spar and arragonite. 

 The simple truth is that if we are unable to discover any 

 correspondence, or, as we may call it, any correlation between 

 the properties of minerals, we cannot make any one arrange- 

 ment which will enable us to treat all these properties in a 

 single system of classification. We must classify minerals 

 in as many different ways as there are different groups of 

 unrelated properties of sufficient importance. Even if, for 

 the purpose of describing minerals successively in a treatise, 

 we select one chief system, that, for instance, having regard 

 to chemical composition, we ought mentally to regard the 

 minerals as classified in all other useful 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 the article on 

 classification in Eees' " Cyclopaedia, " or in the introduc- 

 tion to Lindley's " Vegetable Kingdom." There have been 

 the Fractists, such as Csesalpinus, Morison, Hermann, 

 Boerhaave or Gaertner, who arranged plants according to 

 the form of the fruit. The Corollists, Eivinus, Ludwig, 

 and Tournefort, paid attention chiefly to the number and 

 arrangement of the parts of the corolla. Magnol selected 

 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 classi- 

 fication which have been tried. Of such attempts it may 

 be said that every system will probably yield some infor- 

 mation concerning the relations of plants, and it is only 



