Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         "crystal-kingdom"
           chemical substances
             reactive intermediates
               Class diradicals [X:, X•Y•]
                 Class carbenes [X2C:, X=C:]
                   Class alkylidenes [R2C:]
                     1 methylene [H2C:]
                     2 propylidene [CH3CH2CH:]