Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       naturalia - natural bodies
         "crystal-kingdom"
           chemical substances
             chemical compounds
               Class pyrimidine bases
                 cytosine [~NHC(=O)N=C(NH2)CH=CH~]
                 pyrimidineᵀ [~N=CHN=CHCH=CH~]
                 thymine [~NHC(=O)NHC(=O)C(CH3)=CH~]
                 uracil [~NHC(=O)NHC(=O)CH=CH~]