Natura - nature
   actualia - actual entities
     Mundus Plinius - physical world
       particles
         elementary particles
           elementary fermions - elementary particles with half-integer spin
             quarks M. Gell-Mann, 1964
               1.1 up quark 1964?
               1.2 down quark 1964?
               2.1 strange quark 1964?
               2.2 charm quark 1976
               3.1 bottom quark L. Lederman et al., 1977
               3.2 top quark 1995