 The class as a distinct rank of biological classification having its own distinctive name (and not just called a "top-level genus" "(genus summum)") was first introduced by the French botanist Joseph Pitton de Tournefort in his classification of plants that appeared in his "Eléments de botanique", 1694.Insofar as a general definition of a class is available, it has historically been conceived as embracing taxa that combine a distinct "grade" of organization -- i.e.a 'level of complexity', measured in terms of how differentiated their organ systems are into distinct regions or sub-organs -- with a distinct "type" of construction, which is to say a particular layout of organ systems.