An assemblage of species, having so many fundamental points
   of structure in common, that in the judgment of competent scientists,
   they may receive a common substantive name. A genus is not necessarily
   the lowest definable group of species, for it may often be divided into
   several subgenera. In proportion as its definition is exact, it is
   natural genus; if its definition can not be made clear, it is more or
   less an artificial genus.