In article , JennyC
writes


3. In zoölogy and botany, a species is an ideal group of individuals
which are believed to have descended from common ancestors, which
agree in essential characteristics, and are capable of indefinitely
continued fertile reproduction through the sexes. A species, as thus
defined, differs from a variety or subspecies only in the greater
stability of its characters and in the absence of individuals
intermediate between the related groups.

So even Webster understands the difference!
A genus is a group of species descended from the same ancestors, which
agree in essential characteristics. BUT - the different species within a
genus may or may not be capable of fertile reproduction with each other
(and if they are, then in the wild they are separated by niche or
habitat so such reproduction does not normally occur). And if
reproduction between two species occurs, the result is a hybrid. Still
in the same genus, but not the same as any of the existing species.


--
Kay Easton

Edward's earthworm page:
http://www.scarboro.demon.co.uk/edward/index.htm