Janine;
   I am sorry I worded things so badlyin my previous post.
     The main definition of the word Species is a group os organisms that
display similar characteristics and have the ability to interbreed and produce
viable Fertile offspring.
        The bit about the populations is what I had been taught in High school. I
just read through another textbook and the definition which I had memorized
and given is for the "Biological Species Concept" by none other than the
person others were talking about.. Ernst Mayr. I should have just put down the
part about interbreeding and producing fertile young. The reason the word
Fertile is so important is because similar organisms that are not members of
the same species can mate, but they produce sterile or no young. These crosses
are considered hybrids. A good example is the crossing of the Atlantic
bottlenose dolphin and the Spotted dolphin. They will mate but they produce
badly deformed young which die in utero.
 For more information read some of the posts from about a month ago that are
archived under the name Hybrids or cypraea hybridization. I don't have the
exact url for the home site for that archive but I am sure someone else can
give it to you.
 
Sarah (who never sleeps) Watson