> Paul,
 
I agree with you but the case I cited for a,b,c,d is in fact an actual
case or cases.  It applies to deer mice in the US and cichlids in the
African rift lakes.  I'm not claiming true knowledge here - just parroting
what I have read.  In both cases the animals at the ends of the spread
will not interbreed but they will interbreed with subspecies in-between
(and maybe with each other if forced into an artificial or laboratory
situation).  Some call the a and d populations subspecies but other
experts argue they are really separate species.  As Emilio pointed out,
the entire species concept begins to fall apart if you look too closely at
bacteria and certain other really small guys (that is the technical term -
sorry, can't remember the common one).  It also has to be severely
stretched when we consider mollusks with an active swimming/floating stage
in their development.  Only DNA type testing will ever settle this issue -
but it sure is fun in the mean time!!
 
Oh yeah, one last hooker for the sterile hybrid myth.  Captive breeding of
reptiles has greatly increased over the last 15 years and there have been
some interesting hybrids.  One that comes to mind is actually a cross
between two genera; the common kingsnake (Lampropeltis getula getula) and
the common corn snake (Elaphe guttata).  These species overlap in habitat
in the wild but do not interbreed until put into an artificial situation.
In captivity they are regularly interbred and the result is a fertile
snake - or so I have been told.  So if we think species is a bit tough to
pin down, what about this?
 
The answer has to be a combination of we just don't know enough and nature
just doesn't always fit into a neat little compartment.
 
Tom Eichhorst in New Mexico
 
P.S.  Do you think my Ceratostoma burnetti could really be just a remnant,
isolated  population of my Ceratostoma foliatus?  Or vice versa?  What we
need is for Sarah to get her degree and dig into this stuff!  I'm too old
to work it out, I just watch and enjoy.