Okay, Art, I sympathize with your plight. Here's a loose working definition
of "species":
 
A species is a "kind" of organism. The species is the basic unit of the
taxonomic system, the one that everyone can most easily agree upon.
Distinctions below the level of a species (subspecies) are real, but not so
great as to make the populations into different "kinds". Distinctions above
the level of a species (genera and families) are real also, but are great
enough to make most people uncomfortable with calling the populations the
same "kind."
 
Set aside for now what criteria are used to define species of plants versus
animals versus bacteria, because it all comes to the same thing in the end.
College-educated people usually agree with New Guinea tribesmen on what is
a "kind" of bird or plant, to a high degree of accuracy. So do the
organisms themselves, when they are courting or otherwise interacting. That
tells us something about the reality of species, but it also tells us that
the concept of species is in the head of the observer. Of course it's
important to discuss what criteria we use to base our species concept on,
but it differs among observers and among groups of organisms. What works
for plants does not work for mollusks.
 
I think we can (mostly) agree on what I've said up to this point, but tread
any further than this and you step into a quagmire. I will illustrate by
illuminating just one small part of Gary Rosenberg's dark edifice.
 
Fossil species cannot be defined in the same way as living species. Fossils
are incomplete; in mollusks, typically only the shell is preserved, and not
even its pigment. We know that this would not be enough to distinguish
every living species unambiguously, especially of freshwater gastropods. So
when we refer to any particular fossil species, we understand that it means
something a bit broader and looser than for a living species.
 
Fossil species changed through time; this is undeniable. Each generation of
mollusks led to the next without a single break. Clearly, we are in deep
trouble here with one of the major criteria for determining living species,
i.e., "Do the two populations interbreed?" First, fossils are dead and no
experiments can be run to prove whether two contemporaneous populations
could interbreed. Second, we have many cases where it looks like one
species is ancestral to another, and we don't have any way to know whether
the ancestor could have interbred with the descendant. Usually, there's a
gap between two related populations--in most cases, the rock that might
have contained intermediate forms is missing due to erosion, or was
deposited in a different environment. That makes classification easier,
because we can just place the two populations into different species or
subspecies. It's an artificial solution, but it works most of the time, and
it's the best we can do in the absence of a complete record.
 
But there are many, many cases where the intermediate forms ARE present.
The best examples are of microfossils preserved in deepsea oozes. The oozes
have been deposited nearly continuously for millions of years, as a "gentle
rain" of tiny skeletons, clay, and debris from above, and are reworked in
the cold abyss by slow, long-lived worms and other burrowers. Species of
foraminifera can be shown to change through time, with the intermediate
steps preserved in an unbroken reproductive chain. At what point did
species A become species B? No amount of mathematical analysis is going to
help, no amount of precision in our definition will help, no "further
research" or experiments will help. In fact, the question cannot be
answered, not because data are lacking, but because the question is a bad
one to begin with, like the classic, "Answer yes or no! Have you stopped
beating your spouse?" In practice, of course, an arbitrary division is
made, or useful modifiers like "aff." and question marks.
 
I find that even this brief treatment grossly oversimplifies the case (I
especially ask the cladists for forbearance), but it will do. It's just a
small part of the complete answer that your "simple" question needs. You've
been keeping us on our toes, Art, but give it up: It's time to hit the
books!
 
Andrew K. Rindsberg
Geological Survey of Alabama