Interesting. Thanks.
By this, do you mean the fact that "species" is a somewhat arbitrary classification based on some equally arbitrary traits?
Thal already covered this, but just to add a little bit more...
There is no scientific definition of "species" which is universally accepted. The geneticists, microbiologists, virologists, cladists, phylogenists, classical phyloginists, botanists, and probably a bunch more that I cannot think of right now all have different (and some times more than one per group) definitions of what qualifies as a species.
The problem really has two routs in modern biology (we'll ignore the centuries of conflict between different groups of phylogenists for now...):
1) The vast majority of species on this earth do not reproduce sexually. Meaning the classical definition of a species being "any population who can produce viable (i.e. fertile) offspring" doesn't really work in most cases. Definitions of what qualifies as a species in non-sexually reproducing organisms is up in the air, although most people will accept a >30% genetic divergence as being a truly separate species.
2) Even among sexually reproducing species, the "barrier" between species is foggy. Horses and donkeys can reproduce, but their offspring (mules/hinneys) are generally sterile. Hence, they are considered to be separate species. But, occasionally fertile mules are born, which would mean that the are the same species...
Another example, which causes even more headaches for phylogenists, is the case of the two "species" of European mouse (Mus musculis
castaneus , and Mus musculis domesticus). Take one from England (musculis) and one from East Europe (castaneus), and they cannot breed - so they're different species. But in the Alps you'll find mice which are both a mix of the two, and which can interbreed with both. Similar examples of "foggy boarders" between species are all over - most insects are like this, some of the different species of wild dogs, etc, etc, etc.
In the "real world" species don't really exist - instead you get gradients of differentiation, like the ones I described above. Eventually there'll be enough differences to make breeding impossible, but exactly where on that gradient you define a "species" is purely arbitrary, and quite artificial.
EDIT: I should have finished off the last paragraph by saying: "two sexually-reproducing populations which cannot interbreed and produce fertile offspring are clearly different species, beyond that, its anyones guess."
Bryan