This is one of the reasons I hated Biology as a subject. The best definition I ever got for 'species' was a set of organisms that could interbreed and produce fertile offspring. This guy reminds us "remember, that's not always the case". So, if that's not the definition of species, what is? Poke and prod any Zoology professor long enough and he'll finally say "that's just the way it is, so just memorize it". There's no logical process defined for assigning organisms a place in our taxonomy. The only answer is "They guy credited with finding these arbitrarily decided that it belonged in this phylum, genus, etc" (I forgot the orders because, like I said, it's all arbitrary anyway). There is no sure-fire way to decide whether 2 organisms belong to the same species (much less any more generic taxon) because the reproduce with fertile offspring test is not necessarily the answer. It then comes down to "Well, they look different and they act different enough that I now officially say so".
Bah. Is anything that arbitrary truly a science?