> a creationist can just say that any mutations that we've not observed are the limits to "kinds".
You have almost assuredly come across those who claim that the Neanderthal, Homo Erectus , etc. skeletons are signs not of different species of humans, but of genetic variation in "normal" humans. You know, just like all those *other* Nenderthals and Homo Erecti we see walking around all the time.....
I never asked him about Neanderthal, Homo Erectus or the others. He had already stunned me with his utter ignorance by asserting the "six kinds of evolution" as described in the Chick tract "Big Daddy" (asserting that I was ignorant for not "knowing" that the theory of evolution covers the formation of the cosmos) that I knew that he was too stupid to be trusted with the bigger details.