Expect it eventually will be defined genetically. But meanwhile I liked kuresu's list of hybrids which seemed to fit with what MJFloresta said about how a kind would be determined by ability to interbreed, even if that had to be tested artificially. I wasn't sure about this because I know there are cases where speciation has occurred and interbreeding ability has been lost though the new breed is certainly of the same kind. In any case I thought this made some kind of sense:
However, adding in the part about "certainly the same 'kind'" even though reproductively isolated leaves us back in the same place, without a usable definition. "'Kind' = ability to interbreed except when they can't" isn't very useful.
I know. I should not have added my doubtful thought at the end, which contradicted the first part as you say. Just because they don't doesn't mean they can't. The fact that many don't proves nothing in other words.
I was thinking of a frog species somebody posted about on another forum a long time ago, and I don't remember its name, but it's a member of the frog kind and there is no doubt, not something new. HOw do I know? I don't know. It's just obvious.
I'd also like your opinion concerning the second part of the question: how do you determine whether two fossils are the same "kind"? Obviously, an interbreeding test isn't going to work. So what's the criteria?
I don't know enough fossils myself to ponder the possibilities. Something in the morphological department would have to be relevant. I was impressed at a posting once of the Karoo formation which is full of reptilian swimming things that I'd have to say are clearly of the same Kind although they vary from each other in interesting ways. I may be misremembering.
I'm tempted to say that whatever most people would be inclined to call the thing (at least in most cases -- there would always be exceptions) is probably close to a definition of the Kind.
But the fact is we don't have a definition. We have feelings about what a Kind would include, and I think the list of hybrids kuresu posted is intuitively satisfying. I also think ability, apart from inclination, to interbreed, seems relevant. And morphology too of course, but that's not easy to define either.
Someday it will have to be genetically defined. Or maybe Jesus will come back first.