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.
Well, if you mean "don't in the wild", then you're right - it doesn't prove much. It also doesn't help the definition of "kind" we're trying to reach. MJFloresta insists that the interbreeding criteria is the sole determinant. As you pointed out, that doesn't always work - there are a lot of organisms that are superficially similar (you mentioned a morphological criteria as the "fall back" - which of course ran us smack into the reproductive wall contradiction you recognize here) that simply can't interbreed, regardless of circumstances. Not "don't want to", but actually can't produce fertile hybrids (see next).
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.
This brings to mind an example that might be illustrative of the problem any definition of "kind" needs to overcome. Remember this little cutie?
It is a glass frog of the family Centrolenidae, which includes glass frogs and leaf frogs, that we found in the reserve I'm working in. Clearly, it is "frog kind". Morphologically (at least superficially), they resemble the Hylidae, or tree frogs, as many of the latter are small, green, and dwell in the canopy like the Centrolenidae in Ecuador. They even sound, to the untrained ear, like each other (they're both peepers). However, the details of their morphology (they have multiple different structures, even different organs), behavior, genetics, etc, make it literally impossible for them to mate - even artificially: the chromosomes don't line up properly* - they are not interfertile by any stretch. Your "I just know it" definition of "kind" - an objectively useless distinction - is the only way these two families can be considered the same. Genetically, reproductively, morphologically, behaviorally, and even in terms of ecological role, parental care, egg-laying strategies, etc, they are very different groups of organisms. Where can you draw the "kind" line?
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.
Unfortunately, "whatever most people would be inclined to call the thing" doesn't provide any real possibility of distinguishing one group of organisms from another. For example, what is an "elk"? The common name “elk” (what most people are inclined to call the thing) describes very different organisms in Europe and North America, even though the term is used in both places. A European “elk” is called a moose in North America, whereas the North American “elk” is referred to by Europeans as a “red deer”. Add in recently vanished organisms such as the “Irish Elk” - a type of fallow deer - and anyone attempting to communicate any kind of useful information about the critter is completely at sea, awash in conflicting nomenclature. Common names are pretty worthless outside of really obvious things like "dog", "cat", "sheep", etc. Even here, the differences between what scientists call
Felis catus, and say
Puma concolor or
Leopardis pardalis render "cat" a fairly worthless piece of nomenclature from any practical standpoint. This is pretty much the entire reason why Linnaeus came up with the binomial nomenclature in the first place. If you're going to replace this with "kind" - for whatever reason - it's got to be at least as useful and operationally defined as what is now in use. Until you (generically speaking - I'm aware you personally don't have more than a vague idea on the subject), can do this, I see no reason for anyone to take the "kind" term seriously.
Someday it will have to be genetically defined. Or maybe Jesus will come back first.
If I were you, I'd bet on the latter, since genetics not only doesn't support the concept, but quite clearly refutes its existence.
*Before you ask - no, no one has attempted to perform this experiment. Why would they? The chromosomes are so different, the arrangement of functional genes so different, that there is clearly no way for it to work. Eggs from the one simply physically can't be fertilized by sperm from the other.