At this level, biology is an observational science. In other words, this demand is like:
You can't prove a negative, OTOH, look what happens when you cross a horse and a donkey.
As I pointed out in the previous message, what happens -- it turns out -- is unclear. For a long time it was thought that mules are always sterile, but it turns out that given half a chance some of them aren't. Perhaps under the right circumstances in nature, a race of mules would turn out to be viable. Would that prove that horses and donkeys are actually the same species? As far as I am concerned, that would prove the concept of "species" is largely an artifact of the human imagination. It is a distinction that nature itself does not make with as much clarity as we do.
It is kind of like the difference between a sea and an ocean, or an island and a continent. (Which is Australia? I can never remember.)
It may also turn out that cold and hot fusion are fundamentally the same process. They produce heat and helium in roughly similar proportions. As Chris Tinsley used to say, burning sugar and metabolism look very different but fundamentally they are the same. People love to make distinctions that nature does not make.
- Jed
