Maybe I am just ahead of others. A thousand years ago if someone said DNA, someone else would have said "DNA has no meaning, you are speaking gibberish".
Or... they would've said, "what's DNA?"
Try this: years before viruses were discovered,
Ignaz Semmelweis came up with a method that he thought would reduce mortality in his hospital: getting the doctors to wash their hands after handling cadavers.
His idea was based on the notion that disease was transferred from the cadavers to the doctors hands and then to women in labour through some sort of particle that was so small it was invisible to the naked eye. He had no way of observing this particle directly.
So how did they know there was merit to the idea? By conducting an experiment: when the doctors washed their hands, mortality in the maternity ward decreased from more than 10% to less than 1%. There was a real, observable effect that could be linked to his ideas about an unobservable cause.
Same for genetics: do you think that
Gregor Mendel had any way to observe DNA? No... but he observed that inheritance follows certain laws, and deduced that there was a causal mechanism behind it.
In the same way, if the notion of a "kind" as some sort of limit to evolution has any merit at all, then it would be visible in the evidence. What evidence shows us that "kinds" are real? You can't even tell us what a "kind" is supposed to be.
Here's how science works when you believe that there's some sort of invisible cause at play:
- you come up with predictions:
if the cause is real, what would its observable effects be?
- you conduct experiments to look for those effects.
- if you observe the effects, then this is support for your hypothesized cause (though you should keep on looking for other possible explanations for those effects).
- if you don't observe the effects, then this shows that there's something wrong with your hypothesis and it needs to be revised (or abandoned).
An invisible cause or force is no impediment to science. As long as you have an actual hypothesis with actual predictions, you can investigate it. However, until creationists stop waffling and come up with an actual definition of "kind", none of the steps that follow can happen.
And
that's why this sort of thing is rejected as unscientific: not because the barriers between "kinds" are invisible, but because they're undefined.