Here's an analogy: Why can't I jump to the moon? What barrier prevents me from jumping to the moon? There's no barrier preventing me from jumping to the moon. I can jump an inch, I can jump ten inches. So logically it must follow that since I can jump, and there's no barrier preventing me from reaching the moon, I CAN JUMP TO THE MOON!! Of course its a ridiculous argument. My very nature (physiology, limit to strength) does not allow me to jump to the moon, whether there's a barrier or not...
Actually I think you will find that the limits of your strength, physiology, the force of gravity, etc... would in fact be the barriers to your jumping to the moon. Where your analogy completely falls apart is that we know of no such limits on mutation. you claim that they exist but it is this very fact that is at issue. What are the limits? How do you know they exist?
At the moment mutation is the only known mechanism for the production of true genetic novelty. In the absence of any other mechanism and the absence of any evidence suggesting a limit to the potential of mutation then there is absolutely no need to discount mutation as the mechanism leading to the genetic diversity we see in modern organisms.
If you have evidence for another mechanism or for a limit to mutation then you should provide it. Otherwise you can cry as much as you like about evolutionary biologist's failure to breed pigs with wings but no one will take you any more seriously than the creationist next door demanding we show them a cat give birth to a dog.
TTFN,
WK
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