[galadriel'smaiden]

Lets look at this as if we were dealing with animals.

A donkey and a zebra make a zonkey.

A lion and a tiger make a liger or a tion ( depending on maternity)

Logically, a man and a dwarf would make a hobbit, right?

Wrong.

Now lets looka this as if they would be people.

There arent subspecies of humans, we're all Homo Sapieas Sapiens, so its very difficult to decide wether or not this could happen; personally I doubt it. It was probably a recessive gene for shortness that perpetuated and eventually became hobbits.