| The divine comedy|
Creationists frequently claim that, in order to show speciation, science needs to produce some kind of bridge linking different "kinds". However, they have this backwards. We don't need a bridge if there is no sign of an ocean.
Suppose you were debunking someone who believed in "macrowalking". This guy thinks you can walk to China from Florida! You, however, only accept microwalking, the idea that there are limits to how far you can walk in a single direction because of certain barriers (like, say, the Pacific Ocean).
Okay, first, you can explain the concept of oceans. You can explain how an ocean if it existed, could block walking. You can explain where your theory (which allows only microwalking) says the ocean is. There would be consensus among microwalking scientists as to where this boundary lay. You could link to satellite pictures of the globe, showing the oceans where the microwalking scientists say they are. You could, if all else fails, fly your opponent to the Pacific Ocean and show him the thing. All of this could be used to disprove macrowalking.
Now, let's look at creationism trying to disprove "macroevolution" (i.e., evolution).
Creationists tell us that there is a barrier, a metaphorical ocean that prevents individual mutations from accumulating into macroevolution (the same way that the ocean prevents individual steps from accumulating into macrowalking). Fine. We ask:
How does this barrier work?
We get no answer. No creationist has ever explained what it would be that might stop small changes from accumulating over time and becoming large changes. We get lots of "I don't believe it" (which is the argument by personal incredulity logical fallacy), and a bunch of "Well you can't prove it does" (but then, we don't have to, if we have evidence that it did, as is described throughout this site in abundance). There has not been one proposed mechanism for a barrier.
Okay, but maybe we could forgive that. Let's move on. Is there some evidence for the barrier?
We can point to evidence of the oceans. If nothing else, you get wet if you try walking west of California. Satellite views also show that there's something there even if you denied what it was, there was something.
Okay, so creationists, what evidence do you have for the barrier?
Nothing. More personal incredulity, more demands that we prove that there isn't a barrier (which is even weaker than above, because they are the ones making the positive claim here).
Okay, but we'll forgive that and give creationism one last shot. Even if they don't have any direct evidence for a barrier that they can objectively present, they might have something if they did, in fact, have a consensus on where this barrier was. At the very least, that would give us a place to start looking for this as yet unevidenced limit.
So, what do we get?
Well, some creationists say that it's about a genus (there are four or more different separately created kinds of cat). Some say it's about a family (one kind of cat). Others place it even bigger. Creationists have claimed that all birds are one kind and others insist that woodpeckers must be separately created.
About the only thing that you can get any two creationists to agree on is that there is a barrier and, wherever it is, humans are different from anything else! Of course, they can't really tell us where the barrier is there either!
So, no matter how nice we might try to be, there isn't a shred of support that there is a barrier.
We don't need a bridge. Creationists need to find some water.