One of the main objections to Darwin’s theory, both when the Origin was published and in the minds of many present-day Creationists, is the idea that species (or “fundamental kinds” of organism) are separated from each other by walls. No one doubted, then or now, that natural selection can cause small changes within existing species. The question was whether the process Darwin described can bring about large changes. Maybe a species can be pushed only so far. …
If we focus just on natural selection, it is hard to see why Darwin had the more compelling case. However, if we set natural selection aside and consider instead the idea of common ancestry, the picture changes. Darwin thought he had strong evidence for common ancestry. This is enough to show that insuperable species boundaries (and insuperable boundaries between “kinds”) are a myth; if different species have a common ancestor, the lineages involved faced no such walls in their evolution. And the case for common ancestry does not depend on natural selection at all. ...
Two of the facts mentioned earlier—that humans and monkeys have tailbones, and that human fetuses and fish have gill slits—are evidence for common ancestry precisely because tailbones and gill slits are useless in humans.
I know, tailbones are not useless and human embryos don’t have gill slits. Right now we’re ignoring evolution’s abuse of science and focusing on the theological argument. One fallacy at a time, please.
Now back to the argument. So why are (supposedly) useless or deleterious structures such strong evidence for common ancestry? Would not evolution have done away with such disasters? No, it was busy doing other things.
Nature’s malignant mechanisms prove common ancestry by virtue of refuting separate ancestry. If separate ancestry is false, then what’s left? Exactly, common ancestry is proved by the process of elimination.
And why is it, again, that these mistakes refute separate ancestry? Why that’s simple. As Sober explains, under separate ancestry it would mean that the mistake was made twice (or even multiple times), whereas under common ancestry it was only made once, a much more reasonable and probable conclusion.
Take that creationists. No creator would make a mistake twice. Like the Addam’s Family, nature’s monsters must all be related. There must be no fundamental kinds. The “god did it” hypothesis is not only unscientific, it is obviously wrong. We know because god didn’t do it. That’s just the Stuff of Good Solid Scientific Research.