In Trask’s Historical Linguistics there’s a very illuminating discussion (pp 78-82) of changes in phonological systems, in particular of Latin rhotacization–the change from an intervocalic /s/ to intervocalic /r/. There were several stages to this change, and the change was absolutely regular: every instance of pre-Latin intervocalic /s/ became intervocalic /r/ in Classical Latin. On the other hand, Classical Latin did have instances of intervocalic /s/, e.g. ecclesia, quasi, and visum. All of these turn out to have explanations, such as being borrowed after the shift had occurred (ecclesia, probably rosa), or not having intervocalic /s/ at the time of the shift (quasi, visum). Trask writes (ibid 82-83):
There are two important lessons here. First, a sound change normally happens at some particular time in the history of the language, and then stops. Consequently, the phonological history of a language consists of a series of changes, each acting on what’s left over from the last change. As a result of these accumulating layers of changes, the effects of earlier changes may be increasingly obscured by the effects of later ones. In our Latin example, various later changes reintroduced intervocalic /s/ into the language after the rhoticism had eliminated it; as a result we can’t immediately tell by looking at Classical Latin that the language had, centuries earlier, lost every single intervocalic /s/. We know this only because of patient and careful investigation by historical linguists.
Second, our policy of insisting that sound change must be regular is fruitful. If scholars had thrown their hands into the air and declared the troublesome words to be mere exceptions to rhotacism, there would have been no reason to worry about them. By insisting on their regularity, however, they were forced to find explanations for the odd cases, and, as you can see, they have been very successful in finding those explanations — and, as a result, they have wound up knowing rather more about the history of Latin than might otherwise have been the case. Even the few really nasty cases like rosa remain as puzzles to be investigated, and perhaps a future scholar will manage to find definitive solutions to these, too. But, without the regularity hypothesis as a guiding principle, there would be no reason for anybody even to look for such solutions.
The point I’m making about Intelligent Design should be clear: ID is nothing but an exhortation for scholars (specifically biologists) to throw their hands in the air every time they come across something that seems puzzling. It’s not a scientific hypothesis, despite what its proponents claim, because it is anti-fruitful. It doesn’t require any kind of Popperian view of the philosophy of science or verifiabilty and falsifiability to see that saying “I don’t currently understand how this happened, so it must be beyond explanation” isn’t a research strategy at all. ID is sterile. To claim that biologists should take ID seriously is to claim that biologists should take something like Sidney Harris’s cartoon as a publishable result.
Life on Earth, too, is a series of changes, each acting on what’s left over from the last change. The hypothesis is, must be in order to be fruitful, that this is absolutely regular: following the laws of physics, chemistry, and biology, with no exceptions. As soon as you’re willing to throw your hands in the air and say, well this bit is inexplicable, you might as well pack it in; there’s nothing further that you’re going to learn, because you’re going to stop even looking. To do science, or really any kind of scholarly research, you have to start with the assumption “There’s an explanation for that”; ID’s “hypothesis” is “There’s no explaining that.”
