Johnson and Losos’ next move is to make what is probably the most enduring and powerful metaphysical proof for evolution: biology’s bad designs would not have been intended. In a rebuke to the intelligent design argument, they write:
As you can see in the blown-up image in figure 17.9, the receptor cells are actually facing backward to the stimulus (light). No intelligent designer would design an eye backwards! 
Darwin’s book was full of arguments from bad design. They were the powerful arguments for his theory. People who do not share the premise (that a bad design would not have been intended) fail to appreciate the power of the argument. The premise does not sway them, so they have difficulty understanding the point. The key here is to assume the evolutionary perspective. Pretend you are an evolutionist for a moment. Pretend you genuinely believe the premise: a bad design would not have been intended.
The argument then becomes clear. And its power is obvious. If you believe in this metaphysical premise, then of course, some sort of evolution must be true. As Darwin argued in his book:
We cannot believe, that the similar bones in the arm of the monkey, in the fore-leg of the horse, in the wing of the bat, and in the flipper of the seal, are of special use to these animals. We may safely attribute these structures to inheritance.
QED: evolution must be true. Bad designs, inefficient designs, even designs not of "special use" must not have been intended—they must have evolved. We may not understand how it occurred (the theory), but we know that it did occur (the fact). As Stephen Jay Gould more recently put it:
Odd arrangements and funny solutions are the proof of evolution—paths that a sensible God would never tread but that a natural process, constrained by history, follows perforce. No one understood this better than Darwin. Ernst Mayr has shown how Darwin, in defending evolution, consistently turned to organic parts and geographic distributions that make the least sense.
Our textbook authors could not have demonstrated evolutionary thinking any more clearly. If a designer—who is capable of designing the vast and complicated biological world—would never have designed our backward receptor cells, then of course evolution is a no-brainer. It must be a fact, one way or another.
Evolution is at bottom a non scientific, religious, theory. It states that new forms emerge from biological variation undergoing natural selection—an idea that repeatedly has failed on the empirical science. But no matter, it must be true. Religion drives science, and it matters.