As usual Darwin was creating a rhetorical fog. There is no conflict between design in nature and the operation of fixed laws of nature. Quite the opposite. The Euler spiral is a fixed law of the mathematics that helps hold our universe together, resulting in the design we see. There is no reason to believe that the rat went through hundreds of flopped, fatal designs for whiskers (natural selection acting on random mutation) before hitting on the Euler spiral. It was probably implicit from the beginning because the nature of reality in our universe would enact it. You can call this creationism if you want.