In the depths, underlying the froth of the debate about Darwinism, lie profound metaphysical questions. We can see something of this by looking at an argument of P.T. Geach:
Darwin’s method in The Origin of Species is to put up a challenge of this form: You mention some ostensibly teleological feature of living things, and I will show how it could have arisen through casual variations and the elimination of ill-adapted varieties. And he seems to suppose that the accumulation of many such stories increases the probability of each story. The opposite is the truth. Any one such story is pretty improbable on the face of it; we often have no evidence whatever that such random variation and the perishing of unfit varieties took place as Darwin hypothesizes; and in view of the imperfections of the geological record, the irrecoverable perishing of soft tissues, and the like, we can in many instances be pretty certain that no better evidence is going to be available. The improbability of the whole theory is increased, not diminished, as one unlikely story succeeds another.
There is a great deal which is of interest in Geach’s argument and a lot that could be said in reply to it. For the moment let us note that even if the evidence for Darwinism is in the poor condition ascribed to it by Geach—and one fallacy here is that Geach assumes that the recital of such ‘just-so stories’ constitutes the only possible evidence for the theory 1 —it could still be said that such imaginative reconstructions show that an explanation of the origin of species of a purely mechanistic sort is possible. It was precisely this possibility that was made fun of by thinkers such as Paley.