It is plain that natural selection cannot be considered a cause of variation; and if not of variation, which is as the rain drop, then not of specific and generic modification, which are as the river; for the variations must make their appearance before they can be selected. Suppose that it is an advantage to a horse to have an especially hard and broad hoof, then a horse born with such a hoof will indeed probably survive in the struggle for existence, but he was not born with the larger and harder hoof _because of his subsequently surviving_. He survived because he was born fit--not, he was born fit because he survived. The variation must arise first and be preserved afterwards.
Mr. Darwin therefore is in the following dilemma. If he does not treat natural selection as a cause of variation, the 'Origin of Species' will turn out to have no _raison d'être_. It will have professed to have explained to us the manner in which species has originated, but it will have left us in the dark concerning the origin of those variations which, when added together, amount to specific and generic differences. Thus, as I said in 'Life and Habit,' Mr. Darwin will have made us think we know the whole road, in spite of his having almost ostentatiously blindfolded us at every step in the journey. The 'Origin of Species' would thus prove to be no less a piece of intellectual sleight-of-hand than Paley's 'Natural Theology.'
If, on the other hand, Mr. Darwin maintains natural selection to be a cause of variation, this comes to saying that when an animal has varied in an advantageous direction, the fact of its subsequently surviving in the struggle for existence is the cause of its having varied in the advantageous direction--or more simply still--that the fact of its having varied is the cause of its having varied.
And this is what we have already seen Mr. Darwin actually to say, in a passage quoted near the beginning of this present book. When writing of the eye he says, "Variation will cause the slight alterations;"[349] but the "slight alterations" _are_ the variations; so that Mr. Darwin's words come to this--that "variation will cause the variations."
There does not seem any better way out of this dilemma than that which Mr. Darwin has adopted--namely, to hold out natural selection as "a means" of modification, and thenceforward to treat it as an efficient cause; but at the same time to protest again and again that it is not a cause. Accordingly he writes that "Natural Selection _acts only by the preservation and accumulation_ of small inherited modifications,"[350]--that is to say, it has had no share in inducing or causing these modifications. Again, "What applies to one animal will apply throughout all time to all animals--_that is, if they vary, for otherwise natural selection can effect nothing_"[351]; and again, "for natural selection only _takes advantage of such variations as arise_"[352]--the variations themselves arising, as we have just seen, from variation.