Huxley rested his claim that the "hoary objection...has lost its force" on an article by Fisher which made a fallacious argument claiming to show that highly improbable events do happen and which also made the serious error of conflating complexity and improbability. In the article reproduced here, Huxley makes the same error.
A change of probability may be necessary but is not sufficient; a change in complexity is also needed.
An increase in adaptivity does not entail an increase in complexity.
The claim that NS generates improbability (sic) is an assumption whose validity has never been established. Thus Huxley uses an unproved assumption to dismiss the "hoary objection"!
Huxley's "Hoary Objection" Howler
Julian Huxley (1) made the following remarkable claim about natural selection:
Basing ourselves on this knowledge, and on the interesting branches of mathematics which have been developed to deal with problems of biological selection and transformation, we can now not merely assert with R. A. Fisher that "natural selection is a mechanism for generating an exceedingly high degree of improbability," but, with H. J. Muller, make some quantitative estimate of that degree. Muller has calculated that the most conservative odds against a higher organism, such as a man, a mammal, or even a fruit-fly, coming into existence fortuitously, without the operation of selection, by the union in one stock of all the necessary mutations, are given by a number with so many noughts that it would take an average novel to write it out, a number immensely greater than that of all the electrons and protons in the visible universe. That is a measure of the degree of our own inherent improbability-an improbability of the same order of magnitude as that of a monkey with a typewriter producing the works of Shakespeare. Just as it took the conscious activity of an outstanding human mind to produce the one, so it took two thousand million years of natural selection to generate the other.
After all, what selection does is not only to ensure the survival and establishment of a long series of exceedingly rare events (favourable mutations) but to secure, with the help of sexual reproduction, their combination in a single strain of living matter. If the rarity of a favourable mutation is I in I00,000, then the chance of two favourable mutations occurring in the same strain without the aid of selection is I in (I00,000)2 or I0,000 million, and that of 20 is I in (I00,000)20-"which is absurd."
Thus the hoary objection of the improbability of an eye or a hand or a brain being evolved "by blind chance" has lost its force: indeed, the shoe is now on the other foot, and the most apparently improbable adaptations-provided they can be regarded as conferring a biological advantage-are so many demonstrations of the immense power of natural selection operating over the stretches of geological time.
(1) J. Huxley in Evolution as a Process, J. Huxley, A. C. Hardy, E. B. Ford, Eds. (Allen & Unwin, London, 1954), p. 5.