On the other hand, Huxley all through his life, while holding that natural selection was by far the most probable hypothesis as to the mode in which evolution had come about, maintained that it was only a hypothesis, and, unlike evolution, not a proved fact. In 1863, in a course of lectures to workingmen, he declared:
"I really believe that the alternative is either Darwinism or nothing, for I do not know of any rational conception or theory of the organic universe which has any scientific position at all beside Mr. Darwin's.... But you must recollect that when I say I think it is either Mr. Darwin's hypothesis or nothing; that either we must take his view, or look upon the whole of organic nature as an enigma, the meaning of which is wholly hidden from us; you must understand that I mean that I accept it provisionally, in exactly the same way as I accept any other hypothesis."
In 1878 he wrote:
"How far natural selection suffices for the production of species remains to be seen. Few can doubt that, if not the whole cause, it is a very important factor in that operation; and that it must play a great part in the sorting out of varieties into those which are transitory and those which are permanent."
The difficulty in accepting natural selection as more than a hypothesis is simply that we have no experimental knowledge of its being able to produce the mutual infertility which is so striking a character of species. This difficulty is, in the first place, the difficulty of proving a negative. It might be possible to prove that its operation actually does produce species; it will always be impossible to prove that, in the past, natural selection, and no other known or unknown agency or combination of agencies, had a share in the process. All naturalists are now agreed that, as a matter of historical fact, it was the propounding of natural selection by Darwin that led to the acceptance of evolution, to the fact that evolution "takes its place alongside of those accepted truths which must be reckoned with by philosophers of all schools." The difficulty as to natural selection still exists, and there is no better way to express it than in Huxley's words, written in the early sixties:
"But, for all this, our acceptance of the Darwinian hypothesis must be provisional so long as one link in the chain of evidence is wanting; and, so long as all the animals and plants certainly produced by selective breeding from a common stock are fertile with one another, that link will be wanting; for, so long, selective breeding will not be proved to be competent to do all that is required of it to produce natural species.... I adopt Mr. Darwin's hypothesis, therefore, subject to the production of proof that physiological species may be produced by selective breeding; just as a physical philosopher may accept the undulatory theory of light, subject to the proof of the existence of the hypothetical ether; or as the chemist adopts the atomic theory, subject to the proof of the existence of atoms; and for exactly the same reasons, namely, that it has an immense amount of prima facie probability; that it is the only means at present within reach of reducing the chaos of observed facts to order; and, lastly, that it is the most powerful instrument of investigation which has been presented to the naturalists since the invention of the natural system of classification, and the commencement of the systematic study of embryology."—Man's Place in Nature, p. 149.[E]
FOOTNOTES:
[E] Further details on the subject of this chapter may be obtained in Clodd's excellent volume, Pioneers of Evolution, where an account of the history of the idea of evolution from the earliest times is given; and in Poulton's Charles Darwin and the Theory of Natural Selection, where there is a particularly valuable chapter upon Huxley's relation to Darwinism.