the doctrine of evolution."

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 ev