“As he noted in the Presidential Address to which they had just listened with such well deserved interest, he found it stated, on what was then and at this time the highest authority for them, that as a matter of fact the doctrine of the immutability of species was disposed of and gone.  He found that few were now found to doubt that animals separated by differences far exceeding those which they knew as species were yet descended from a common ancestry.  Those were their propositions; those were the fundamental principles of 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: