In order to appreciate the influence of Darwin upon Huxley, we must find out the beliefs of the latter upon the “species question” before the appearance of the “Origin.” In his chapter “On the Reception of the ‘Origin of Species’” (“Life and Letters,” Vol. II.) Huxley says that, before 1858, he took up an agnostic position as regards evolution “... upon two grounds: firstly, that up to that time, the evidence in favour of transmutation was wholly insufficient; and, secondly, that no suggestion respecting the causes of the transmutation assumed, which had been made, was in any way adequate to explain the phenomena.” It is obvious that these two grounds are entirely distinct, and that the logical foundation of the first is far more secure than that of the second.
The effect of the “Origin” was completely to convince Huxley on the first ground: from that time he never doubted the truth of evolution, however it may have been brought about. With regard to the second ground, it is quite clear that Huxley had a very high opinion of natural selection: he thought it incomparably the best suggestion upon the subject that had ever been made, and he firmly believed that it accounted for something--that it may even have taken a dominant part in bringing about evolution. On the other hand, he never felt quite confident about the entire sufficiency of the evidence in its favour. It is probable that he was far more interested in the establishment of evolution as a fact than in natural selection as an explanation of it. He saw the vast amount of research in all kinds of new or almost neglected lines, which would be directly inspired by evolution. And his own investigations in some of these lines soon afforded some of the most weighty evidence in favour of the doctrine. Natural selection had not the same personal interest for him; no one has expounded it better or defended it more vigorously and successfully, but Huxley’s own researches never lay in directions which would have made them available as a test of the theory. Of natural selection he might have used the words of Mercutio--it may not be “so deep as a well, nor so wide as a church door,” to contain the whole explanation of evolution, “but ’tis enough ’twill serve”; it will, at any rate, prevent him from feeling the second ground on which he had maintained an agnostic position.