In this high function the publication of the Origin of Species gave him his opportunity. That was in 1859. As with Hooker and Bates, his experiences as a traveller, and, more than this, his penetrating inquiry into significances and relations, prepared his mind for acceptance of the theory of descent with modification of living forms from one stock. Hence the mutability, as against the old theory of the fixity, of species. In the chapter On the Reception of the Origin of Species, which Huxley contributed to Darwin's Life and Letters, he gives an interesting account of his attitude toward that burning question. He says-- * * * * * "I think that I must have read the Vestiges (see p. 119) before I left England in 1846, but if I did the book made very little impression upon me, and I was not brought into serious contact with the 'species' question until after 1850. At that time I had long done with the Pentateuchal cosmogony which had been impressed upon my childish understanding as Divine truth with all the authority of parents and instructors, and from which it had cost me many a struggle to get free. But my mind was unbiassed in respect of any doctrine which presented itself if it professed to be based on purely philosophical and scientific reasoning.... I had not then and I have not now the smallest _a priori_ objection to raise to the account of the creation of animals and plants given in Paradise Lost, in which Milton so vividly embodies the natural sense of Genesis. Far be it from me to say that it is untrue because it is impossible. I confine myself to what must be regarded as a modest and reasonable request for some particle of evidence that the existing species of animals and plants did originate in that way as a condition of my belief in a statement which appears to me to be highly improbable....