In his biological studies his immense knowledge was firmly fixed in his mind by practical investigation; as is said above, he would take at second hand nothing for which he vouched in his teaching, and was always ready to repeat for himself the experiments of others, which determined questions of interest to him. The citations, analyses, maps, with which he frequently accompanied his reading, were all part of the same method of acquiring facts and setting them in order within his mind. So careful, indeed, was he in giving nothing at second hand, that one of his scientific friends reproached him with wasting his time upon unnecessary scientific work, to which competent investigators had already given the stamp of their authority. "Poor--," was his comment afterwards, "if that is his own practice, his work will never live." On the literary side, he was omnivorous--consuming everything, as Mr. Spencer put it, from fairy tales to the last volume on metaphysics. Unlike Darwin, to whom scientific research was at length the only thing engrossing enough to make him oblivious of his never-ending ill-health, to the gradual exclusion of other interests, literary and artistic, Huxley never lost his delight in literature or in art. He had a keen eye for a picture or a piece of sculpture, for, in addition to the draughtsman's and anatomist's sense of form, he had a strong sense of colour. To good music he was always susceptible. (To one breaking in upon him at certain afternoon hours in his room at South Kensington, "a whiff of the pipe" (writes Professor Howes), "and a snatch of some choice melody or a Bach's fugue, were the not infrequent welcome.") He played no instrument; as a young man, however, he used to sing a little, but his voice, though true, was never strong. But he had small leisure to devote to art. On his holidays he would sometimes sketch with a firm and rapid touch. His illustrations to the "Cruise of the Rattlesnake" show what his untrained capacities were. But to go to a concert or opera was rare after middle life; to go to the theatre rarer still, much as he appreciated a good play. His time was too deeply mortgaged; and in later life, the deafness which grew upon him added a new difficulty.