IN Prof. Semper's recently-published work on the “Conditions of Existence as they Affect Animal Life,” a review of which from the pen of Prof. Lankester appeared in your columns a fortnight ago (vol. xxiii. p. 405), allusion is made on pp. 67, 68, and elsewhere to John Hunter's celebrated experiment of feeding a gull with corn. Prof. Semper, however, seems not to have been aware of the precise nature of the result of Hunter's experiment. He says: “The English anatomist Hunter purposely fed a sea-gull for a whole year on grain, and he thus succeeded in so completely hardening the inner coat of the bird's stomach, which is naturally soft and adapted to a fish diet, that in appearance and structure it precisely resembled the hard, horny skin of the gizzard of a pigeon.”