We must now pass on to consider the sense of hearing in animals. That the mammalia have this sense well developed is a matter of familiar observation, and in some of them, such as the horse and the deer, it is exceedingly acute. The form and movements of the external ear also enable many of the mammalia to collect and attend to sounds from special directions. The mammalia possess also the power of tone-discrimination, as is shown by the fact that our domesticated animals recognize different modulations of the human voice, and that wild creatures distinguish tones or noises of different quality. A Newfoundland dog, possessed by a friend of mine, always howled when the tenor D was struck on the piano, or sung. And Théophile Gautier reports that one of his cats could not endure the note G, and always put a reproving and silencing paw on the mouth of any one who sang it. In birds the sense of hearing is not only very sensitive, but the power of discrimination is exceedingly delicate. No one who has watched a thrush listening for worms can doubt that her ear is highly sensitive. The astonishing accuracy with which many birds imitate, not only the song of other birds, but such unwonted sounds as the clink of glasses or the ring of quoits, shows that the delicacy in discrimination has reached a high level of development. In birds, however, the cochlear canal has not the same development that it has in mammals, and there are no arched rods--no organs of Corti. Nothing special is to be noted concerning the sense of hearing in the reptiles, amphibia, and fishes. In all (with the exception of the lowly lancelet) the auditory organ is developed. We shall, however, presently see reason to question whether the possession of an "auditory organ," with well-developed semicircular canals, necessarily indicates the power of hearing. And Mr. Bateson's recent experiments at Plymouth[EX] seem to indicate that fishes are not so sensitive in this respect as anglers[EY] are wont to believe. "The sound made by pebbles rattling inside an opaque glass tube does not attract or alarm pollack; neither are they affected by the sharp sound made by letting a hanging stone tap against an opaque glass plate standing vertically in the water." Carp at Potsdam are, indeed, said to come to be fed at the sound of a bell. But Mr. Bateson well remarks that this "can scarcely be taken to prove that the sound of the bell was heard by them, unless it be clearly proven that the person about to feed them was hidden from their sight." There is clearly room for further observation and experiment in this matter.