In the animal kingdom various odours and sounds are closely connected with the reproduction of the species. During the season of love a musky odour is emitted by the submaxillary glands of the crocodile, and pervades its haunts. At the same period the anal scent-glands of snakes are in active function, and so are the corresponding glands of the lizards. Many mammals are odoriferous. In some cases the odour appears to serve as a defence or a protection, but in other species the glands are confined to the males, and almost always become more active during the rutting season. Again, a great many insects have the power of producing stridulous sounds. In two families of the Homoptera and in three of the Orthoptera, the males alone possess organs of sound in an efficient state, and these are used incessantly during the pairing season. Some male fishes have sound-producing instruments, and the fishermen of Rochelle assert that the males alone make the noise during the spawning-time. Of frogs and toads the males emit various sounds at the pairing time, as in the case of the croaking of our common frog. During the rutting season, and at no other time, the male of the huge tortoise of the Galapagos Islands utters a hoarse bellowing noise, which can be heard at a distance of more than a hundred yards. Professor Aughey states that on two occasions, being himself unseen, he watched from a little distance a rattle-snake coiled up with head erect, which continued to rattle at short intervals for half an hour; at last he saw another snake approach, and when they met they paired. Among Birds the power of song, or of giving forth strange cries, or even instrumental music, is exceedingly common, particularly in the males during the pairing season; and almost all male mammals use their voices much more during that period than at any other time. Some, as the giraffe and porcupine, are stated to be completely mute except during the rutting season.