As cold drives certain animals, insects, and reptiles to a state of torpidity, so heat and lack of water bring about the same condition in others. The animal or reptile that hibernates, or goes to sleep in cold weather, arranges its body so that it will conduce to the greatest warmth, while those that estivate, or become torpid in warm weather, place themselves in positions that show that they want all the coolness the climate will permit. The tenric, a tropical animal, carnivorous and insectivorous, becomes torpid during the greatest heat, and lies on its back with its body drawn to its greatest length, and its limbs spread wide apart. Snakes estivate in the South, all kinds together, just as snakes hibernate in the North, but instead of rolling themselves in great balls, as the northern snakes do, they lie singly, and stretched to their full length. Want of water will cause the common garden snail to go into a state of the most complete and curious lethargy. This is the snail of the genus Limax, not the larger one of the genus Helix. In the latter the phenomenon of hibernation is especially remarkable. In November the snail forms just a soft, silky membrane across the external opening of its shell. On the inner surface of that it deposits a coating of carbonate of lime, which immediately hardens the gypsum. This partition is again lined with a silky membrane. The snail then retires a little further into the shell and forms a second membranous partition, retiring again and again until there are six of these partitions between the snail and the lime-coated door at the entrance of the shell.

In the recess behind all these partitions the snail lies torpid until May. All this time it lives without motion, without heat, without food, without air, without circulation or the exercise of any of its functions. If this snail is prevented from hibernating for several seasons by keeping it in a warm room, it will gradually waste away and die. A case is known where several snails of this genus were shut in a perforated box without food or water. They retired into their shells and closed them with a thin membrane. They remained so for three years, but revived when put into torpid water. They had been driven into torpidity by drought. The blood of this animal is white. It may be of interest to state in connection with these animals who pass half the year, or less, in sleep, that there are several species of fish, reptiles, and insects which never sleep during their stay in this world. Among fish it is now positively known that pike, salmon, and gold-fish never sleep at all. Also that there are several others of the fish family that never sleep more than a few minutes during a month. There are dozens of species of flies which never indulge in slumber, and from three to five species of serpents which the naturalists have never been able to catch napping.