I. _The Physiology of Hibernation. Hibernation and Aestivation_.--The physiology of hibernation, as exemplified in mammalia, has been worked out in detail by several observers in the case of some European species, notably bats, hedgehogs, dormice and marmots. Of the physiology of aestivation nothing definite appears to have been ascertained. It seems probable, however, from observations upon the dormant animals that the physiological accompaniments of winter and summer sleep are to all intents and purposes the same. The state of hibernation, for example, in the European hedgehog (_Erinaceus europaeus_) is not distinguished by external signs from the state of aestivation of the allied Mascarene genus, the tenrec (_Centetes ecaudatus_). The lethargy in both cases appears to be directly due to fall in the temperature of the organisms; and the fall in temperature proceeds _pari passu_ with the slowing down and weakening of the respiration and with retardation in the circulation of the blood. Similarity, moreover, between hibernation and aestivation is shown not only in their physiological accompaniments but also in the species of animals which become seasonally dormant. Birds neither hibernate nor aestivate. The tenrec (_Centetes_) of Madagascar, which aestivates, closely resembles the hedgehog (_Erinaceus_) in habits and belongs to the same order of mammalia. In the case of reptiles and batrachians, snakes, lizards, tortoises, frogs and toads sleep the winter through in cold countries; and some species of these groups habitually bury themselves in the sand or mud in tropical latitudes where drought is of periodical occurrence. Terrestrial molluscs lie dormant in the winter in cold and temperate latitudes and their tropical allies aestivate in districts where conditions enforce the habit. Some fresh-water molluscs bury themselves in the mud at the bottom of ponds when the surface is covered with ice; others take refuge in the same way when pools and tanks become exhausted during the dry season in the tropics. In temperate and north temperate countries insects and arachnida either die or retire to winter quarters during the cold weather, and in the tropics they similarly disappear during times of drought.