The nature of hibernation, and probably also of aestivation, and the principal physiological phenomena connected with them, may be briefly summarized as follows:-- 1. During hibernation death from starvation and wasting of the tissues is prevented by the absorption of fat, which, at least in the case of mammalia, is stored in considerable quantities, sometimes in definite parts of the body, during the weeks of activity immediately preceding the period of winter sleep. 2. Every gradation seems to exist between ordinary sleep and hibernation; the differences between the ordinary diurnal or nocturnal sleep in summer of hibernating animals and their prolonged and lethargic quiescence in winter are merely differences of degree, differences, that is to say, of intensity and duration. 3. The physiological accompaniments of hibernation are: (a) Cessation of all activities associated with alimentation and excretion; (b) lowering of the body temperature to that of the surrounding medium or to within a few degrees of it; (c) total or almost total cessation of respiration, accompanied by power to survive immersion for a considerable time in water or asphyxiating gases, which prove rapidly fatal to the same animals when normally active; (d) marked increase in the irritability of the muscles, especially of those of the left side of the heart, whereby the pulsations of that organ, although retarded, are uninterruptedly maintained; (e) a slight exchange of gases in the lungs is kept up by the cardio-pneumatic movement. 4. Amongst cold-blooded animals, both vertebrate and invertebrate, devoid of the faculty of temperature adjustment, the phenomenon of hibernation or aestivation is of general occurrence wherever the conditions of existence accompanying the onset of cold or drought are inimical to active life. In hot-blooded vertebrates, on the contrary, the phenomena are non-existent so far as birds are concerned; aestivation is of very rare occurrence in mammalia, while hibernation is practised by a comparatively small number of species; and in these the faculty of temperature adjustment appears to be temporarily at all events in abeyance.