_Predisposing Causes of Hibernation._--The likeness between hibernation and aestivation and the coincidence of the one with cold and of the other with heat arrest the conclusion that the temperature of the surrounding medium, whether atmospheric or aquatic, is the prime, much less the sole, cause of either. The effect of extreme cold is to rouse the hibernating animal from its slumber; and its continuance thereafter brings about a state of torpor which proves fatal. This at least appears to be the case with mammals, where actual freezing of the tissues is followed by death because the gases are expelled from the fluids as bubbles and the salts separate in the form of crystals. Some cold-blooded animals, however, may be cooled to 0 deg. C. Fish have been resuscitated after solidification in blocks of ice, and frogs have been known to recover when ice has been formed in the blood and in the lymph of the peritoneal cavity (Landois). For the reasons given, all hibernating mammals take precautions against exposure to extreme cold. They either bury themselves in the soil or under the snow or seek the shelter of hollow trees or of caves, not infrequently congregating in the same spot so that the temperature is kept up by corporeal contact. Again the hibernating instinct may be suspended unless the conditions are favourable for safely entering upon winter sleep. It is alleged that bears in Scandinavia do not hibernate unless food has been sufficiently plentiful during the summer and autumn to fatten them for their winter fast; and hedgehogs and dormice in captivity have been known to remain active in the cold until warm sleeping-quarters were insured by placing hay and cotton-wool in their cages. Finally the wood-chucks (_Arctomys monax_) in the Adirondacks retire to winter quarters at about the time of the autumnal equinox, when the weather is warm and pleasant, and emerge at the vernal equinox before the snows of winter have vanished from the ground. These and other facts justify Marshall Hall's conclusion that cold is merely a predisposing cause of hibernation in the sense that it is a predisposing cause of ordinary sleep. It has also been shown that the state of hibernation cannot be forced upon snails in summer by submitting them to artificial cold even almost to freezing point; but that at the proper season they prepare for winter quarters at temperatures varying from 37 deg. to 77 deg. Fahr. Again insects sometimes retire to winter quarters in the autumn when the temperature of the atmosphere is higher than that of preceding days during which they retain their activity.