Thus the oncoming and ceasing both of winter and summer sleep depend to a considerable extent upon conditions of existence other than those of temperature. Darwin saw scarcely a sign of a living thing on his arrival at Bahia Blanca, Argentina, on the 7th of Sept., although by digging several insects, large spiders and lizards were found in a half-torpid state. During the days of his visit when nature was dormant the mean temperature was 51 deg., the thermometer seldom rising above 55 deg. at mid-day. But during the succeeding days when the mean temperature was 58 deg. and that of the middle of the day between 60 deg. and 70 deg. both insect and reptilian life was in a state of activity. Nevertheless at Montevideo, lying only four degrees further north, between the 26th of July and the 19th of August when the mean temperature was 58.4 deg. and the mean highest temperature of mid-day 65.5 deg. almost every beetle, several genera of spiders, land molluscs, toads and lizards were all lying dormant beneath stones. Thus the animal-life at Montevideo remained dormant at a temperature which roused that at Bahia Blanca from its torpidity. Darwin unfortunately does not record whether the species observed were identical in the two localities. The temperature of animals in a profound state of hibernation is approximately the same as that of the surrounding medium or at most a degree or two higher. If, however, the temperature of the chosen hibernaculum (winter quarters) falls as low as freezing point, life is endangered at least in the case of mammals.