Possessed of important bearing upon the possibility of life on Mars is the rather recent appreciation that the habitat of both plants and animals is conditioned not by the minimum, nor by the mean temperature of the locality, but by the maximum heat attained in the region. Not only is the minimum thermometric point no determinator of a dead-line, but even a mean temperature does not measure organic capability. The reason for this is that the continuance of the species seems to depend solely upon the possibility of reproduction, and this in turn upon a suitable temperature at the critical period of the plant’s or animal’s career. Contrary to previous ideas on the subject, Merriam found this to be the case with the fauna of the San Francisco Peak region in northern Arizona. The region was peculiarly fitted for a test, because of rising a boreal island of life out of a sub-tropic sea of desert. It thus reproduced along its flanks the conditions of climates farther north, altitude taking the part of latitude, one succeeding another until at the top stood the arctic zone. Merriam showed that the existence of life there was dependent solely upon a sufficiency of warmth at the breeding season. If that were enough the animal or plant propagated its kind, and held its foothold against adverse conditions during the rest of the year. This it did by living during its brief summer and then going into hibernation the balance of the time. Nature in short suspended its functions to a large extent for months together, enabling it to resurrect when the conditions turned. Hibernation proves thus to be a trait acquired by the organism in consequence of climatic conditions. Like all such it can only be developed in time, since nature is incapable of abrupt transition. An animal suddenly transported from the tropic to a sub-arctic zone will perish, because it has not yet learnt the trick of winter sleeping. While still characterized by seasonal insomnia it is incapable of storing its energies and biding its time. But given time enough to acquire the art, its existence is determined solely by the enjoyment of heat enough at some season to permit of the vital possibility of reproducing its kind. Diurnal shutting off of the heat affects the process but little, provided the fall be not below freezing at the hottest season. So much is shown by the fauna of our arctic and sub-arctic zones, but still more pertinently to Mars by the zones of the San Francisco Peak region, since the thinner air of altitude, through which a greater amount of heat can radiate off, is there substituted for the thicker one of latitudinally equal isotherms. Here again with the diurnal as before with the seasonal it is the maximum, not the mean, or, till low, even the minimum temperature, that tells.