ing organisms implied by the existence of a liability to be fatally injured by it, may be found in the fact that Nature has amply provided for maintaining all very young animals at an elevated temperature, and protecting them from external cold. Note, first, how animals usually breed during the seasons of spring and summer; and observe, further, how the bird feathers her nest for the reception of her young, and shelters them under her wings, at the same time imparting heat by the contact of her own warm body. Rabbits and other animals tear off the fur from their own skins in order to provide a warm bed for the young while the parent is away in search of food. Frequently, too, animals are born in broods, or litters, especially those that are nearly nude at birth, and incapable of generating heat by exercise, and thus warmth is generated, or at least maintained, by the crowding together of a number of individuals in a small space.
Whether the liability, on the part of young animals, to be injuriously influenced by cold, is owing to their vital forces being so taken up with the process of growth as to leave a smaller surplus of vitality to resist the chemical agency of a diminished temperature, or whether it is that the lack of muscular exercise in them prevents the development of heat, we may not be able to determine; but this question is immaterial so far as the fact itself is concerned, that cold acts injuriously. It seems not improbable that the liability to be unusually affected by cold may depend upon rapidity of organic change. Thus those organs are most readily affected whose evolution is in most rapid progress; hence the digestive organs of the child and its pulmonary tissues are more apt to suffer from a depressed temperature than its reproductive organs; the former are undergoing rapid development, the latter are in a state of almost complete quiescence. Similarly the increased rate of tissue-change incident to violent activity of function appears to increase the susceptibility to cold; thus any one who has unusually exercised certain muscles will, after exposure, find those muscles become painful, "stiff," tender, and inflamed, while the remaining muscles of the body will have escaped any such manifestations. Buds that have withstood the severest cold of winter are often killed by the late but more moderate frosts of spring, because at this latter period they are in a state of more rapid tissue-change. The egg of the fowl will bear a considerable degree of cold without losing its vitality so long as its evolutionary processes are at a stand-still; but, when the rapid changes of structure incident to embryonic development have been set up, exposure even to the ordinary atmospheric temperature of spring and summer, if at all prolonged, is sufficient to destroy its life.
The greatest security against injury, therefore, from exposure to cold, would seem to be (comparative) structural stability—organic rest. But, in whatever manner to be explained, the fact remains that processes of physiological development may be disastrously embarrassed
