Still, however, this subject is open to inquiry, for the great variety of species, and other circumstances belonging to the animal creation, so modify the phenomena as to create an almost endless field of investigation. When warm-blooded animals are exposed by their parents to the atmospheric influence at an early age, they are better provided against the perils of cold, being born with an abundant source of heat. But, if the cold exceeds their powers of generating heat, the mortality is so much readier. Hence arises the danger of animals being born in the winter season. [p305] Two circumstances are distinguishable, the refrigeration of the body, and the temperature it is capable of sustaining. The cooling is so much less injurious with the young. If two young animals of the same species be cooled down equally, the youngest suffers the least. But, in order to lower to the same number of degrees the temperature of bodies of different ages, the external heat should be lowered in proportion to the advancement of the animal towards maturity, in order to compensate for the difference which the modification of age produces. While it is true that the younger animals suffer least from cold, it is, at the same time, to be considered that they cool down more rapidly. On this principle depends the mortality of our domestic fowls and other animals, the management of which requires so much observation and experience in order to rear them. In regions where the temperature is liable to great alterations in the course of the year, man and other vertebrated animals of warm blood are liable to suffer in their health; for, though _cold_ should produce the resistance derived from the necessary constitutional developement of heat, this increase of caloric, having its limits, often exposes the constitution to the effects of too great reduction of temperature, as is exemplified in the frozen regions of the North Pole, in Siberia, and in Russia. The young of mammiferæ, in general, were found by Dr. Edwards to differ very materially in the duration of their lives, in a state of asphyxia, often being limited to from five to eleven minutes, according to their developement at birth, the most advanced in organization living the longest period. The author proved these facts by placing animals in a state of asphyxia under water; and it is remarkable that, in all his experiments, the _voluntary_ motions were always first destroyed, the _involuntary_ outliving them. With dogs, cats, and rabbits, sensibility existed only three or four minutes. A puppy showed _automatic_ signs of life nearly half an hour. The best divers appear to be able to remain under water from three to four minutes.