Cells cannot remain alive outside certain limits of temperature and much narrower limits mark the boundaries of effective functioning. Enzyme systems of mammals and birds are most efficient only with a narrow range around 37℃; a departure 1of a few degrees from this value serious impairs their functioning. 2Even though cells can survive wider fluctuations the integratedactions of bodily systems are impaired. Other animals have a wide 3tolerance for changes of bodily temperature. For century it has been recognized that mammals and birds 4differ other animals in the way they regulate body temperature. 5Ways of characterizing the difference have become more accurateand meaningful over time, but popular terminology still reflects an 6old division into " warm-blooded" and " cold-blooded" species; warm-blooded included mammals and birds whereas all other creatures were considered cold-blooded. As more species were studied, it became evident that this classification was adequate. A 7cold-blooded fence lizard usually has a body temperature only adegree or two below those of humans and so is not cold. Therefore 8the next distinction was taken between animals that maintain a 9constant body temperature, called homeotherms, and those whose body temperature varies with their environments, calledpoikilotherms. But this classification was also proved inadequate, 10because among mammals there are many that vary their body temperatures during hibernation. Furthermore, many invertebrates that live in the depths of the ocean never experience change in the depths of the ocean, and their body temperatures remain constant.