绿叶鸟网（www.lvyeniao.cn）原创整理的 > 考研英语作文 Cells cannot remain alive outside certain limits of temperature and much narrower limits mark the boundaries of effective nctioning. Enzyme systems of mammals and birds are most efficient only within a narrow range around 37C;a departure of a few degrees from this value seriously impairs their nctioning. Even though cells can survive wider fluctuations the integrated actions of bodily systems are impaired. Other animals have a wider tolerance for changes of bodily temperature.
For centuries it has been recognized that mammals and birds differ from other animals in the way they regulate body temperature. Ways of characterizing the difference have become accurate and meaningl over time, but popular terminology still reflects the old division into "warm-blooded" and "cold-blooded" species; warm-blooded included mammals and birds whereas all other creatures were considered cold-blooded. As species were studied, it became evident that this classification was inadequate. A fence lizard or a desert iguana-each cold-blooded----usually has a body temperature only a degree or two below that of humans and so is not cold. Therefore the next distinction was made between animals that maintain a constant body temperature, called home0therms, and those whose body temperature varies with their environments, called poikilotherms. But this classification also proved inadequate, because among mammals there are many that vary their body temperatures during hibernation. Further, many invertebrates that live in the depths of the ocean nr experience change in the depths of the ocean nr experience change in the chill of the deep water, and their body temperatures remain constant.
