英语短文：Cells and Temperature

　　Cells cannot remain alive outside certain limits of temperature and much narrower limits mark the boundaries of effective functioning. Enzyme¹ 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 impairs² their functioning. Even though cells can survive wider fluctuations³ the integrated actions of bodily systems are impaired⁴. Other animals have a wider tolerance⁵ 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 more accurate and meaningful over time, but popular terminology⁶ 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 more species were studied, it became evident that this classification was inadequate⁷. A fence lizard⁸ 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 hibernation⁹. Furthermore, many invertebrates¹⁰ that live in the depths of the ocean never experience change in the depths of the ocean never experience change in the chill of the deep water, and their body temperatures remain constant.

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