The term biological clock is applied to the means, which living things 【M1】______ adjust their activity patterns, without any obvious cue, the time of day, or 【M2】______ the month, or the year. They are affected but little, if at all, by drugs, chemical, or wide temperature differences—factors may alter substantially 【M3】______ the rates of all ordinary processes of the body. The nature of the biological clocks mechanism is still a mystery. Two 【M4】______ quite different theories have been advanced to account to them. According 【M5】______ to the first of these theories, each individual have evolved, aided by natural selection, an adaptation to the rhythmic environment. It has now 【M6】______ become independence on the environment. According to this view, the clocks 【M7】______ are not perfect timers. They require regular corrections by the natural light and tide cycles and the changed lengths of the day throughout the year. 【M8】______ The other theory holds up living things react continuously to their 【M9】______ rhythmic physical environment. The setting of their biological clocks, therefore, involves a constant adjustment to subtle clocks are potential 【M10】______ perfect timers. 【M1】