The term biological clock is applied to the means which living __1__ things adjust their activity patterns, without any obvious cue, the __2__ time of day, or the month, or the year. They are affected but little,if at all, by drugs, chemicals, or wide temperature differences—factors may alter substantially the rates of all ordinary processes of __3__ the body.
The nature of the biological clocks mechanism is still a mystery __4__ two quite different theories have been advanced to account to __5__ them. According to the first of these theories, each individual have envolved, aided by natural selection, an adaptation to the rhythmic __6__ environment. It has now become independent on the environment __7__ according to this view, the clocks 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. __8__ The other theory holds up living things react continuously to __9__ their rhythmic physical environment. The setting of their biological clocks, therefore, involves a constant adjustment to subtle clocks which are potential perfect times. __10__

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