The term biological clock is applied to the means, which living things 67. ______ adjust their activity patterns, without any obvious cue, the time of day, or 68. ______ the month, or the year. They are affected but little, if at all, by drugs, chemical, or wide temperature differences—factors may alter substantially 69. ______ the rates of all ordinary processes of the body. The nature of the biological clocks mechanism is still a mystery. Two 70. ______ quite different theories have been advanced to account to them. According 71. ______ to the first of these theories, each individual have evolved, aided by natural selection, an adaptation to the rhythmic environment. It has now 72. ______ become independence on the environment. According to this view, the clocks 73. ______ 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. 74. ______ The other theory holds up living things react continuously to their 75. ______ rhythmic physical environment. The setting of their biological clocks, therefore, involves a constant adjustment to subtle clocks are potential 76. ______ perfect timers.