Test 11
Section C
The term biological clock is (36) ______ to the means by which living things (37) ______ their activity patterns, without any (38) ______ cue, to the time of day, or the month, or the year. The biological clocks seem to be beautifully (39) ______ to the needs of living things. They are (40) ______ but little, if at all, by drugs, chemicals, or wide temperature difference-factors which may alter substantially the rates of all ordinary (41) ______ of the body.
The nature of the biological clocks' mechanism is still a (42) ______ . Two quite different theories can (43) ______ for them. According to the first of these theories, (44) ___________________________________ . This is believed to have evolved, aided by natural selection, as an adaptation to the rhythmic environment. It has now become independent of the environment. (45) ________________________________________________ . The setting of their biological clocks, therefore, involves a constant adjustment to subtle environment forces. But the search for the specific timing system has been futile thus far. (46) ____________________________________________________ .

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