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回忆：新东方Harvey  
 
four rhythms in nature: day-night's rhythm, tidal rhythm and seasonal rhythm.
＋ another rhythm that can't be seen: gravitational rhythm.
 
day-night's change is periodical and rhythmical.
called day-night's rhythm of nature.
Some animals do activities at night  and rest at daytime, for example, owl, vole, roach, bat
Some animals do activities at daytime and rest at night, for example, chicken, duck, pig
day-night's biorhythm of animal is controlled by biological clock in its body.
Scientists did a test for roaches in order to set biological clock of animal back or forward.
Roach is an insect which rests at daytime and works at night.
Scientists put some roaches into a test room and reversed artificially the order of day and night in the test room.
finally, the roaches just worked in the artificial "dark night" time even though it was a daytime in fact.
 
Tidal rhythm of nature.
Tide comes in or goes out by a regular way
Every tide always comes later 50 minutes than last one. Namely, the cycle of tidal rhythm is 24 hours 50 minutes .
 
small crab in seabeach; living in a tidal environment; avoid tidal strike for life.
By training and adaptation for a long time, it follows the tidal rule.
It usually gets out of its hole for finding food after every tide just goes out.
It always gets into its hole 10 minutes in advance when next tide comes in again.
It's just later 50 minutes than last day when the small crab gets into its hole everyday.
 
seasonal changes forms when the earth moves a circle around the sun.
Each change is periodical and rhythmical, which is called seasonal rhythm of nature.
Some animals can form a rhythmical action with the seaconal changes,
e.g. birds' migration, the hibernation of snake or frog
 
a rhythm which man can not see in nature---gravitational rhythm
caused by relative movements between the earth and any of other celestial bodies
All of matters in nature are attracted by the gravitation from the moon.
So, gravitational rhythm from the moon should belong to a natural rhythm.
e.g. woman’s menstrual rhythm.