Skin color<span> is due primarily to the presence of a pigment called melanin , which is controlled by at least 6 genes. Both light and dark complexioned people have melanin. However, two forms are produced--pheomelanin , which is red to yellow in</span>color<span>, and eumelanin , which is dark brown to black</span>
The answer is the nervous system.
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If the inner lining of the air sacs neither thin nor highly vascularized, then it can be inferred that AIR SACS ARE CANNOT BE THE SITES OF GASEOUS EXCHANGE BETWEEN AIR AND BLOOD. Air sacs are generally lined with mucus and surrounded with blood capillaries.
In case of birds, air sacs play an important role in respiratory system.
Because you cannot test it or observe it.
A scientific theory are created through the scientific method, in the scientific method you need to be able observe it and experiment.
