Iris
The iris is the part of the eye that controls the size of the pupil. With the change in the pupil size, the amount of light entering the eye can be regulated (controlled). The iris is also the part of the eye that has colour. The colour is caused by the pigment known as melanin.
There is a theory that people who live in the equatiorial belt around the world possess more melanin in their iris than those living in the northern regions. This is to prevent excessive sunlight from getting into the eye. Consequently, people in the equatorial region have dark brown eyes. People living further north are less exposed to sunlight and thus, have lesser melanin making their iris blue, green or bluish green.
