托福阅读机经背景：眼睛变色的原因

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对很多人来说，眼睛的颜色从生下来那一天开始就确定了，而且一辈子不变。但对另外一些人来说，却不是这样。究竟是什么原因会导致眼睛变色呢?
　　It can bend light， bring the world into focus， and next to the human brain may be our most complicated organ. But for many people the most intriguing feature of the human eye is simply its color.
　　Can it really change for no apparent reason?
　　In most people， the answer is no. Eye color fully matures in infancy and remains the same for life. But in a small percentage of adults， eye color can naturally become either noticeably darkerorlighter with age.
　　What determines eye color is the pigment melanin. Eyes that have a lot of it in the connective tissue at the front of the iris， called the stroma， are darker， while those that have less tend to be lighter.
　　The levels of melanin generally remain the same throughout life， but a few things can change them permanently.
　　The first is a handful of ocular diseases like pigmentary glaucoma. Another is a condition called heterochromia，ormulticolored eyes， which affects about 1 percent of the population and is often caused by traumatic injuries. An example of this can be seen in the rock star David Bowie， who attributes his contrasting eye colors， hazel and light blue， to a blow to the face as a child.
　　The third cause appears to be genetics. A study in 1997， for example， looked at thousands of twins and found that 10 percent to 15 percent of the subjects had gradual changes in eye color throughout adolescence and adulthood， which occurred at nearly identical rates in identical twins.
　　The bottom lines Eyes can change color in some people because of geneticsorinjury.

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