the descent of man, and selections relating to sex.
darwin wrote it about twelve years after he whote his more famous book... it entirely discounts natural selection as a cause of racial variation amongst humans.
the below information is paraphrased from the third chimpanzee which i already suggested you read.
tasmanians have very dark skin and their area recieves very little sunlight. (btw. australian aborigines and their tasmanian compatriots have lived there for 50,000 years. more than long enough to adapt to the lower uv since most europeans have been in their indigenous home much less time. par example. scandinavia has only been inhabited for 9,000 years.)
no one in the new world has very dark skin immaterial of uv exposure.
parts of asia with excessive uv exposure boast only medium-toned peoples.
"when one takes cloud cover into account, the world's most dimly lit areas, recieving a daily average of under 3.5 hours of sunlight, include parts of equatorial west africa, south china, and scandinavia, inhabited respectively by some of the world's blackest, yellowest, and palest peoples!" (diamond 115).
in the solomon islands (sorry. i thought it was new guinea... wrong place it's been a while since i read it.) there are indigenous people with very dark skin and very light skin living within short distances of each other.