If all the humans on the planet were lined up by a single physical trait, say, height or color, in ascending or descending order, tallest to shortest, darkest to lightest, it would confound us to choose the line between these arbitrary divisions. One human would blend into the next and it would be nearly impossible to make the cutoff between, say, the San people of South Africa and the indigenous people along the Marañón River in Peru, who are scientifically measured to be the same color, even though they live thousands of miles apart and do not share the same immediate ancestry.
