Factors such as skin color, hair color and texture, shape of the eyes and nose, brain size, and blood type have been suggested, but not one of these has proved to be entirely satisfactory as a classifier of the varieties of mankind.
ऐसा इसलिए है क्योंकि प्राकृतिक रूप से लोगों का कोई ऐसा समूह नहीं होता है जिसमें सभी सदस्य ऐसी विशेषताओं में एक समान हों ।