In prosecuting this inquiry, one method to be adopted is to review the principal external differences observable among mankind, as to complexion, structure, and stature; and examine, whether analogous diversities appear among the lower animals within the limits of the same species. If it is ascertained that corresponding phenomena to the human variations occur in the case of animals belonging to an identical species, the chief objection is obviated to the unity and common origin of the human kind. 1. The most obvious distinction displayed by mankind is that of _color_, in relation to the skin, hair, and eyes, which, with few exceptions, are well known to have a certain correspondence, intimating their dependence on a common cause. Thus light-colored hair is very generally in alliance with light blue or gray eyes; but a relation of the complexion of the skin to the hue of the hair is still more invariable. Persons of light hair have a fair and transparent skin, which assumes a ruddy tint by exposure to the light and heat of the sun, while the complexion of black-haired individuals is of a darker cast, and acquires a bronze shade in proportion to the intensity of the solar influence admitted to it. The dark-haired women of Syria and Barbary are indeed frequently very white; but this is owing to the careful avoidance of exposure to the effect of climate, which Prichard calls a being “bleached by artificial protection from light, or at least from the solar rays.” He discriminates three principal varieties of mankind, taking the color of the hair as the leading character, which he styles the _melanic_, the _xanthous_, and the _leucous_. The melanic or black variety, includes all individuals or races who have black or very dark hair; the xanthous or fair class embraces those who have either brown, auburn, yellow, flaxen, or red hair; and the leucous or white variety comprises those who are commonly called albinos, whose hair is either pure white or cream-colored.