As we have now shewn it to be highly probable, from the facts which have been advanced, that climate is the cause of the difference of colour which prevails in the different inhabitants of the globe, we shall now shew its probability from so similar an effect produced on the mucous substance before-mentioned by so similar a cause, that though the fact does not absolutely prove our conjecture to be right, yet it will give us a very lively conception of the manner, in which the phaenomenon may be caused.

This probability may be shewn in the case of freckles, which are to be seen in the face of children, but of such only, as have the thinnest and most transparent skins, and are occasioned by the rays of the sun, striking forcibly on the mucous substance of the face, and drying the accumulating fluid.  This accumulating fluid, or perspirable matter, is at first colourless; but being exposed to violent heat, or dried, becomes brown.  Hence, the mucosum corpus being tinged in various parts by this brown coagulated fluid, and the parts so tinged appearing through the cuticle, or upper surface of the skin, arises that spotted appearance, observable in the case recited.

Now, if we were to conceive a black skin to be an universal freckle, or the rays of the sun to act so universally on the mucous substance of a person’s face, as to produce these spots so contiguous to each other that they should unite, we should then see, in imagination, a face similar to those, which are daily to be seen among black people:  and if we were to conceive his body to be exposed or acted upon in the same manner, we should then see his body assuming a similar appearance; and thus we should see the whole man of a perfect black, or resembling one of the naked inhabitants of the torrid zone.  Now as the feat of freckles and of blackness is the same; as their appearance is similar; and as the cause of the first is the ardour of the sun, it is therefore probable that the cause of the second is the same:  hence, if we substitute for the word “sun,” what is analogous to it, the word climate, the same effect may be supposed to be produced, and the conjecture to receive a sanction.

Nor is it unlikely that the hypothesis, which considers the cause of freckles and of blackness as the same, may be right.  For if blackness is occasioned by the rays of the sun striking forcibly and universally on the mucous substance of the body, and drying the accumulating fluid, we can account for the different degrees of it to be found in the different inhabitants of the globe.  For as the quantity of perspirable fluid, and the force of the solar rays is successively increased, as the climates are successively warmer, from any given parallel to the line, it follows that the fluid, with which the mucous substance will be stained, will be successively thicker and deeper coloured; and hence, as it appears through the cuticle, the complexion successively darker; or, what amounts to the same thing, there will be a difference of colour in the inhabitants of every successive parallel.