The colour of the skin is justly considered one of the chief characteristics of race. Now it is quite impossible to assign any definite reason why one race is white, another black, brown, or yellow. Nobody has yet been able to prove that the colour of the skin is of any direct use to man, and it certainly is not the immediate result of long exposure to a certain climate. But we know that there exists an intimate connection between the colour of the skin and bodily constitution. “Les colorations diverses,” says M. Godron, “qui distinguent les différentes variétés de l’espèce humaine, tiennent beaucoup moins aux agents physiques, qu’aux phénomènes les plus intimes de l’organisation qui dans l’état actuel de la science, nous échappent et resteront peut-être toujours couverts d’un voile impénétrable.”[1597] Thus the alteration in the customary physiological functions called acclimatization, seems often to be connected with some change of colour not directly depending upon the influence of the sun. Dr. Mayer observed that a European at the tropics loses his rosy complexion, the difference in colour between arterial and venous blood being strikingly diminished on account of the smaller absorption of oxygen, which results from the feebler process of combustion.[1598] According to Dr. Tylor, it is asserted that the pure negro in the United States has undergone a change which has left him a shade lighter in complexion;[1599] whilst a long medical experience at New Orleans showed Dr. Visinié that the blood of the American negro has lost the excess of plasticity which it possessed in Africa.[1600] A negro boy brought to Germany by Gerhard Rohlfs, changed his colour after a residence of two years, from deep black to light brown.[1601] Klinkosch mentions the case of a negro who lost his blackness and became yellow; and Caldani declares that a negro, who was a shoemaker at Venice, was black when brought, during infancy, to that city, but became gradually lighter, and had the hue of a person suffering from a slight jaundice.[1602] In the ‘Philosophical Transactions,’ there is even a record of a negro who became as white as a European.[1603] On the other hand, we are told of an English gentleman Macnaughten by name, who long lived the life of a native in the jungle of Southern India, and acquired, even on the clothed portions of his body, a skin as brown as that of a Brahman.[1604] These statements, if true, certainly refer to exceedingly exceptional cases, but their accuracy cannot be _à priori_ denied. We know that certain organisms are much better able than others to undergo the change which constitutes acclimatization, and we have no positive reason to doubt that this power may, in abnormal cases, be extraordinarily great. At any rate, it is beyond doubt that a close connection exists between the colour of the skin and the physiological functions of the body, on the one hand, and between these and the conditions of life on the other.