OF THE HUMAN RACK. 205 



diversities may appear, they are equally to be met with in the varieties of 

 several other kinds of animals that can be proved to have been produced from 

 a single species, and, in one or two instances, from a single pair. 



The chief causes we are acquainted with are the four following . climate, 

 food, manner of life, and hereditary diseases. 



I. The influence which CLIMATE principally produces on the animal frame 

 is on the colour of the skin and on the extent of the stature. All the deepest 

 colours we are acquainted with are those of hot climates ; and all the lightest 

 those of cold ones. In our own country we perceive daily, that an exposure 

 to the rays of the sun turns the skin from its natural whiteness to a deep 

 brown or tan; and that a seclusion from the sun keeps it fair and unfreckled. 

 In like manner the tree-frog (rana arborea) while living in the shade is of a 

 light yellow, but of a dark green when he is obliged to shift from the shade 

 into the sunshine. So the nereis lacustris, though whitish under the dark- 

 ness of a projecting bank, is red when exposed to the sun's rays. And that 

 the larves of most insects that burrow in the cavities of the earth, of plants, 

 or of animals, are white, from the same cause, is clear, since being confined 

 under glasses that admit the influence of solar light, they exchange their 

 whiteness for a brownish hue. 



The same remark will apply to plants as well as to animals ; and hence 

 nothing more is necessary to bleach or whiten them, than to exclude them 

 from the light of day. Hence the birds, beasts, flowers, and even fishes of 

 the equatorial regions are uniformly brighter or deeper tinctured in their spots, 

 their feathers, their petals, and their scales, than we find them in any other 

 part of the world. And hence, one reason at least for the deep jet which, 

 for the most part, prevails among mankind under the equator ; the dark-brown 

 and copper colours found under the tropics; and the olive, shifting through every 

 intermediate shade to the fair and sanguine complexion, as we proceed from 

 the tropic of Cancer northwards. Hence, too, the reason why the Asiatic 

 and African women, confined to the walls of their seraglios, are. as white as 

 Europeans ; why Moorish children, of both sexes, are, at first, equally fair, 

 and <vhy the fairness continues among the girls, but is soon lost among the 

 boys. 



As we approach the poles, on the contrary, we find every thing progres- 

 sively whiten ; bears, foxes, hares, falcons, crows, and blackbirds, all assume 

 the same common livery ; while many of them change their colour with the 

 change of the season itself. For the same reason, as also because they have 

 a thinner mucous web, the Abyssiniaris are less deep in colour than the negro 

 race ; for though their geographical climate is nearly the same, their physical 

 climate differs essentially : the country stands much higher, and its tempera- 

 ture is far lower. 



The immediate matter of colour, as I had occasion to observe more fully 

 in a preceding lecture, is the mucous pigment which forms the middle layer 

 of the general integument of the skin ; and upon this the sun, in hot climates, 

 appears to act in a twofold manner ; first, by the direct affinity of its colorific 

 rays with the oxygen of the animal surface, in consequence of which the 

 oxygen is detached and flies off ; and the carbon and hydrogen being set 

 at liberty, form a more or less perfect charcoal according to the nature of 

 their union; and next, by the indirect influence which its calorific rays, like 

 many other stimulants, produce upon the liver, by exciting it to a secretion 

 of more abundant bile, and of a deeper hue. I have formerly remarked that 

 this second or colouring layer of the general integument of the skin, differs 

 (as indeed all the layers of the skin do) in its thickness, not only in different 

 kinds of animals, but very frequently in different species, varieties, and even 

 individuals. Thus in our own country we find it more abundant in some 

 persons than in others ; and wherever it is most abundant, we find the com- 

 plexion also of a darker and coarser and greasier appearance, upon a com- 

 mon exposure to the solar light and heat: and we find also, that the hair is 

 almost uniformly influenced by such increase of colour, and is proportionally 

 copper and darker. 



