But for these shades of colour, or for any other, we should look in vain in the animal of the Tower Menagerie, which, in consequence of a particular conformation, not unfrequent in some species of animals, and occasionally met with even in the human race, is perfectly and purely white. In order to explain this phenomenon, which is one of the most curious, but at the same time one of the most simple in physiology, it is necessary to observe that there exists beneath the epidermis, or outer covering of the skin, both in man and animals, a peculiar membrane of very fine and delicate texture, which is scarcely visible in the European but sufficiently obvious in the Negro, termed by anatomists the rete mucosum. In this net-work is secreted, from the extremities of the minute vessels which terminate upon its surface, a mucous substance which varies in colour according to the complexion of the individual, of the varieties in which it is the immediate cause; and from the substance thus secreted the colouring matter of the hairs and of the iris is derived. The pure whiteness then of the covering of the animal in question, and of all those which exhibit a similar variation from their natural tinge, is attributable solely to the absence of this secretion from whatever cause. It is always accompanied, as in the present instance, by a redness of the eyes, arising from the blood-vessels of the iris being exposed to view in consequence of the want of the usual coating formed by this secretion, by which they are naturally protected from the too great influence of the light. In the human race the individuals who are thus afflicted, characterized by the dull whiteness of their skins, the deep redness of their eyes, and their colourless, or, as it is generally termed, flaxen, hair, are called Albinos. They are generally timid in disposition, languid in character, and weak both in mind and body. The same original conformation, for it is always born with the individual and never acquired in after life, although sometimes prolonged beyond its limits in the shape of an hereditary legacy, is common to many animals. Perhaps the most familiar instances among these are the white mice, the white rabbits, and the white pigeons, which are known to every one. But it has also been occasionally seen in many other species, as monkeys, squirrels, moles, pigs, and even cows and horses, and, to come a little closer to our present subject, in goats and deer. Not even that massive and stupendous beast the Elephant is exempted from its influence. It can hardly be necessary to recall to the reader the title on which the ruler of millions of not uncivilized Asiatics, the Burmese monarch, prides himself more than on any other, inasmuch as it is the emblem of power and prosperity, that of Lord of the White Elephant; a title, which, while it demonstrates the fact of the existence of this deviation in the Elephant as well as in other animals, proves also the extreme rarity of its occurrence. It has moreover been noticed in many species of birds.
Both these animals, although lively and tolerably good humoured when young, become mischievous in their dispositions and disgusting in their habits as they advance in age. The voice of the latter closely resembles the bark of a dog.
The well known group of which the Horse, the Ass, and the Zebra constitute the leading species, is distinguished from all other quadrupeds by the form of their hoof, which is single and undivided, rounded in front, of considerable thickness, and enveloping the extremity of their only apparent toe. They have in each jaw six powerful cutting teeth, accompanied on either side by the same number of grinders with square crowns flattened at the top: the males have two canines in the upper jaw, and frequently in the lower also; and this structure is sometimes shared by the females of the domesticated races. Between the canines and the molars there is a vacant space, which, our readers scarcely need to be reminded, receives the bit, the small but irresistible instrument by means of which man has for ages exercised the most complete control over the services of these useful animals. Although purely and essentially herbivorous, their anatomy, as well as their habits, separates them most thoroughly from the Ruminants, and approximates them in several respects to the Pachydermatous order, with which, in spite of their many discrepancies, both physical and moral, M. Cuvier has associated them. It is needless to point out the incongruity of this union, and it would be equally so to say more of the general form and external characteristics of a group, the principal species of which are so constantly before our eyes.
Ursus americanus. Pallas.