In a large number of mammals hairs of one kind only are scattered pretty evenly over the surface; but in many there are two kinds, one longer, stiffer, and alone appearing on the surface, and the other shorter, finer, and softer, constituting the under fur, analogous to the down of birds. This under fur, or _pashm_ as it is called by the natives of Kashmir, is especially abundant in the mammals inhabiting the cold plateau of Tibet and the adjacent regions. In many cases hairs of a different character from those of the general surface grow in special regions, forming ridges or tufts on the median dorsal or ventral surface or elsewhere. The tail is very often completed in this way by variously disposed elongated hairs. The margins of the eyelids are almost always furnished with a special row of stiffish hairs, called _cilia_ or eyelashes; and in most mammals specially modified hairs, constituting the _vibrissæ_ or whiskers, and endowed, through the abundant nerve supply of their basal papillæ, with special tactile powers, grow from the lips and cheeks. In some mammals the hairy covering is partial and limited to particular regions; in others, as the Hippopotamus and the Sirenia, though scattered over the whole surface, it is extremely short and scanty; but in none is it reduced to so great an extent as in the Cetacea, in which it is limited to a few small bristles confined to the neighbourhood of the lips and nostrils, and often only present in the young or even fœtal condition. Some kinds of hairs, as those of the mane and tail of the Horse, appear to persist throughout the lifetime of the animal; but more generally, as in the case of the body hair of the same animal, they are shed and renewed periodically, generally annually. Many mammals have a longer hairy coat in winter, which is shed as summer comes on; and some few, which inhabit countries covered in winter with snow, as the Arctic Fox, Variable Hare, and Ermine, undergo a complete change of colour in the two seasons, being white in winter, and gray or brown in summer. The several species of Cape Mole (_Chrysochloris_), the Desmans or Water Moles (_Myogale_), and _Potamogale velox_, are remarkable as being the only mammals whose hair reflects those iridescent tints so common in the feathers of tropical birds. The principal and most obvious purpose of the hairy covering is to protect the skin against external influences, especially cold and damp. Its function in the hairless Cetacea is supplied by the specially modified and thickened layer of adipose tissue beneath the skin, called “blubber.”