The hairs are greatly modified in form in different mammals and in different parts of their bodies. It is very commonly the case that a soft under-fur can be distinguished from the longer and coarser hairs, which to some extent hide the latter. Thus the "sealskin" of commerce is the under-fur of the _Otaria ursina_ of the North. The coarser hairs may be further differentiated into bristles; these again into spines, such as those of the Hedgehog and of the Porcupine. Again, the flattening and agglutination of hairs seems to be responsible for the scales of the _Manis_ {10} and for the horns of the _Rhinoceros_. It is a matter of common knowledge that upon the head of various animals, _e.g._ the Domestic Cat, long and sensitive hairs are developed, which are connected with the terminations of nerves, and perform a sensory, probably tactile function. These occur on the snout, above the eyes, and in the neighbourhood of the ears. It is an interesting fact that a tuft of quite similar hairs occurs on the hand of many mammals close to the wrist, which, at least in the case of _Bassaricyon_, are connected with a strong branch from the arm-nerve. These tufts also occur in Lemurs, in the Cat, various Rodents and Marsupials, and are probably quite general in mammals who "feel" with their fore-limbs;--in which, in fact, the fore-limbs are not exclusively running organs. That the last remaining hairs of the Cetacea are found upon the muzzle, is perhaps significant of the importance of these sensory bristles. The entire absence of hairs is quite common in this order, although traces of them are sometimes found in the embryo. The Sirenia, too, are comparatively hairless, as are also many Ungulates. Whether the presence of blubber in the former case and the existence of a very thick skin in the latter animals are facts which have had anything to do with the disappearance of hair or not, is a matter for further inquiry.