teeth are so intimately connected with the jaw as to appear outgrowths from it, this is not the case really. They originate in the skin which covers the jaw, and the most effective part of their structure—the enamel—is derived from the epidermis, the outer layer of the skin. The centre of each tooth is filled with pulp, around which is the bone-like dentine with an outer coat of hard, glossy enamel. In the incisors or cutting teeth of the Rodents, while the front of the tooth is protected by a thick plate of hard enamel, the back portion consists only of dentine which wears away whilst the enamel front maintains a chisel-like cutting edge. In the grinding teeth or molars, especially noticeable in the Ungulates, the enamel is thrown into ridges and tubercles, so that the action of these in the upper and lower jaws upon each other is like that of "the upper and the nether millstones" in grinding corn.

Four forms of teeth are recognised in the Mammals: the incisors in the front of the jaw, the pointed, round canines or "eye-teeth" next to them, and at the sides the cheek teeth, separated into premolars and molars. In describing the teeth in any species a simple formula is adopted which shows at a glance the number of each kind in one side of each jaw. Taking our own normal dental equipment as an example, it would be expressed in this fashion:—

i 2/2, c 1/1, pm 2/2, m 3/3 = 32

the upper figures representing the number of each kind in the upper jaw and the lower figures the teeth of the lower jaw, and the total being reached by multiplying by two for the two sides of the skull. Often in our rambles we may come across the skull of some animal, and an examination of the teeth will help us to the identity of its late owner.

For the purposes of the present work it is unnecessary to enter minutely into all the characters that distinguish the Mammals from the other backboned animals. One is really sufficient—the possession of glands (teats) in the skin of the female which secrete milk for the nourishment of the new-born young. There are, in addition, differences in the structure of the skull and the articulation of the lower jaw. The skin is always more or less clothed with hair. The heart has a single left aortic arch, the blood is hot, and the heart and lungs are lodged in a special cavity separated from the abdomen by a muscular partition known as the diaphragm.

Respecting one item in the foregoing—it has been said truly that the possession of a few or many true hairs as outgrowths from pits in the skin is alone sufficient to distinguish a Mammal from any other animal. Although these hairs may take different forms, they are alike in their origin—even, to take an extreme case, the spines of the Hedgehog. Each hair consists of an outer wall enclosing a central cavity filled with pith, in which is the dark pigment which gives the hair its colour. In the Mammals this pigment is always brown, and the varying tints of the hairs—black, brown, tawny, cream-colour or white—depends upon the amount of pigment and its disposition in the pith, combined with differences in the density of the envelope. In some cases, as about the mouth, eyes, and ears of the Cat, long sensitive hairs are connected with the terminations of nerves, which help the animal to feel its way. There are no marked colour differences in the fur of the sexes, such as we find in the plumage of Birds; though we do find such discrepancies in the presence or absence of horns in Deer, and in the manes and hair-tufts of some exotic Mammals. Certain species, such as the Alpine Hare and the Stoat, undergo a marked seasonal change of colour in the fur under the influence of low temperature. This may be quite sudden, owing to a rapid fall of temperature, and—as shown by Metchnikoff—is effected by the pigment granules being consumed by a sort of phagocyte. By Metchnikoff's researches an old controversy appears to have been settled finally.

ANIMAL LIFE
OF THE BRITISH ISLES.

INSECT-EATERS: MOLE, SHREWS, AND HEDGEHOG

Hedgehog (Erinaceus europæus, Linn.).

The Hedgehog, Urchin or Hedgepig is so distinct from every other British mammal, that anybody could correctly name it at sight. The development of many of its hairs into long, stiff spines gives it an individuality that is not to be confused with any other; but there are other peculiarities, such as the extreme shortness of the head and neck in comparison with the bulk of its body, and the muscular power that enables it to remain rolled into a ball with every part protected by erected spines. But for the fact that the Hedgehog is frequently introduced into houses and gardens to keep down insect pests, few town-dwellers would have had the opportunity of seeing the Hedgehog alive; for it is a nocturnal beast coming from its retreat only at dusk and hunting through the night. There are, however, exceptions to this rule when a heavy summer downpour of rain has drenched the herbage and caused the snails and slugs to show considerable activity. Then the Hedgehog wakens also, and reduces their numbers; for it is with such fare, plus insects, worms, mice, rats, frogs, lizards and snakes, that the Hedgehog maintains his portliness. He passes the day under a heap of dead leaves or moss in a spinney or thick hedgerow, and the solitary observer in such places may sometimes be guided to this retreat by his snoring!

The winter time is spent as a rule in continued sleep; though he has been known on mild nights in winter to wake up and prowl around for the very few good things then to be found. But he is no intermittent hibernator like the Squirrel and Dormouse; therefore he makes no provision by laying up winter stores, which are only possible for seed-feeders. For his winter retreat he looks out for a hole in the bank—perhaps one that has been gradually enlarged by a colony of wasps to accommodate their continually increasing nest—and this he lines with dry leaves and moss, carried in by the mouth. Then he snuggles into his bed and goes to sleep until the spring.

The Hedgehog's eyesight does not appear to be very good, but this is made up to him by a very acute sense of smell. He hunts along the hedgebottoms and the sides of ditches, and in some localities he is frequently to be seen in such situations. But we have met with signs of his presence high up on the moors where he finds dense cover among the heather and bilberry. His common diet of snails and beetles is varied by the eggs of the robin and meadow pipit, and occasionally he stumbles upon a huge store of food in the shape of a dozen or more eggs of pheasant or partridge. By depressing his spines he may even find his way between the bars of a hen-coop, but after eating a great part of the hen he may be too portly to get out, and then falls a victim to the enraged poultry-farmer. He is, of course, too short-legged to accomplish the operation formerly attributed to him—that of milking cows—unless, of course, the cow assented to the robbery and laid down to it. But no evidence has been given in support of the charge, which is of kindred nature to the aspersions of Pliny, Ælian and other of the ancients that it climbed apple and fig trees, gathering and throwing down the fruit, then throwing itself down so that its spines would impale its plunder with which it walked off. One weak point in the story is the fact that the Hedgehog has no use for such fare as apples, and as for the milk—any one inspecting the small gape of his mouth would exonerate him from the charge of getting a cow's dug into it.