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.).