structural conditions in living members of a single class. What is true for the limbs and the bony arches which support them is true for the backbone also, for the ribs, and to some extent for the skull. The neck in the Whale is shortened almost beyond recognition. In the Giraffe the same seven vertebræ are elongated into a marvellous neck; so that in the technical definition of a mammal both are said to have seven neck vertebræ. Yet exceptions show a capacity for variation. One of the Sloths reduces the number to six, while another has nine vertebræ in the neck; proving that there is no necessary difference between a mammal and a reptile when judged by a character which is typically so distinctive of mammals as the number of the neck bones. The skull varies too, though to a less extent. The Great Ant-eater of South America is a mammal absolutely without teeth. The Porpoises have a simple unvarying row of conical teeth with single roots extending along the jaw. And the dental armature of the jaws, and relative dimensions of the skull bones, exhibit such diversity, in evidence of what may be parted with or acquired, that recognition of the many reptilian structures and bones in the skull of Ornithorhynchus, the Australian Duckbill, demonstrates that the difficulties in recognising an animal by its bones are real, unless we can discover the Animal Type to which the bones belong; and that there is very little in osteology which may not be lost without affecting an animal's grade of organisation. VARIATION IN SKIN COVERING OF MAMMALS Even the covering of the body varies in the same class, or even order of animals, so that the familiar growth on the skin is never its only possible covering. The Indian ant-eater, named Manis, which looks like a gigantic fir-cone, the Armadillo, which sheathes the body in rings of bone, bearing only a scanty development of hair, are examples of mammalian hair, as singular as the quills of a Porcupine, the horn of a Rhinoceros, or the growth of hair of varying length and stoutness on different parts of the body in various animals, or the imperfect development of hair in the marine Cetacea. Among living animals it is enough for practical purposes to say that a mammal is clothed with hair, but in a fossil state the hair must usually be lost beyond recognition from its fineness and shortness of growth. VARIATION IN SKIN COVERING OF BIRDS