atlas) by a single, and in the Mammals (and Amphibia) by a double, condyle at the back of the head. In the former the lower jaw is composed of several pieces, and connected with the skull so that it can move by a special maxillary bone (the quadratum); in the Mammals the lower jaw consists of one pair of bony pieces, which articulate directly with the temporal bone. Further, in the Sauropsids the skin is clothed with scales or feathers; in the Mammals with hair. The red blood-cells of the former have a nucleus; those of the latter have not. In fine, two quite characteristic features of the Mammals, which distinguish them not only from the birds and reptiles, but from all other animals, are the possession of a complete diaphragm and of mammary glands that produce the milk for the nutrition of the young. It is only in the Mammals that the diaphragm forms a transverse partition of the body-cavity, completely separating the pectoral from the abdominal cavity. It is only in the mammals that the mother suckles its young, and this rightly gives the name to the whole class (mamma = breast). (FIGURE 2.267. Skull of a Triassic theromorphum (Galesaurus planiceps), from the Karoo formation in South Africa. (From Owen.) a from the right, b from below, c from above, d tricuspid tooth. N nostrils, NA nasal bone, Mx upper jaw, Prf prefrontal, Fr frontal bone, A eye-pits, S temple-pits. Pa Parietal eye, Bo joint at back of head, Pt pterygoid-bone, Md lower jaw.) From these pregnant facts of comparative anatomy and ontogeny it follows absolutely that the whole of the Mammals belong to a single natural stem, which branched off at an early date from the reptile-root. It follows further with the same absolute certainty that the human race is also a branch of this stem. Man shares all the characteristics I have described with all the Mammals, and differs in them from all other animals. Finally, from these facts we deduce with the same confidence those advances in the vertebrate organisation by which one branch of the Sauromammals was converted into the stem-form of the Mammals. Of these advances the chief were: (1) The characteristic modification of the skull and the brain; (2) the development of a hairy coat; (3) the complete formation of the diaphragm; and (4) the construction of the mammary glands and adaptation to suckling. Other important changes of structure proceeded step by step with these.