These relations are so clear and so closely connected with a gradual transformation of the whole skull, and with the progressive differentiation of the Primate-form, that we are justified in saying that the pedigree of the Primates, from the oldest Eocene Lemures upwards to man, is now so well known, its principal features so firmly fixed within the Tertiary age, that there is no missing link whatever. Quite different, and much more incomplete, is the palæontological evidence, if we go further back into the Secondary or Mesozoic age, and look there for the older ancestors of the mammalian series. There we meet everywhere with wide gaps, and the scarce fragments of fossil Mesozoic mammals (excessively rare in the Cretaceous formation) are too poor to permit definite conclusions as to their systematic position. Indeed, comparative anatomy and ontogeny lead us to the hypothesis that the oldest Cretaceous Mammalia--the Prochoriata--are descended from Jurassic marsupials, and these again from Monotremes. We may also suppose with high probability that among the unknown Cretaceous Prochoriata there have been Lemuravida and forms intermediate between these and the Jurassic Amphitheriidæ, and that these marsupials in their turn are descendants of Pantotheria or similar monotreme-like creatures of the Triassic age. Any certain evidence for these hypotheses is at present still wanting. One important fact, however, is established--namely, that these interesting and oldest Mammalia--the Pantotheria of Marsh, the Triassic Dromatheriidæ, and the Jurassic Triconodontidæ of Osborn--were small insectivorous mammals with a very primitive organization. Probably they were Monotremes, and may be derived directly from Permian Sauromammalia, an ill-defined mixture of Mammalia and Reptilia. This generalized characteristic supports our view that _the whole class of Mammalia is monophyletic_, and that all its members, from the oldest Monotremes upwards to man, have descended from one common ancestor living in the older Triassic, or perhaps in the Permian, age. To acquire full conviction of this important conception, we have only to think of the hair and the glands of our human skin, of our diaphragm, the heart and the blood corpuscles without a nucleus, our skull with its squamoso-mandibular articulation. All these singular and striking modifications of the vertebrate organization are common to mammals, and distinguish them clearly from the other Craniota. This characteristic combination and correlation proves that they have been developed only _once_ in the history of the vertebrate stem, and that they have been transferred by heredity from one common ancestor to all the members of the class of Mammalia.