Among these numerous fossil Lemures which have been discovered within the last twenty years, there exist, indeed, all the connecting forms of the older series of Primates, all the 'missing links' sought for by comparative odontology.
The oldest Lemures of the tertiary age are the Eocene Pachylemures, or Hyopsodina. They possess the complete dentition of the Prochoriata—namely, forty-four teeth (3.1.4.3/3.1.4.3). Then follow the Eocene Palæolemures, or Adapida, with forty teeth, they having lost one pair of incisors in each jaw. To these are attached the younger Autolemures, or Stenopida, with thirty-six teeth, they thus possessing already the same dentition as the Platyrrhinæ. The characteristic dentition of the Catarrhinæ is derived from this formula by the loss of another premolar.
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.
The next step, as we trace our human phylogeny to its origin, leads us further back into the lower Vertebrata, into that obscure Palæozoic age the immeasurable length of which (much greater than that of the Mesozoic) may, according to one of the newest geological calculations, have comprised about one thousand millions of years.[16]
The first important fact we have to face here is the complete absence of mammalian remains. Instead of these we find in the later Palæozoic period, the Permian, air-breathing reptiles as the earliest representatives of Amniota. They belong to the most primitive order of that class, the Tocosauria; and besides them there were the Theromorpha, which approach the Mammalia in a remarkable manner. These reptiles in turn were preceded, in the Carboniferous period, by true Amphibia, most of them belonging to the armour-clad Stegocephali. These interesting Progonamphibia were the oldest Tetrapoda, the first vertebrates which had adapted themselves to the terrestrial mode of life; in them the swimming fin of fishes and Dipneusta was transformed into the pentadactyle extremities characteristic of quadrupeds.
To appreciate the high importance of this metamorphosis, we need only compare the skeleton of our own human limbs with that of the living Amphibia. We find in the latter the same characteristic composition as in man: the same shoulder and pelvic girdle; the same single bone, the humerus or the femur, followed by the same pair of bones in the forearm and leg; then the same skeletal elements composing the wrist and the ankle regions; and, lastly, the same five fingers and toes.
The arrangement of these bones, peculiar and often complicated, but everywhere essentially the same in all the Tetrapoda, is a striking evidence that man is a descendant from the oldest pentadactyle Amphibia of the Carboniferous period. In man the pentadactyle type has been better preserved by constant heredity than in many other Mammalia, notably the Ungulata.
The oldest Carboniferous Amphibia, the armour-clad Stegocephali, and especially the remarkable Branchiosauri discovered by Credner, are now regarded by all competent zoologists as the indubitable common ancestral group of all Tetrapoda, comprising both Amphibia and Amniota. But whence this most remote group of Tetrapoda? That difficult question is answered by the marvellous progress of modern palæontology, and the answer is in complete harmony with the older results arrived at by comparative anatomy and ontogeny. Thirty-four years ago Carl Gegenbaur,[17] the great living master of comparative anatomy, had demonstrated in a series of works how the skeletal parts of the various classes of Vertebrata, especially the skull and the limbs, still represent a continuous scale of phyletic gradations. Apart from the Cyclostomes, there are the fishes, and among them the Elasmobranchi (sharks and rays), which have best preserved the original structure in all its essential parts of organization. Closely connected with the Elasmobranchi are the Crossopterygii, and with these the Dipneusta or Dipnoi. Among the latter the highest importance attaches to the ancient Australian Ceratodus. Its organization and development is now, at last, becoming well known. This transitional group of Dipnoi, 'fishes with lungs' but without pentadactyle limbs, is the morphological bridge which joins the Ganoids and the oldest Amphibia. With this chain of successive groups of Vertebrata, constructed anatomically, the palæontological facts agree most satisfactorily. Selachians and Ganoids existed in the Silurian times, Dipnoi in the Devonian, Amphibia in the Carboniferous, Reptilia in the Permian, Mammalia in the Trias. These are historical facts of first rank. They connote in the most convincing manner that remarkable ascending scale in the series of vertebrates for our knowledge of which we are indebted to the works of Cuvier and Blainville, Meckel, Johannes Mueller and Gegenbaur, Owen and Huxley. The historical succession of the classes and orders of the Vertebrata in the course of untold millions of years is definitely fixed by the concordance of those leading works, and this invaluable acquisition is much more important for the foundation of our human pedigree than would be a complete series of all possible skeletons of Primates.