PEDIGREE OF MAMMALS. 275 of the aboriginal genus is repeated, and Philogenesis is unequivocally expressed in Ontogenesis. The Anchitherium is a three-toed horse, in which, however, the middle toe has already undertaken the chief task. But in the Hipparion the two side toes are entirely raised from the ground, and by disuse are brought to the condition of arrest which is completed in the horse. In the constitution of the molar teeth the tapirs have remained most faithful to the ancestral type. The circumstance that the tapir has four toes in front, whereas the Palæotheridæ known to us, have three shows, however, that the genus Palæotherium cannot have been the ancestral stock of the tapirs. For the supposition that the tapir acquired the fourth toe is contrary to all experience respecting the formation of the extremities. Rhinoceroses are also four-toed in front, and their close kindred with the tapirs is testified by the structure of their toes and a series of details in the skeleton. Hippopotami. Pigs. Tragulidæ. Deer. Antelopes. Oxen. Anoplotheridæ. An isolated branch of the Palæotheridæ seems to be the fossil genus Macrauchenidæ, which combines the characteristics of the horse and rhinoceros with those of the camel. How far the latter, as ruminants, are directly connected with the Macrauchenidæ, or whether the form of their skull, approaching that of the horse, points to actual homology, it is for the present impossible to say. The Anoplotheridæ are likewise distinguished by a sort of undifferentiated dentition, from which a number of specific forms might deviate in different directions. The Tragulidæ are descended from them in a direct line; they form a small group not unlike the musk animals, and are confined to South Africa and Southern Asia. As chewing the cud, they are more nearly allied to the other typical ruminants with which we are acquainted; but, on the other hand, they occupy an intermediate position towards the other non-ruminants of the division, of which the whole was united in the pre-historic world through the Anoplotheridæ. The Suidæ, or piglike animals, were very profusely represented in the Eocene and Miocene periods. From a side branch of their predecessors, reaching up to the Anoplotheridæ, are descended the river-horses, or hippopotami. The function of ruminating is, as we know, correlated with a complex structure of the stomach as well as a peculiar mechanism of the œsophagal groove. It is naturally impossible to determine in which fossil animals these arrangements originated; yet it seems to have occurred at a very early period. Perhaps the more highly integrated structure of some non-ruminating genera, such as the hippopotamus and the peccary, may have been transmitted from the age of the Anoplotheridæ, and the very conspicuous accordance of the ruminating Tragulidæ with PEDIGREE OF MAMMALS. 277 the Anoplotheridæ stamps the latter with tolerable certainty as ruminants. Disregarding the camels, already mentioned as of doubtful origin, the typical ruminants separate into the deer-like and the horned. Through the hornless musk animals, the deer are connected with the Tragulidæ and the older genera. The giraffes form a side branch. But although the Helladotherium, nearly allied to the giraffe and at one time inhabiting the Athenian territory in herds, and the colossal Sivatherium, found in the spurs of the Himalayas, afford some clue to the position of the giraffes, so entirely isolated in the present world, the details of their derivation still remain very obscure. From the antelopes to the closely allied-in fact, scarcely separable - genera of goat and sheep, and similarly to the oxen, the systematic as well as the palæontological series, and likewise the ontogenetic phases, present transitions undeniably evincing family relationship. Besides the relations of the milk dentition of the filial to the ancestral genera, which Rütimeyer has also followed minutely, great interest attaches to the gradual transformation of the skull, which reaches its extreme in the oxen, and advances from the antelope and sheep, through Ovibos, Bubalus (buffalo), Bison, to Bos (ox). In the latter, the erect position of the frontal bone attains its utmost grade, and this transformation of the skull of the antelope is repeated in the calf. The usual classification of the Sirenia, or sea-cows, with the Cetacea, was decidedly a systematic misconception, arising from one-sided and, moreover, merely superficial consideration of the locomotive organs. All other characteristic indications-above all, the structure of the skull and the nature of the teeth-remove them from the Cetacea as much as they approximate them to the Ungulata. In the hippopotamus we have a member of this order nearly converted into an aquatic animal. We must think of the Sirenia as originally emanating from some unknown genera, which probably branched off at a very early period. A very uncertain position is occupied by the Hyracoidæ, now represented only by a few species of the genus Hyrax. To say that their characteristics recall at once the Ungulates, the Rodents, and the Insectivora, affords no explanation. Considering the great importance of the molar teeth in deciding derivation, the chief stress should perhaps be laid on their similarity in the hyrax and the rhinoceros, and we hence regard the hyrax as an offshoot of an old Ungulate family. With respect to the progenitors of the Proboscidæ, we refrain from any conjecture. Later than the Graminivora, the Carnivora, and especially the beasts of prey, seem to have appeared on the scene of arctic animal life. Granting the possibility (and it is scarcely possible to do otherwise) that placental formations may have originated in various ways, the possibility likewise exists that the Carnivora, and indeed other orders too, such as the Rodents especially, may be direct descendants of carnivorous Marsupials. The oldest beasts of prey known are feline, or resemble the Viverridæ and hyenas. Then come the Canidæ, and latest of all the Ursidæ. In skull, dentition, and extremities, the seals and walruses (Pinnipedea) constitute a side branch. Although there can be no idea of PEDIGREE OF MAMMALS. 279 any special affinity between the otter and the seal, the comparison of the two will aid us in imagining how from true beasts of prey and terrestrial animals the strange figure of seals and walruses must have proceeded. If the conjecture already propounded should be confirmed, that the detachments and ejections of the placenta, which constitute the phenomena of the decidua, assume very heterogenous forms in groups belonging to the same family, and may be alike in others no more nearly related, the Cetacea would be installed in our pedigree in the vicinity of the beasts of prey. Between a lion and a whale an angle is enclosed, containing a countless multitude of intermediate forms. But we must always bear in mind that our business is, not to bridge over the chasms between the present peripheral ends of the series of development representing the extreme forms, but to discover the points of derivation and attachment. Fossil whale-like animals are known in the Tertiary period, such as the Zeuglodon and Squalodon. The remains of the former colossal genus are kept in good preservation at Berlin, where Johannes Müller discovered their relations to both seals and whales. The dentition is seal-like; in the skeleton there is much similarity with the whales; and although the Zeuglodons must have been preceded by a great series of species, and followed by another of considerable, if not equal, length, before the present Cetacea proceeded from them, a development of this sort seems, nevertheless, extremely probable and natural. By their still perfect dentition and the still proportionate dimen sions of the skull, the Delphinoidæ are the oldest members of the true Cetacea. They were joined by the |