Page:Popular Science Monthly Volume 8.djvu/164

152 rare in other parts of the world, the kangaroos and almost all of the great variety of animals of Australia belong to this group. Thus it appears they are mostly tropical.

The earliest fossil mammals known appear to be marsupials allied to the opossum. In the bone-caverns of Brazil quantities of bones of opossums, such as live in that country now or similar, are found. One species of Didelphys was found fossil in the Paris Basin, of Eocene formation. Other relatives of the opossum have been found in a fossil state, associated with the palæotherium, anoplotherium, and. other extinct pachydermous quadrupeds; but the most remarkable are found in Jurassic rocks, as the earliest fossil mammals known. Their discovery in this ancient reptilian age in the limestone of Stonesfield was so extraordinary that attempts were made, on the one hand, to prove that their remains were reptilian; on the other, to prove that the rocks were of Tertiary origin; but it has been established, beyond all doubt, that these animals originated in this early reptilian age, and, probably, by descent, either directly or indirectly, from not very remote reptilian ancestry. This relationship is indicated, not only by the fossil remains of marsupials, but also by the anatomical and embryonic characters of marsupials and monotremes, so far as known. The organization of marsupials seems to be a kind of reptilian and mammalian combination, as has been shown by the valuable investigations of Prof. Owen, Dr. Coues, and others.

The monotremes present the lowest grade of mammalian organization, in many respects approaching closely to the oviparous classes of birds and reptiles. It is probably through these that the marsupials have gained some reptilian characters. The opossum, for example, has "a genuine reptilian skull," as Dr. Coues has remarked in his estimable memoir on the anatomy of this animal.

The main difficulty in tracing out the genealogy of marsupials is that our knowledge of them is confined chiefly to the living forms, while these must be but a small remnant of the whole group as it existed in ancient times, when its members inhabited every land on the face of our globe. Even in the imagination we cannot resurrect the manifold varieties of the past. But, in all probability. Prof. Haeckel is right in believing that this group affords a series of forms connecting the lower apes or lemuroids above them with the monotremes below. This would bring some of the marsupials within the lineage of human ancestry, and, before all others, the opossums seem most closely allied to the lemuroid apes. Indeed, they have already been grouped with man and the apes, although their structure hardly warrants such a classification. Storr congregated into one group all mammals with an opposable thumb. Also, Ogilby adopted the name cheiropeds for the same group, and subdivided it into Bimana (men), Quadramana (monkeys), and Pedimana (Semiadæ and opossums).

The characters of groups are generally arranged into categories