Didelphidæ (opossums) have a well-developed thumb; in some of the Dasyuridæ it becomes very small, while a tolerably distinct thumb characterizes the Phascogales; a rudimentary thumb in Dasyurus; no external thumb in D. Maujei, but its metatarsal exists, while in Thylacinus even its metatarsal is gone.
Fig. 4.—A Young Female Opossum (Didelphys Virginiana). Natural Size. B. Marsupium, clitoris, and vent of the same, enlarged; C. Marsupium, penis, and vent of a male of the same litter, enlarged.
Below the marsupials stands the group of Monotremes, including the remarkable Australian Ornithorhynehus and Echidna. In the former the openings of the milk-glands on the abdomen are not marked by any elevation or depression; but in Echidna we find a similar pair of glands, the opening of each becoming depressed at maturity, so as to form a small pit, into which the nose of the young is inserted and attached, where it remains pendant and nourished while its development advances. This pair of little pits may be regarded as the beginning of the bilateral pocket so largely developed in some marsupials. If we can imagine that these depressions have become so deep as to envelop not only the nose of the young, but also its whole body, we can understand the evolution of a marsupial from something lower. At the same time we should notice that these depressions are just the opposite of what we find in the higher mammalia, where the mammary glands form larger or smaller abdominal or pectoral prominences. The milk-glands of Ornithorhynehus seem primitive, while the depressed glands of Echidna and the marsupials, and the elevated glands of higher mammals, may be viewed as differentiations of the same.
The opossum is the animal on which the first observations of marsupial reproduction were made. At first the young, found in an imperfect condition within the pouch, were not examined closely enough to disclose their real nature. They were regarded as formless and inanimate. Even in the "Natural History of New York," Part I., the young is spoken of as "a small gelatinous body, not weighing-more than a grain." But these ideas of the early observers still exist in the popular mind, and are as imperfect as their explanations as to how the young originated. The peculiar character of the young led to the belief that they must have developed from the parents' teats, by a kind of metamorphosis or budding process. This gemmiparous
