MARSUPIALIA, a subclass of Mammalia (q.v.) in which (with some exceptions) the young for some time after birth are kept in a pouch (marsupium) or bag of skin on the under side of the body of the female. Outstanding examples of the group are the kangaroos of Australia and the opossums of America. The marsupials are exceptionally important and interesting to the student of evolution for reasons which will appear later.
Discovery.—The South American opossums were seen and described by the first explorers of the New World, the Pinzons having taken a live one in Brazil in 150o, which was exhibited in Granada and described in a work by Trivigiano published in As to the North American opossums, Captain John Smith in his Description of Virginia (1612) states that "An opossum bath a head like a Swine, and a taile like a Rat, and is of the bignesse of a Cat." Apart from the description by a Dutch voyager Pelsart (1629) of wallabies seen on islands off western Australia, the Australian marsupials remained practically unknown until Captain J. Cook's voyage. In 177c) Captain Cook saw kangaroos in the region now known as North Queensland. The kangaroo was at first regarded as a gigantic leaping rodent. The discovery of other Australian marsupials such as the beaver-like wombat, the rabbit like bandicoot, the wolf-like thylacine, at first brought con fusion into the gradual process of classifying the mammals of the world. Even Cuvier did not at first realize that the marsupials formed an independent series, having only a general and super ficial resemblance to the carnivorous, insectivorous and gnawing animals of the northern world, and separated from them by pro f ound differences in the mode of reproduction. It remained for De Blainville in 1816 and 1834 to take the steps of putting the marsupials by themselves in a subclass of mammals which he named Didelphia, coordinate in rank with the Ornithodelphia (the egg-laying mammals or monotremes) and with the Monodelphia, which are the placentals or ordinary mammals of the northern world.
Reproduction.—The most famous character of typical mar supials is the pouch in which the females carry their young. In its extreme form, as in the kangaroo, the pouch consists of a capacious elastic bag, lined with fur. The fold which grows up to form the pouch may be compared to a gigantic teat surrounding the entire mammary field. In some Australian dasyures the pouch-fold arises only around the front end of the mammary field; the presence of a scrotal pouch in the male thylacine shows how easily the abdominal wall sinks in. In its complete form the pouch has a circular aperture opening downward. As the number of nipples increases the mammary field becomes an elongate oval and the pouch gives place to low longitudinal ridges and finally disappears. It has been suggested that the lack of a pouch is really a primitive character and that we can trace many stages in the evolution of the pouch among living members of the group. Since the pouch is reduced or wanting in members of different families of marsupials, the foregoing hypothesis must assume also that when it is present the pouch has arisen independently in different families. However, before such an hypothesis can be seriously considered it would be necessary to disprove the older view that marsupials, like all other mammals, are subject to the occasional degeneration or loss of even highly evolved structures, that the pouch is normally an important part of the marsupial mode of caring for the young but that in special circumstances, such as a marked increase either in the number of young at a birth or in their size, the pouch becomes unnecessary and is reduced or eliminated. The newer view seems to rest in part also on the erroneous assumption that the banded anteater (Myrmecobius) is a little modified survivor of the Mesozoic mammals. In the marsupials of the polyprotodont division (see below) the pouch frequently opens downward or backward but in the kangaroos and phalangers, which rest in a sitting position, the pouch opens forward or upward.