Natural History: Mammalia/Marsupialia
SUB-CLASS II. MARSUPIALIA.
(Pouched Animals.)
Most zoologists are now agreed that the animals we are about to consider, constitute a group, which, however limited in extent, is shewn by the importance of the characters by which it is distinguished to be equal in rank, not to an Order of Mammalia, but to all the other Mammalia combined. The most striking of these are the immature condition of the young at the time of birth, and its reception into a pouch (marsupium) or fold of the skin on the abdomen of the female, in which it is protected from exposure to the air and injury, while, suspended from the teat, to which it is very early attached, it gradually assumes the form of its adult condition, and acquires the powers necessary for its independent existence. For some time, however, after it is able to procure its own living, and to run and play by the side of its mother, the young marsupial instinctively flees to the maternal pouch for protection on the approach of danger.
But these are far from being the only characters in which the Marsupialia differ materially from the Placentalia. Important peculiarities in the reproductive organs, in the arterial system, and in the structure of the brain; the open condition of the skull, the bones of which remain permanently separate: the tendency to a multiplication of the teeth; the presence of marsupial bones in the skeleton, even where the marsupium itself is not developed; and the absence of a true voice,—all manifest a departure from the high development of the placental Mammalia, and an approach to that of the oviparous Vertebrata in general, and to that of the Reptilia in particular.
The Sub-class before us, though not to be compared with the Placentalia in the number of its members, contains animals differing widely from each other in the modifications of their structure, in their habits, and in the nature of the food whereby they are sustained. They even exhibit analogies, more or less distinct, to the principal Orders of the great division we have already considered: thus the Opossums, in their opposible thumbs, seem to represent the Quadrumana, the little Myrmecobius the Insectivora, and the Kangaroos the Ruminantia; while more strongly drawn analogies exist between the Dasyuri (the "Zebra-wolf, native-devil," &c., of the Australian colonists) and the Carnivora, between the Phalangistæ and Petauri, and the Rodentia, and between the Duck-bill and Spiny ant-eater, and the Edentata. These, however, are not real affinities; the characters which distinguish the marsupial from the placental animals, and which bind them together among themselves, being of greater importance than those in which they respectively resemble the Orders just enumerated.
It is remarkable that this division of Mammalia is almost entirely confined to one region of the globe, Australia, including New Guinea, and the islands immediately adjacent. An exception exists in the case of the Opossums, the most highly developed form of Marsupialia, whose home is South America, one or two species reaching into the southern provinces of the northern continent: and it is worthy of observation that the same region is the great abode of the lowest order of Placentalia, the Edentata; which we have just dismissed.
Mr. Waterhouse, whose attention has been much directed to these interesting animals, divides the sub-class into eight Families,—Didelphidæ, Dasyuridæ, Myrmecobiidæ, Peramelidæ, Macropodidæ, Phalangistidæ, Phascolomyidæ, and Monotremata. It may be doubted, indeed, whether these might not with propriety, take the rank of Orders, were it not for the limited number of the species, which makes it more convenient to consider them as Families. Perhaps, however, we may more correctly group the whole into two Orders; as the animals designated by the last term, seem to differ much more widely from all the others than those do from each other.
