intended to show how groups are distinct from each other; but, if it is equally fair to arrange those characters in such a way as to show the affinities of groups with each other, and what they have in common, we may say briefly that the placental mammals are connected with the marsupials by having—1. Nipples; 2, Free clavicles; 3. An embryonal cloaca, and by these characters both groups are distinguished from the monotremes below them; the marsupials and monotremes are united by having in common—1. Marsupial bones; 2. Undeveloped bigeminal bodies; 3. No placenta, and by these characters
Fig. 3.—Young Opossum. Natural Size.
are distinguished from the placental mammals above; while the monotremes join with the reptiles in possessing—1. United clavicles; 2. A permanent cloaca; 3. No nipples, and by these characters are distinguished from the marsupials above. A great many more characters and facts from the comparative anatomy, embryology, and palæology, could have been used in this comparison; but those given are enough to show how characters usually regarded as distinctive only may also at the same time be viewed as connective.
The order of living marsupials presents remarkable diversity of structure and habits, containing herbivorous, insectivorous, and carnivorous species; yet we find all these traits combined in one and the same species, the opossum. It is probable that, by adaptation to similar modes of life, the marsupials have developed groups parallel to those of the placental mammals. However, it is certain the Quadrumana seem represented by the Phalangers, the Carnivora by the Dasyuri, Insectivora by the Phascogales, Ruminantia by the kangaroos, and Edentata by the Monotremes. Rodents and bats are numerous in Australia, but only one of the former is marsupial, and none of the latter. The subdivisions of the order are indicated by the modifications of the extremities and digestive system. A gradual transition is found passing from the Phalangers through the Paramelidæ to the kangaroos. All arboreal species have an opposable thumb. This thumb is rudimentary or wanting in the terrestrial species, but in both the carnivorous and herbivorous groups we find a gradual transition to the species possessing a well-developed thumb; thus the
