The opossums of America are marsupials, though not showing anomalies as great as kangaroos and bandicoots (in their feet), and Myrmecobius (in the number of teeth).
Fossil bones of extinct kangaroo species are met with; these kangaroos must have been of enormous size, twice or thrice that of any species now living.
These kangaroos are largely arboreal in their habits, but they descend to the ground to feed.
Then come the rat-kangaroos, or kangaroo-rats, constituting the sub-family Potoroinae; while the tree-kangaroos (Dendrolagus), rock-wallabie's (Petrogale), and wallabies and kangaroos (Macropus) form the Macropodinae (see Kangaroo).
Huxley in 1880 briefly suggested the arboreal origin, or primordial treehabitat of all the marsupials, a suggestion abundantly confirmed by the detailed studies of Dollo and of Bensley, according to which we may imagine the marsupials to have passed through (r) a former terrestrial phase, followed by (2) a primary arboreal phase - illustrated in the tree phalangers - followed by (3) a secondary terrestrial phase - illustrated in the kangaroos and wallabies - followed by (4) a secondary arboreal phase - illustrated in the tree kangaroos.
They were then arboreal; but they speedily entered upon a rapid, although short-lived, course of evolution, during which leaping terrestrial forms like the kangaroos were developed.
The Arunta hold that the spirits of kangaroos are expelled by human blood from certain rocks.
Although intimately connected with the cuscuses and phalangers by means of the musk-kangaroo, the kangaroos and wallabies, together with the rat-kangaroos, are easily distinguishable from other diprotodont marsupials by their general conformation, and by peculiarities in the structure of their limbs, teeth and other organs.