Several smaller forms of the same general appearance are known as wallabies, and are common everywhere.
The rock wallabies again have short tarsi of the hind legs, with a long pliable tail for climbing, like that of the tree kangaroo of New Guinea, or that of the jerboa.
There are some twenty smaller species in Australia and Tasmania, besides the rock wallabies and the hare kangaroos; these last are wonderfully swift, making clear jumps 8 or io ft.
With respect to their size they are distinguished as large wallabies and small wallabies, some of the latter being no bigger than a rabbit.
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.