such forms as the deer (_Cervus_), the
weasel (_Viverra_), the pig (_Sus_), the Macaque monkey
(_Cercopithecus_), and others. As we proceed in a south-westerly
direction, from Celebes to Amboina and thence to New Guinea, we find the
Indian types diminishing in number, and the Australian (_i. e._
marsupial forms) increasing. Thus in New Guinea seven species of pouched
quadrupeds have been detected, and among them two singular
tree-kangaroos; yet only one species of the whole seven, viz. the flying
opossum (_Petauris ariel_), is common to the Indian archipelago and the
main land of Australia. The greater the zoological affinity, therefore,
between the latter and the New Guinea fauna, although it seems in some
way connected with geographical proximity, is not to be explained simply
by the mutual migration of species from the one to the other.

8thly. When _Australia_ was discovered, its land quadrupeds, belonging
almost exclusively to the marsupial or pouched tribe, such as the
kangaroos, wombats, flying opossums, kangaroo-rats, and others, some
feeding on herbs and fruits, others carnivorous, were so novel in their
structure and aspect, that they appeared to the naturalist almost as
strange as if they were the inhabitants of some other planet. We learn
from the recent investigations of Mr. Waterhouse,[875] that no less than
170 species of marsupial quadrupeds have now been determined, and of the
whole number all but thirty-two are exclusively restricted to Australia.
Of these thirty-two, nine belong to the islands in the Indian
archipelago before mentioned, and the other twenty-three are all species
of opossum inhabiting the tropical parts of South America, or a few of
them extending into Mexico and California, and one, the Virginian
opossum, into the United States.

9thly. It only remains for me to say something of the mammiferous fauna
of _North_ and _South America_. It has often been said that, where the
three continents of Asia, Europe, and North America, approach very near
to each other towards the pole, the whole arctic region forms one
zoological and botanical province. The narrow straits which separate the
old and new world are frozen over in winter, and the distance is farther
lessened by intervening islands. Many plants and animals of various
classes have accordingly spread over all the arctic lands, being
sometimes carried in the same manner as the polar bear, when it is
drifted on floating ice from Greenland