A journey to the southward from Britain or Japan or Illinois, or any point within the holarctic realm, would show the successive changes in the character of life, though gradual, to be more rapid. The barrier of frost which keeps the fauna of the tropics from encroaching on the northern regions once crossed, we come on the multitude of animals whose life depends on sunshine, the characteristic forms of the neotropical realm.
The neotropical realm includes South America, the West Indies, and the hot coast-lands of Mexico and Central America. To the northward, this realm overlaps the holarctic in the transition regions of Sonora, Arizona, Texas, and Florida; but to the southward the barrier of the broad ocean keeps it practically distinct from all others. The richness of this fauna in forms and species makes the great forests of the Amazon the dream of the naturalist. Joaquin Miller gives a vivid picture of the life of tropical America:
Birds hung and swung, green-robed and red,
Or drooped in curved lines dreamily,
Rainbows reversed from tree to tree,
Or sang—low hanging overhead,
Sang soft as if they sang and slept,
Sang low like some far waterfall,
And took no note of us at all.
Corresponding to the neotropical realm in position, but with a less rich and varied fauna, is the Ethiopian realm. This includes the greater part of Africa, merging gradually on the north into the holarctic realm, through the transition regions of Barbary, Italy, and Spain. In monkeys, herbivorous mammals, and reptiles, this region is wonderfully rich. In variety of birds and fishes the neotropical region far surpasses it.
The Indian realm comprises southern Asia and the neighboring islands. Its rich fauna has much in common with that of Africa, and it is, moreover, surrounded by transition districts which lead on the north to the holarctic, and on the west to the Ethiopian. On the east the Indian realm is lost in the islands of Polynesia, which represent each one its own degree of transition and isolation.
The Australian realm of Australia and its islands is more isolated than any of the others. It shows a singular development of low types of life, as though in the progress of evolution this continent had been left a whole geological age behind the others. It is certain that, could the closely competing fauna of the holarctic or Indian realms have been able to invade Australia, the dominant mammals and birds of that region would not have been marsupials and parrots. In the words of Prof. Bergen, "the antiquated forms of life are found in abundance only in regions where they