3 2o THE POPULAR SCIENCE MONTHLY. 



A journey to the southward from Britain or Japan or Illinois, 

 or any point within the holarctic realm, would show the success- 

 ive 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 In- 

 dies, 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 south- 

 ward 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 natural- 

 ist. 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 neighbor- 

 ing 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 iso- 

 lated than any of the others. It shows a singular development of 

 low types of life, as though in the progress of evolution this con- 

 tinent 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 marsu- 

 pials and parrots. In the words of Prof. Bergen, "the antiquated 

 forms of life are found in abundance only in regions where they 



