THE DISTRIBUTION OF ANIMALS. 237 
tropical regions are well known to every intelligent 
person. In the same zone, too, different regions vary 
greatly in the life they support. The bison and grizzly 
bear belong to North America. The gorilla is found 
only in the forests of the western coasts of Africa. 
New Zealand has no native mammals except bats. 
Monkeys with prehensile tails live only in the New 
World. Continents have animals very different from 
those living on islands very near their borders. 
_On the other hand, the domestic fly flourishes every- 
where, and the house-sparrow seems to thrive as well 
in America as in Europe. Distribution over the earth’s 
surface is often called Geographical Distribution. 
All the animals inhabiting a given region constitute 
its fauna, just as the plants inhabiting a locality consti- 
tute its flora. 
The entire region inhabited by a certain kind of 
animal is often called its area of distribution. In the 
case of animals which migrate or wander from place 
to place the area of distribution may be spoken of as 
the range. 
Dispersals. In modern as well as in ancient times 
many animals have greatly extended the areas in which 
they live. The changes made in recent years have 
been carefully studied, to furnish a guide in explaining 
the movements of animals in the past. Doubtless 
many of the same causes which influence animal dis- 
tribution now have been always active. 
The Milkweed-butterfly. The milkweed-butterfly 
is an American insect. In about the year 1845 it first 
appeared in the Hawaiian Islands. At about the same 
time its special food-plant (Asclepias) appeared and 
became a troublesome weed. This butterfly soon 
spread over many of the Pacific islands, following the 
spread of its food-plant. It has also extended eastward 
to Bermuda, and is sometimes found in England and 
France. In this case the dispersal has been aided by 
commerce; for the seeds of the food-plant as well as 
