238 ANIMAL ACTIVITIES. 
the eggs, or young, of the butterfly were doubtless 
transported in cargoes carried by ships. It is interest- 
ing to note, however, that the animal follows the 
spread of the plant on which its larva lives. 
The Colorado Beetle. This insect, which was first 
described in 1824, was then found only in the neigh- 
borhood of the Rocky Mountains. Its food then was a 
wild plant, the sand-burr (Solanum rostratum). When 
the potato in its westward journey reached this beetle, 
the insect eagerly adopted the new food. For about 
forty years the potato-beetle has been extending its 
ravages, until it now flourishes in large numbers 
throughout the United States and Canada, apparently 
defying man’s best efforts to keep it in check. Here 
a change in the food-plant seems to have been respon- 
sible for the change in distribution. 
Some Causes of Dispersal. As shown by the cases 
just described, dispersal is often brought about by 
changes in food. Indeed, most of the wanderings of 
animals are doubtless prompted by some impulse con- 
nected with food-supply. Other causes, however, like 
pressure by enemies, change, in climate, overcrowding, 
perhaps even the curiosity of the animals themselves, 
have been instrumental in bringing about migrations. 
The regular migrations of birds, during which many 
thousands of our common birds travel half the length 
of a continent in a few weeks, or possibly a few days, 
or the somewhat similar migrations of fish, have not yet 
been fully explained. 
Barriers. Rivers, seas, and sometimes mountains 
have often prevented some of the animals of a region 
from getting far away from their home areas. These 
restricted animals often characterize a region. In a 
somewhat arbitrary manner naturalists have divided 
the surface of the earth into areas of distribution, for 
greater convenience in studying the causes which have 
brought our present faunas to their present places. 
These areas are often shown by maps. 
