













128 THE GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 
often, that they are now catalogued as British species, 
although they are never known to breed there; and Euro £ 
pean species have frequently been taken upon the America 
coast. It is not very probable that land birds have cros: 
the Atlantic in this way and become established in the oppe f 
site country, but in the case of northern aquatic birds, itii 
by no means impossible that whole flocks may have crossel 
the ocean, and become inhabitants of both shores of he 
there is a still greater chance of being carried from country. 
to country by winds. That they have never been known ti 
new country, the chances of their becoming perman 
established are very much greater than for birds, for a sing# 
female with eggs might be sufficient to introduce the species | 
Some of the facts mentioned below in regard to the intro 
duction of insects through man’s agency, show how 
they may become established. 
Of the organic causes in animal distribution, the infi 
of animals themselves is very slight compared with that 
man. Still, many species, carried by the winds or by 
influence from their original homes into other regions, 
destroyed by native carnivorous species, their perm 
introduction prevented, and the mingling of far-sepa™ 
parasites from place to place. A species is seldom 
duced without some of its parasites, and it might 
Introduce them without becoming introduced itself, for 
sited cocoons and eggs of insects, or living insects and 
animals infested by parasites, might be carried great © 
