THE DISTRIBUTION OF ANIMALS 463 



been found to contain eggs and cysts of many small crustaceans and 

 protozoans. Lice, ticks, and fleas hitch rides on their host animals. 

 Clams, which move about very little as adults, produce larvae which 

 attach themselves to the gills of fish and are carried great distances by 

 these more active hosts. Many animals make their way downstream 

 floating on logs and debris caught by the crest of floods. Even in the 

 ocean there are currents which carry many sea animals for great dis- 

 tances. Winds play their part also. Hurricane winds in Florida bring 

 salt marsh mosquitoes and other insects far inland. The beaches of the 

 North Atlantic states may be covered with many thousands of Por- 

 tuguese-men-of-war after a hurricane in the Atlantic has blown them 

 in from the Gulf Stream. There are forty-four species of European 

 birds that have been found in North America, apparently blown across 

 the ocean, since many of them were found after a storm. In most cases, 

 the animals which are dispersed in these ways do not become established 

 in their new environment, but there is no doubt that such accidents do 

 occasionally expand the range of animals. 



Man has been one of the greatest aids to dispersal of animals, since 

 he has developed means of rapid locomotion to all parts of the earth. 

 Early explorers let some of their horses escape and these established 

 herds of wild horses in the southwestern region of the United States. 

 The Norway rat has been carried as a "stowaway" in the holds of ships 

 to every port in the world. The European periwinkle clings to the hulls 

 of ships and drops off in our own coastal waters. The Colorado potato 

 beetle formerly lived on wild plants belonging to the potato family in 

 the foothills of the Rocky Mountains. When man planted a bridge of 

 potato plants all the way across the country, this beetle found a means 

 of spreading all the way to the Atlantic Coast. 



Man has deliberately introduced animals in many parts of the world. 

 In Kentucky there are two wild life reservations into which the Euro- 

 pean red deer has been successfully introduced. In the Bernheim reser- 

 vation near Louisville, the red deer have become so numerous that they 

 damage farm crops and an open season was declared on them. The 

 ubiquitous English sparrow was carefully introduced in several Ameri- 

 can cities in about 1860. It rapidly spread all over the United States 

 and became an obnoxious pest, especially in the cities. With the pass- 

 ing of the horse the English sparrow has gradually become less common 

 in the cities, for they do not thrive on the bolts and gasoline fumes as 

 they did on the droppings from the horse. An introduced species often 

 reproduces much more rapidly than native species because it has out- 

 distanced its natural enemies, including many parasites and diseases. In 

 recent years, whenever an insect pest has become established, the Bureau 



