718 TEXTBOOK OF ZOOLOGY 



Among land animals tlie birds and flying insects appear to be 

 least restricted in their ranges. Even these, however, are often 

 limited by mountains, deserts, and large rivers which act as the 

 most effective barriers to the wide distribution of land animals. 

 Many of the birds and mammals found on the south rim of the 

 Grand Canyon of the Colorado River are not able to reach the north 

 rim of the Canyon. For example, the Abert squirrel is found in 

 New Mexico and Arizona, south of the Canyon, while the Kaibab 

 squirrel occurs only north of the Canyon. Small islands, which are 

 located great distances from the mainland, are often inhabited by 

 small mammals and reptiles similar to those found on the mainland. 

 Such forms have probably reached the islands by way of floating 

 rafts of vegetation, and hollow trees which are known to be carried 

 several thousands of miles by oceanic currents. 



Wind currents may carry birds across the Atlantic Ocean. It is 

 not uncommon for American birds to land on the coast of England 

 after severe storms. These same currents also carry spores of Pro- 

 tozoa, small insects, and the eggs of many invertebrates. 



Effects of Man Upon Distribution 



The advent of man upon the earth ai^d the development of his 

 more efficient means of travel have greatly enhanced the wide dis- 

 tribution of many species of animals. Marine invertebrates attach 

 themselves to the bottoms of ships (ship fouling) and are carried 

 to practically all parts of the earth. Rodents and insects are acci- 

 dentally distributed from one country to another by ships. During 

 the past man has purposely transported animals from one country 

 to another. In some instances the animals multiply more rapidly 

 in the new environment than they did in the original habitat. 

 English sparrows were introduced into North America in 1859, and 

 since that time they have become so numerous as to be a great pest 

 in this country. The starling, which was introduced at about the 

 same time is rapidly increasing in numbers and in recent years has 

 been reported as far south and west as Central Texas. By cultivat- 

 ing extensive prairie lands, man has created insurmountable bar- 

 riers to grazing animals which once roamed these areas. Deforesta- 

 tion of wooded regions has destroyed the shelter necessary for such 

 animals as deer, foxes, wolves, bears, and many others. 



