ANIMAL DISTRIBUTION 477 



6. The Lower Sonoran zone is less homogeneous in its flora and 

 fauna than are the other zones. In the Southwest it includes most 

 of Texas, Oklahoma, southern New Mexico, southern Arizona, and 

 southern California. The eastern third of Texas is typical of the 

 lower Sonoran of the southeastern states which are characterized 

 mainly by long-leafed pines, magnolia, and live oaks. Some animals 

 of the eastern portion are mockingbirds, painted buntings, and wood 

 rats. The western lower Sonoran is inhabited by typical desert and 

 semidesert flora and fauna. 



7. The Tropical zone is of very little significance in the south- 

 western United States, but many tropical plants, such as the Texas 

 ])alms and bananas, grow in a narrow strip of the lower Rio Grande 

 Valley. 



Migration of Animals 



The migrations of many animals are not well understood but in 

 most cases they involve breeding habits, food, or shelter. Most of 

 these migrations are seasonal, but many permanent changes of loca- 

 tion have been made by groups of animals as a result of permanent 

 changes of climate on the earth. 



The seasonal migrations of the North American caribou and the 

 bison are among the most noted examples of migrations of animals 

 which move about in search of food. The caribou migrate south- 

 ward in Canada during the winter and follow the melting snow 

 northward in the summer. 



The remarkable migrations of the eels of the Mediterranean area 

 and of the eastern coast of the United States are among the best 

 known examples of breeding migrations. The eels from the rivers 

 of these two regions migrate into the Caribbean Sea and spawn. 

 Although the two spawning territories overlap, the young eels of 

 each species journey hundreds of miles back to the rivers inhabited 

 by their ancestors. A great many species of birds migrate from the 

 tropics and semitropics and breed in northern regions. The scarlet 

 tanager spends the winter in northwestern South America and mi- 

 grates across the Gulf of Mexico into northeastern United States and 

 southern Canada and breeds during the summer months. The upland 

 plover is familiar in the Southwest, since it migrates through this 

 region twice each year on its journeys between Argentina and British 

 Columbia. 



