278 AN INTRODUCTION TO ZOOLOGY 



mately permanent area is occupied. The region from which 

 migration took place is termed the center of dispersal. The 

 following criteria have been given to determine the center of dis- 

 persal of a species: the abundance and size of individuals, the 

 location of closely related forms, and the migration routes now 

 selected by the species (225). 



THE LAW OF BARRIERS AND HIGHWAYS. Animals are confined 

 to certain habitats by barriers. They are prevented from gaining 

 access to a new region by the change in the media, by dearth of 

 food, and the interference of other animals. Common barriers 

 are mountains, bodies of water, open country for forest animals, 

 and forests for prairie-inhabiting species. For example, the cray- 

 fish, Cambarus, migrates up and down streams, but cannot travel 

 overland to neighboring streams, the honeybee cannot fly across 

 the ocean, nor can Hydra enter the sea. 



The reverse of a barrier is a highway. Apparently there are 

 routes of migration which are especially favored. This may be 

 illustrated by the flight of birds southward in the autumn, and 

 northward again the following spring. Many birds migrate 

 up and down the Mississippi Valley, and along the Atlantic 

 coastal plain. 



COSMOPOLITAN GROUPS OF ANIMALS. Some species of animals 

 have wide ranges, e.g. some are found inhabiting practically 

 every large land area on the earth's surface. Sixteen families 

 of birds, including doves, owls, and ducks, and one family of 

 mammals, the bats, are cosmopolitan groups. Doubtless the 

 wings of the birds have facilitated their dispersal, since they give 

 them remarkable powers of locomotion. 



RESTRICTED GROUPS OF ANIMALS. In a number of cases certain 

 species are restricted to very limited areas. The mountain goat 

 is found only in the higher Rocky and Cascade mountains to 

 Alaska. Islands are famous for the presence of restricted species. 

 Darwin's descriptions of the animals he found in the Galapagos 

 Islands read like fairy tales (235). 



DISCONTINUOUS DISTRIBUTION. Whenever a species occurs in 



