Geographic Distribution of Animals and Plants 595 



hibernate. (2) Migration to warmer regions. Certain species of birds 

 migrate from the arctic regions to the tropics. The golden plover (bird) 

 illustrates such a migration. (3) Continued activity in their cold habi- 

 tats. If animals remain in the cold habitats in winter, they frequently in- 

 crease their fat layers as well as their coats of hair to help withstand the 

 attacks of the cold. They may also change their diets to include foods 

 which will produce greater amounts of heat. (4) Animals may die be- 

 cause of the extreme temperature. Many species naturally expect this 

 and have carefully taken the precaution of depositing their eggs so that 

 the developing offspring may take their places when more favorable 

 temperatures return. 



The principle of harriers and highways is one of the most important 

 in animal dispersal. What may prove to be a barrier for one species may 

 serve as a favorable highway for another. Some of the more common 

 barriers are as follows : ( 1 ) There may be a lack of the proper quality 

 and quantity of foods along the route of migration. (2) Water may be 

 a barrier for terrestrial forms but may successfully be used by aquatic 

 types. The size, depth, temperature, acidity, and pressure in bodies of 

 water all may be important factors in dispersal of even aquatic animals. 

 The aridity and humidity of terrestrial environments may act as high- 

 ways or barriers, depending on the type of animal in question. Salt water 

 may be a barrier for fresh-water forms, and fresh water likewise may be 

 one for marine forms. Amphibia are rarely found in salt water. (3) 

 The various kinds of land may serve as barriers or highways, depending on 

 the type of land as well as the species of animal. For instance, tracts of 

 land may act as barriers for aquatic forms; forests may act as barriers 

 for open-country or prairie-inhabiting species; deserts and open country 

 may act as such for forest-inhabiting types. Mountains with their char- 

 acteristic temperatures, moisture, oxygen supply, and food supply may 

 act as barriers to many types in their attempt to migrate over them. (4) 

 The interference by other animals either through bodily struggles or com- 

 petition for foods may influence the dispersal of certain kinds of organ- 

 isms. (5) Winds, especially strong winds, tend to carry species of flying 

 habits in the direction of the wind blow. This may result in their migra- 

 tion into more favorable or less favorable habitats as the case may be. 

 (6) Temperature may prevent the dispersal of many animals, either by 

 its direct effect on the migrant or by its effect on the vegetation upon 

 which the migrant must depend for food and shelter. (7) The lack of 

 adaptive ability of the animal may result in its inability to adapt itself 

 quickly enough to new and changed environmental conditions. The 



