656 General and Applied Biology 



Animal migrations may take place ( 1 ) in order to meet the emergency 

 of overpopulation in any particular area, (2) in order to ensure a better 

 quantity and quality of food for themselves or their offspring, (3) in 

 order to find more suitable environment in which to develop their off- 

 spring, or (4) in order to escape certain types of climate which are not 

 highly satisfactory for their well-being. 



The salmon (male and female adults) migrate from the ocean up the 

 Yukon and Columbia rivers, possibly for distances up to two thousand 

 miles. In the fresh water of these rivers, the adults spawn and then 

 die. The young salmon migrate from these fresh waters to the salt 

 water of the ocean where they mature and spend several years. Even- 

 tually, som.e of these adults again migrate up the rivers to spawn. 



In the case of eels, the young are born in the salt water of the ocean 

 and they migrate up fresh-water rivers, sometimes journeying over three 

 thousand miles. After a few years, they return to salt water and breed, 

 eventually dying, because adult eels do not return to fresh waters. 



In the case of such mammals as fur-bearing seals, great herds of adult 

 males and females migrate each spring to islands in the Bering Sea 

 where they remain from about May 1 to September 1. During this 

 time their young are produced. Great herds of the young seals migrate 

 from these islands to other regions of the North, while other herds 

 migrate long distances to cold regions of the South. Since seals are 

 valuable because of their fur, they are protected by laws in many coun- 

 tries. The "bachelors" (three-year-olds) are caught for their fur. 



We are familiar with the seasonal migrations of certain species of 

 birds. A unique migration is illustrated by the Arctic tern which breeds 

 in northern North America and migrates across the Atlantic Ocean to 

 Europe, southward past Africa to the Antarctic, returning by a circui- 

 tous route to cross the Atlantic again to the northern habitat. The dis- 

 tance between their summer and winter habitats is over 10,000 miles, 

 thus making a journey of over 20,000 miles each year. 



QUESTIONS AND TOPICS 



1. Attempt to give from your own observations as many illustrations as possible 

 of (1) unity between various species of living organisms, (2) plant and ani- 

 mal antagonisms, resulting in a struggle for existence and a survival of the 

 fittest, (3) plant and animal successions, and (4) Web of Life or Balance 

 in Nature. 



2. From your studies would you say that a living organism can live a life of com- 

 plete isolation? Give reasons why you say so. 



