302 



LIVES OF PLANTS AND ANIMALS 



new countries. One of the best instances of animals having 

 spread when introduced by man is the case of rabbits in Aus- 



Wrighi Pierce 



If this desert region of southern California should be thoroughly irrigated, what kind of 

 plant society might succeed these Joshua trees ? 



tralia. They have now become so numerous that they are a 

 serious pest. The English sparrow in America and the English 

 starling in New Zealand are other examples of introductions of 

 animals which have become pests because of lack of enemies to 

 hold them in check. 



Practical Exercise 13. Make a list of all the new forms of life introduced 

 by man into your own environment. 



Food relations. Animals are confined to certain localities, 

 because a food supply is there, and may migrate to new localities 

 when the food supply gives out. Overpopulation with subsequent' 

 lack of food brought about great migrations of the house rat 

 across Russia in 1727, which was the beginning of the occupation 

 of all Europe by this species of rat. 



Food cycles exist, one animal being dependent upon another! 



