MAN AND THE BALANCE OF LIFE 289 



and mistletoe among plants, and many insects and worms, among 

 animals. In every case, the parasite lives on another plant or 

 animal known as its host. In some cases, the host is a temporary 

 one, and in others, it is a permanent one, the parasite remaining 

 with it until death. Many plant and animal diseases are caused by 

 parasites, and man has come into the picture to such an extent that 

 he is now engaged in wiping parasitic disease off the face of the 

 earth. 



Practical Exercise 4. Give as many examples of symbiosis as you can, using 

 references and museum material. Explain how symbiosis in a large sense 

 exists in the world about you. 



Self-Testing Exercise 



Symbiosis is a living (1) for (2) (3). It 



is a (4), sometimes between (5) and (6) as 



in the hydra and algae and sometimes between two plants as in the 



(7) and the (8) ; or between two animals as in the 



(9) (10) and (11) anemone. A (12) 



lives on another (13) organism known as a (14), 



I taking food from it but giving (15) in (16). Many 



parasites do (17), causing (18) or (19) of 



j their hosts (20) is continually combating (21). 



PROBLEM IV. HOW DOES MAN DISTURB THE BALANCE OF 



NATURE? 



Man and the balance of life. Man has come to disturb the 

 balance of life in many ways. He has introduced water to regions 

 1 and made them support plant and animal life some of which 

 are parasites ; he has placed new plants in new localities and had 

 them exterminate the native plants ; and unwittingly he has dis- 

 turbed the balance which nature had established. He has brought 

 into this country, insects which are doing millions of dollars of 

 damage every year, witness the browntail moth and the gypsy 

 moth in New England, and he is now going to the ends of the 

 earth to find the natural enemies of these imported pests ; as is seen 

 in the importation of the ladybird beetle from Australia to feed 

 upon the imported citrus scale insects brought to California from 

 Australia in 1868. He is killing off wolves and coyotes which 



