wastes threaten to obstruct life as they accumulate in the earth or in the 



water? . u u 



When we recover from some diseases, we become immune to them, but 

 other diseases one may have again. Plants and animals recover from in- 

 juries. What changes take place in the body when it is sick? How does 

 vaccination work? Why can we immunize against certain diseases, but not 



against others? 



We may understand some of the similarities among plants and animals 

 which we include under the broad idea of "life". But many questions are 

 raised by the variety of living forms, and especially by the complexity of 

 our own bodies and of other familiar species. How do such totally different 

 things as a man and a clam, a bird and a mushroom, all manage to carry 

 on essentially the same processes ? 



162 



