284 ELEMENTS OF GENERAL SCIENCE 



tops the birds contend for nesting places. The English spar- 

 row has driven many native birds away from their former 

 nesting places. Agriculture has occupied the territory formerly 

 occupied by many kinds of native plants and animals, and 

 has introduced other kinds of plants and animals into the old 

 haunts of native living things. 



312. Not food enough for all. The city man who would 

 make a fortune growing chickens finds that his expense for 

 feed is large, often so large that he cannot afford to raise 

 chickens ; and if the possible rates of increase are made real 

 for even a short period, it is soon evident that neither space 

 nor food is sufficient. In nature the tragedy of the shortage 

 of the food supply is frequently seen. It would be seen con- 

 stantly if it were not for the fact that one kind of living thing 

 may'serve as food for one or many other kinds. When salmon 

 become quite abundant, other kinds of fish, or other animals, 

 may devour them in large numbers. In our fresh-water streams 

 and lakes the black bass may produce hundreds of eggs and 

 guard them until the young are hatched and are a few days 

 old. After the mother fish has left her young, she as well as 

 other fish may catch and devour any of them that are not so 

 fortunate as to escape when pursued. In this way not so 

 much food is needed for young salmon and young bass, be- 

 cause some of these fish have become food for others. The 

 few of any kind of living things that reach maturity are to 

 be looked upon as the ones that have " run the gauntlet " and 

 have been able to secure enough food to nourish them and 

 enable them to grow, while at the same time avoiding the 

 catastrophe of serving as food for some other hungry animal. 



313. Climatic dangers. An unexpected and severe storm 

 in early summer leaves in its path a mass of wreckage, as 

 broken trees, flattened grain, upturned nests of birds, drowned 

 nestlings, and insects killed by wind or water. The small 

 and inconspicuous forms of life that are killed are more nu- 

 merous than the larger ones. At other times sudden changes 



