THE STRUGGLE FOE EXISTENCE 285 



in temperature prove extremely destructive. When migratory 

 birds are caught in snowstorms, many die. The American 

 golden plover flies southward in the autumn migration over 

 the Atlantic Ocean from Nova Scotia to the Lesser Antilles or 

 even to South America. Often these birds are caught in heavy 

 storms, many doubtless perishing at sea and many being driven 

 ashore in an injured condition. Rainstorms accompanied by 

 hail cause the death of young birds, insects, and some larger 

 animals, and of large, numbers of plants. Excessive heat, 

 drought, and other climatic causes are important factors in 

 the elimination of living things. 



314. Destruction by disease. A visit to a corn field or wheat 

 field will usually enable one to discover a parasitic plant known 

 as smut growing upon the corn or wheat. This smut is secur- 

 ing its nourishment from the host plant. After a time it pro- 

 duces its reproductive spores, by means of which new smut 

 parasites may get started on new host plants. But the host 

 plant suffers and may be killed by the smut. Its supply of 

 nourishment may be taken by the smut or the smut may pro- 

 duce poisonous substances that severely affect the host. The 

 smut is merely growing in its own way, but in so doing it may 

 cause the death of other plants. 



Bacteria, fungi, and some animals cause the disease and 

 death of other living things. So universal is disease that we 

 scarcely know what is meant by " natural death." In nature, 

 when animals begin to get old and their bodies operate less 

 effectively than when they were younger, such animals are 

 more readily captured by food-seeking animals or are more 

 susceptible to disease. In nature, so called natural death of 

 animals is rarely if ever seen. 



315. The balance of life. The grass of the field is eaten by 

 herbivorous animals, and herbivorous animals are eaten by car- 

 nivorous animals. It would seem, therefore, if we are to look 

 no farther into the matter, that the number of carnivorous ani- 

 mals is limited by the available grass for grass-eating animals. 



