INTRODUCTION 13 



perpetuated their kind, while others have perished. This 

 has been termed the survival of the fittest in the struggle 

 for existence. 



25. Parasites. — Both plants and animals are subject 

 to being preyed upon by other, usually smaller, plants 

 and animals, that live upon or within their bodies, con- 

 suming the tissues of their bodies or their reserve food. 

 Plants or animals that derive their nourishment from other 

 plants or animals are called parasites or parasitic. The 

 plant or animal from which a parasite derives its nourish- 

 ment is called a host. Parasites are often microscopic in 

 size. They are generally more or less injurious to their 

 host, and form one of the most common sources of disease 

 (270). Some, however, as the micro-organisms of the 

 roots of clover and other leguminous plants, are beneficial 

 (112). 



