Introductory. 21 



different from each other, and this variability may con- 

 tinue through several generations. 



24. The Theory of Evolution, now generally accepted 

 by naturalists, assumes that the higher plants and ani- 

 mals have been gradually evolved from lower forms, 

 through the principle that those individuals possessing 

 peculiarities best fitting them to endure the adverse con- 

 ditions of environment have survived and perpetuated 

 their kind, while others have perished. 



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, consuming 

 the tissues of their bodies or their reserve food. Plants 

 or animals that derive their nourishment from other 

 plants or animals are called parasites ( par' -a- sites) or 

 parasitic. The plant or animal from which a parasite 

 derives its nourishment 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 fruitful 

 sources of disease (271). Some, however, as the micro- 

 organisms of the roots of clover and other leguminous 

 plants, are beneficial (113). 



