Introductory. 21 



is a hybrid between the horse and the ass, is a familiar 

 example of a sterile hybrid. Sterile hybrids are not 

 uncoEQmon in plants. A hybrid that is capable of re- 

 production is called a ferti^.e hybrid. 



Hybrids and crosses may resemble both parents about 

 equally or they may resemble one parent more than the 

 other. They sometimes differ materially from either 

 parent. The offspring of crosses and fertile hybrids are 

 generally variable in proportion as their parents were 

 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, 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 (par'-a- 

 sites) or parasitic. The plant or animal from which a 

 parasite derives its nourishment is called a host. Para- 

 sites 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 (270). Some, how- 

 ever, as the micro-organisms of the roots of clover and 

 other leguminous plants, are beneficial (112). 



