20 Principles of Plant Culture. 
Hybrids and crosses may resemble both parents about 
equally or they may resemble either parent more than the 
other. They sometimes differ materially from either parent. 
The offspring of crosses and fertile hybrids is generally 
variable in proportion as their parents were different from 
each other, and this variability may continue througa several 
generations. 
24. The Theory of Evolution, now generally ac- 
cepted by naturalists, assumes that the higher plants and 
animals have been gradually evolved from lower forms, 
through the principle that those individuals possessing pecu- 
liarities best fitting them to endure the adverse conditions 
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 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 fruitful sources of disease. 
Some, however, as the bacteroids of the roots of clover 
(113) and other leguminous plants, are beneficial. 
