20 THE NATUEE AND WOEK OF PLANTS 



are grown or cultivated in great numbers. It is to 

 be borne in mind that the species was not devel- 

 oped for the purpose of being useful to anything 

 except itself. Man has learned to take advantage 

 of the capacity of the plant for forming certain 

 substances, and cultivates these species in order to 

 get their products. This intelligent action is shared 

 by certain lower animals which cultivate crops of 

 moulds and use them for food. 



