COMPOSITION AND PUBPOSES OF PLANTS 7 



a great many years individuals are produced quite 

 unlike tlie originals ; yet there are no abrupt changes 

 or jumps from one form to another. Thus, for in- 

 stance, the belief, quite common in some farming 

 communities, that wheat can be converted into 

 " Chess," or " Cheat," is founded on an utter impos- 

 sibility. It will be shown later (§ 197) that plants 

 have two generations, or two forms which alternate 

 with each other, and these two forms are quite 

 prominent in the mosses ; but in such cases the two 

 forms are equally invariable and may not be changed 

 quickly or suddenly. 



The chief point of interest in the idea of a species 

 in connection with the work to be followed in this 

 book is the fact that a species is a group of individu- 

 als that are trying to adapt themselves to a certain 

 mode of life by means of the special organs in the 

 forms of stems, leaves, roots, and flowers in the 

 higher plants, and by organs which do the same 

 work among the lower forms. And furthermore, 

 that no two species try to live in exactly the same 

 manner. 



6. Scope of work described. — The studies de- 

 scribed in the following pages make up an outline 



