232 VAKIATION AND ITS RESULTS 



380. EVOLUTION. Variation, heredity, natural selec- 

 tion, and other agencies bring about a gradual change in 

 the plant 'kingdom; this change is evolution. The hy- 

 pothesis that one form may give rise to another is now 

 universally accepted amongst investigators; but whether 

 the vegetable kingdom has all arisen from one starting 

 point is unknown. Only a few of the general lines of 

 the unfolding of the vegetable kingdom, with numberless 



384. The progeny of little and big plants. 



details here and there, have been worked out. Not every 

 form or kind of plant can be expected ever to vary into 

 another kind. Some kinds have nearly run their course 

 and are undergoing the age-long process of extinction. 

 It is believed, however, that every kind of plant now liv- 

 ing has been derived from some other kind. Evolution 

 is still in progress. Variation and heredity are the most 

 important facts in organic nature. 



REVIEW. What is a variation? A variety? Agricultural vari- 

 ety? How may variations arise? Explain each of the three cate- 

 gories. "What are some of the causes of variation? What is heredity? 

 Selection? What are essentials in plant-breeding? What is evolution? 



