240 VARIATION AND ITS RESULTS 



This persistence of the best-fitted and loss of the least- 

 fitted is the process designated by Darwin's phrase "natural 

 selection" and by Spencer's "survival of the fittest." Natural 

 selection is also known as Darwinism. 



407. By a similar process, the cultivator modifies his 

 plants. He chooses the variations that please him, and from 

 their offspring constantly selects for seed-bearing those 

 that he considers to be the best. In time he has a new 

 variety. Plant-breeding consists chiefly of two factors 

 or processes; producing a variation in the desired direction; 

 selecting, until the desired variety is secured. 



408. Evolution. — Variation, heredity, natural selection 

 and other agencies bring about a gradual change in the 

 plant kingdom; this change is evolution. The hypothesis 

 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 vege- 

 table kingdom, with numberless 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 thought, however, that every 

 kind of plant now living has been derived from some other 

 kind. Evolution is still in progress. Variation and heredity 

 are two of the most important facts in organic nature. 



Review. — What is a variation? A variety? Agricultural vari- 

 ety? How many variations arise? Explain each of the three categories. 

 What are some of the apparent causes of variation? What is heredity? 

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



