266 PRINCIPLES OF PLANT CULTURE 



food and moisture. They are, therefore, able to reach 

 higher stages of development than is possible in nature, 

 where plants are constantly restricted by environment. 



The principle of selection has doubtless been more or 

 less operative since the beginning of culture (19). All 

 of our cultivated plants must have existed originally 

 in the wild state. The most satisfactory plants of any 

 desirable species have been most carefully guarded, 

 and when the art of propagation became known, these 

 plants were most multiplied. In each successive gen- 

 eration, the most desirable individual plants of each 

 species were protected and multiplied, or at least were 

 permitted to perpetuate themselves. Since the offspring 

 tends to resemble the parent (18), the persistent propa- 

 gation from the best has resulted in more or less marked 

 improvement. Chance crossings have aided the process 

 (445). These facts furnish hints for the further im- 

 provement of plants. 



434. Variability. — The variability of plants renders 

 their improvement possible. In a species of which the 

 individual plants are all practically alike, as in many wild 

 plants, we can do little in the way of plant-breeding, 

 except to give treatment that promotes variability. In 

 a species in which the individuals manifest different 

 qualities, however, we may hope to secure improvement 

 by using the more desirable plants as parents from which 

 to secure still further variability. 



435. Reversion. — Variations are not always permanent. 

 If we find a chance seedling of the wild blackberry, for ex- 

 ample, that has remarkably fine fruit, the plants grown 

 from seeds of this fruit are not always equal in quality 

 to the parent. The tendency, in such cases, is for the 

 seedling plants to revert or go back to the ordinary type 



