TEE BREED IirO OF PLANTS 419 



sprung. The process which has caused them to be 

 more and more unlike the parents is continued se- 

 lection,— the choosing of the best or most coveted, 

 generation after generation. Sometimes this selec- 

 tion has been intended, the person desiring to im- 

 prove the breed. For the most part, however, it 

 has been unintended and generally unconscious, the 

 person thinking only of the excellence of the present 

 crop; but the result, nevertheless, has been the im- 

 provement of the race. 



533. The kind of lettuce plants which are left 

 is determined by the person who thins the row. 

 One person may like broad -leaved lettuce and an- 

 other may want long-leaved lettuce. As no two 

 persons have the same likes and dislikes, so no two 

 would thin the row in exactly the same way. The 

 result is that, in time, two persons would obtain 

 different types or strains of lettuce, if they contin- 

 ually raised their own seed. That is, the evolution 

 or amelioration of domestic plants is directed by 

 personal ideals. The plant- breeder must first con- 

 ceive of his variety, then produce it. 



534. Neither natural selection nor human selec- 

 tion can operate until variations have occurred. We 

 have said (485a) that naturalists are disputing as to 

 the original causes of variation, but the horticul- 

 turist knows how to start off variations. Any 

 change in the conditions in which the plant grows 



