How Domestic Varieties Originate 219 



that he handles are very closely alike, so long there will 

 be little hope of obtaining new varieties. He must, 

 therefore, cause his plants to vary. In plants that 

 are comparatively unvariable, it is frequently impossible 

 to produce variations in the desired direction at once, but 

 it is more important to "break" the type, that is, to 



FIG. 55. Field of wilt-resistant watermelons, growing free from disease 

 on infected land. (From Yeai'book.) 



make it depart markedly from its normal behavior in 

 any or many directions. If the type once begins to vary, 

 to break up into different forms, the operator may expect 

 that it will soon become plastic enough to allow of modi- 

 fication in the ways he desires. But whilst it is impor- 

 tant or even necessary to break a well-marked type into 

 many forms, it would no doubt be unwise to encourage this 



