606 Mutations 



avoidable cross-fertilizations with the parent- 

 form, or with neighboring varieties, and conse- 

 quent impurity of the new strain. This impurity 

 we have called vicinism, and in a previous lec- 

 ture have shown its effects upon the horticul- 

 tural races on one hand, and on the other, on the 

 scientific value that can be ascribed to the ex- 

 perience of the breeder. We have established 

 the general rule that stability is seldom met 

 with, but that the observed instability is always 

 open to the objection of being the result of vicin- 

 ism. Often this last agency is its sole cause; 

 or it may be complicated with other factors 

 >\^ithout our being able to discern them. 



Though our assertion that the practice of the 

 horticulturist in producing new varieties is lim- 

 ited to isolation, whenever chance affords them, 

 is theoretically valid, it is not always so. We 

 may discern between the two chief groups of 

 varieties. The retrograde varieties are con- 

 stant, the indi\dduals not differing more from 

 one another than those of any ordinary species. 

 The highly variable varieties play an important 

 part in horticulture. Double flowers, striped 

 flowers, variegated leaves and some others yield 

 the most striking instances. Such forms have 

 been included in previous lectures among the 

 ever-sporting varieties, because their peculiar 

 characters oscillate between two extremes, viz.: 



