1 86 PLANT-BREEDING 



ciple of selection resting simply on the ground of ordinary 

 or fluctuating variability. Its possibilities, however, .are 

 limited, and since the breeder is always on the lookout to 

 widen the range of his material, it is but seldom thaTlie~Is~" 

 content with this process of pure selection. In almost all 

 cases, he will try to increase the elements of his choice, and in 

 order to do this, he takes to the process of hybridization. 



Hybridization, however, is not always a means of increas- 

 ing variability. In some instances hybrids are as constant 

 and uniform as their parent species even when propagated 

 from seed. A certain number of wild types formerly consid- 

 ered and described as pure species have since been proved to 

 be of hybrid origin, the types having been artificially produced 

 by repeating the assumed original cross. Kerner von Mari- 

 laun has described quite a number of such instances, and 

 Janczewsky and others have produced hybrids which cannot 

 be distinguished from real species otherwise than by the his- 

 torical record of their birth. In such cases the breeder has 

 to be content if his hybrid proves to excel its parent species 

 in some industrial quality, but without renewed crosses his 

 work is limited to its production and propagation. 



Some such cases have occurred in Burbank's work also. 

 As an instance I will adduce the case of a bramble. A cross 

 has been made between the wild Californian dewberry 

 (Rubus Californicus) and the Siberian raspberry, or Rubus 

 Sibiricus. The first is a small species of bramble and the 

 second closely allied to the common raspberry. But in both 

 the fruits are small and worthless. The hybrid, however, 

 surpasses both its parents in this important respect, having - 

 large black berries, which are produced abundantly and 

 ripen several weeks earlier than both parents. As the 

 first notable Improvement among brambles, it received the 

 name of primus-berry. Under this it has found its way into 

 the market. Other constant hybrids have since been pro- 



