178 LUTHER BURBANK 



The following season a few of the plants bore 

 one or two fruits having two or three drupelets 

 each, like mere fragments of a normal raspberry. 

 But not a seed was found. The plants were as 

 sterile as mules. So here the experiment ended, 

 and the hybrid strawberry-raspberries followed 

 the hybrid dewberries to the brush heap. 



WHY THE EXPERIMENTS FAILED 



If we now consider the results of these various 

 experiments, it will be clear that they have 

 certain elements in common. In all cases the 

 hybridizing was effected between species that 

 are botanically related. But in no case was the 

 relationship between the mated forms very close. 

 And this fact is of course of salient importance 

 in enabling us to comprehend the results. 



It is almost axiomatic to say that the hybrid- 

 izing of plants generally becomes increasingly 

 difficult in proportion as the attempt is made to 

 cross more and more distantly related species. 

 Even within the same genus it is very often 

 impossible to produce a hybrid that is not 

 sterile. 



I might cite in further illustration of these 

 difficulties the experiments through which I have 

 hybridized the apple with the pear, and with the 

 quince; the cherry with the plum; and the peach 



