SUMMARY OF THE WORK 339 



special description; yet some other hybrids are 

 worthy of much study/' A list of other species 

 that had been mutually hybridized begins with 

 the peach and almond, and names more than 

 twenty crosses between the various types of 

 orchard fruits apricot, plum, quince, and apple, 

 as well as peach in various combinations. 



Without detailing further examples, it may 

 be said that this body of evidence was over- 

 whelming. It could be supplemented indefi- 

 nitely, of course, by examples from other plants 

 in my experiment gardens. But, without further 

 elaboration, the examples cited in these first two 

 catalogues sufficiently establish the fertility of 

 hybrids of many species of widely different 

 families. 



Thenceforth there could never be any doubt 

 in the minds of practical plant developers that 

 true species, within certain limits of affinity, may 

 be interbred and produce fertile offspring. 



On the other hand, the examples of the straw- 

 berry-raspberry, and the petunia-tobacco might 

 be cited in proof that species too widely removed 

 from each other produce sterile hybrids. 



Thus the experiments as a whole show on one 

 hand the method through which material is sup- 

 plied for the operation of natural selection; 

 while, on the other hand, they show how barriers 



