216 LUTHER BURBANK 



In response to a further query I mentioned 

 for my friend, among plants that often cross in 

 a state of nature, the various species of the genus 

 Rufous, including the blackberry, raspberry; in 

 the tribe of wild roses and crab apples; the 

 California lilac, the various members of the oak 

 tribe, the willow, the strawberry and the huckle- 

 berry; nor are these all the list might be almost 

 indefinitely extended. 



Indeed, it is my firm conviction that crossing 

 between natural species is a phenomenon of 

 almost universal occurrence. 



No other equally plausible explanation has 

 been given of the appearance of what may seem 

 to be spontaneous varieties or forms that 

 furnish the material for the operation of natural 

 selection, and are thus among the bases of 

 organic evolution. 



It is true that such a suggestion as this would 

 have seemed heretical not very long ago; but 

 vast numbers of experiments in the combination 

 of different species, and even representatives of 

 different genera, in my orchards and gardens 

 have afforded a mass of evidence that no one 

 can ignore. So to-day it is coming to be recog- 

 nized quite generally that the combination of 

 wild species is one of nature's conventional 

 methods of producing variability, and, as it were, 



