in the Bromes and their Brown Rust. 315 
stitution of plants can be so modified by breeding and selec- 
tion, that disease-resisting varieties should be no more difficult 
to- evolve than varieties which refuse to cross with the pollen 
of certain other forms. Only, we must not forget that the 
Fungus is also capable of being bred and selected, and pre- 
potent varieties of spores are just as much realities as pre- 
potent pollen. 
Y 2 
