164 Controversial Questions. 



periment should result in the origin of apetalous, fruit- 

 less, glabrous, spineless, white-flowered, unisexual or 

 sterile plants and so forth. Whereas of course on the 

 mutation theory this would not happen; provided that 

 crossing was rigidly excluded from the experiment. 



14. What we must aim at is a complete control of 

 variation. We must become so thoroughly acquainted 

 with the underlying factors that we can predict the re- 

 sults of our experiments. 



