I40 WHAT EVOLUTION IS 



merely wabbles back and forth on one 

 side, so evolution makes no progress 

 through variations. Only when mu- 

 tations occur, when the cube rolls over 

 on to a new face, is evolution taking 

 place. Darwin recognized mutations 

 as conditions in nature and used for 

 them the breeder's common name of 

 sport. He was doubtful, however, 

 whether they had any significance in 

 evolution. To de Vries they are 

 the only real factor in evolutionary 

 progress. 



No one can have bred plants or an- 

 imals for any length of time without 

 having noticed the frequency with 

 which mutations occur. Morgan, in 

 his exhaustive study of inheritance in 

 the fruit fly, has recorded many scores 

 of mutations, and there is no reason 

 to suppose that they do not occur as 

 frequently in open nature as in the 

 laboratory. 



