THE FRAGRANT CALLA 23 



Professor de Vries noted a hitherto unde- 

 scribed variety of this plant in a field near 

 Amsterdam. He took specimens of the plant 

 to his experimental gardens and carefully 

 watched the development of successive genera- 

 tions of seedlings. 



To his astonishment he produced in the course 

 of a few generations more than a dozen divergent 

 types of evening primrose, all descended from the 

 original plant, each of which bred true to the new 

 form suddenly assumed. Professor de Vries 

 spoke of these sudden and wide variations from 

 type on the part of his evening primrose as con- 

 stituting "mutations." 



He conceived the idea that similar mutations 

 or sudden wide variations had probably consti- 

 tuted the material on which natural selection had 

 worked in the past. Such mutations being ob- 

 served to occur in the case of the evening prim- 

 rose, it is not unnatural to argue that similar 

 mutations might occur in the case of other organ- 

 isms; and it requires no argument to show that 

 such w^ide variations offer better material for the 

 operation of the laws of natural selection than 

 could be offered by the minute and inconspicuous 

 variations that had hitherto been supposed to 

 mostly constitute the basis of evolutionary 

 changes. 



