VIEWS OF DE VRIES ON SELECTION 



73 



thrips, the plant being scarcely capable of self-pollination. 

 This explains why constant selection is required to maintain 

 a high standard. Hybridization constantly occurs and for 

 this reason fully stable types cannot be obtained. 



De Vries is also led to adverse conclusions concerning se- 

 lection as an agency in producing racial changes by experi- 

 ments of his own, one of the most extensive of which was an 



7 8 9 10 II 7^ 13 



Pig. 14. Variation of the buttercup (Ranunculus bulbosus) in number 

 of petals preceding and following selection. H 1387, variation curve of 

 unselected race. E 1891 and 1892, curves for successive generations of the 

 selected race. A 1891, curve for parent plants of the 1892 generation. 

 (After De Vries.) 



attempt to increase by selection the number of petals in the 

 common meadow buttercup (Ranunculus bulbosus). This 

 regularly has five-petaled flowers, but an occasional flower 

 contains one or more extra petals. See Fig. 14. When this 

 plant was cultivated in his garden, De Vries found the aver- 

 age number of petals to be 5,Q. After five successive selec- 

 tions the average was raised to 8.6, the upper limit of 

 variation from eight to thirty-one, and the mode (or com- 

 monest condition) from five to nine. De Vries concludes that 

 the change thus produced could be maintained only by con- 

 tinued selection, and that further progress could probably 

 not be made. This conclusion seems to me unwarranted, but 

 I state it as illustrative of the general view of De Vries, who 

 maintains that when a permanent racial change occurs it is 

 due to something different from fluctuating variability, viz., 

 to a discontinuous variation or sport, a process which De Vries 



