' We can only say generally, with Darwin, that selection works by the accumulation of 

 very slight variations, and conclude from this that these ''slight variations" must possess 

 selection-value. To determine accurately the degree of this selection-value in individual 

 cases is, however, as yet impossible.' 



The All- sufficiency of Natural Selection, by Professor Aug. Weismann, 

 Contemporary Review, September 1893, p. 322. 



