The Operation of Nutrition and Selection. 543 



Let us now turn to the amplitude of variation (Q). 

 This was, as the above table shows, the same in both 

 cultures in 1892 and double as large as in the horn-flour 

 culture of 1891. In this latter it was, in fact, smaller 

 than in the ordinary cultures (p. 539). The amplitude 

 of fluctuation is well known to be brought about by the 

 multiformity of internal and external conditions which 

 afifect development, and it is obvious that heavy manuring 

 will tend to level these differences down. We shall refer 

 to a parallel result when we come to describe the contin- 

 uation of the short-fruited race. 



Finally, it will be seen that Q\ and Qo have remained 

 equal to one another and therefore that the curve, in spite 

 of the shifting of its apex, has remained symmetrical. 



Regarded from the methodological point of view, 

 this experiment contains a warning to keep the external 

 conditions, particularly those of manuring, as constant 

 as possible; and not to be too ready to interpret any 

 changes that may occur as the effects of selection. 



As already stated I have cultivated the two races for 

 two more years under exactly the same treatment ( 1S^\>. 

 1894). The long-fruited race underwent no further im- 

 provement; in fact they deteriorated a little. This result 

 is an illustration of Hallett's principle (see ]). ll(V), 

 which enabled him to evolve his new varieties of cereals. 

 During the first year of his experiments notable progress 

 was made; but after that, further selection either made 

 ver}^ slight further progress, or only served to fix what 

 had already been attained. 



In 1891-1892 I left pollination to the agency of in- 

 sects, but in the summer of 1893 I pollinated the fl<nvers 

 artificially after having excluded the visits of insects by 

 bags. But this has not led, so far as I have observed. 



