Variability as a Nutritional Phenomenon. 521 



same or in opposite directions. We can, so to speak, 

 add their effects or subtract the one from the other. If 

 this experiment succeeds it proves that the two plienom- 

 ena are of the same order, and suggests a method of 

 determining their relative importance. 



I shall therefore describe in this chapter a series of 

 experiments carried out on this principle. They deal 

 with measurable or countable characters which are ca- 

 pable of experimental as well as of statistical treatment. 

 I chose for this purpose the length of the fruits of the 

 ordinary Oenothera Lamarckiana (Figs. 114 and 115, 

 pp. 529 and 530), and also the material employed by 

 LuDWiG which is afforded by the ray florets of Compo- 

 sites and the rays in the umbels of Umbelliferae (Figs. 

 117-119, pp. 561-565). In the case of the fruits I tried 

 both the addition and subtraction of the factors ; but in 

 that of the ray-florets and the rays of the umbels only 

 the simultaneous operation in opposite directions of heavy 

 manuring and negative selection. The result of the ex- 

 periment was that sometimes the one factor and, at other 

 times, the other predominated. 



The inquiry into the effect of nutrition (manuring, 

 plenty of room, light and water, etc.) has led to the dis- 

 covery of two principles (foreshadowed in the discus- 

 sions in the first section p. 137) which I think ought 

 to be enunciated here in the interest of a clear under- 

 standing of the whole range of phenomena. 



These two principles are the following: 



1. The younger a plant is the greater is the influence 

 of external conditions on its variability, that is, on the 

 place which its various characters will occupy in the 

 curves of variability of the wdiole culture or race. 



2. In connection with this principle the nutrition (^f the 



