F. KiDD AND C. West 115 



mental experience the conclusion that exteinal influences exert their 

 effect on the development of organs during their youth, that is to say, 

 during the so-called susceptible period. He finds that in many cases 

 the most susceptible stage seems to be that of the young embryo in the 

 ripening seed. It is in this stage that external conditions exercise a 

 powerful pre-determining effect upon the whole subsequent course of 

 the development of seed plants. He was led to enunciate two principles 

 which we may quote fully in view of their importance. 



1. ''The younger a jdant 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 whole culture or race.' 



2. "/« connection with this principle the nutrition of the seed on the 

 mother plant has, in many cases at any rate, a greater effect upon variability 

 than nutrition during germination and vegetative life itself.'' 



In an experiment with Oenothera Lamarckiana, after showing that 

 the lengths of the fruits fall on a Quetelet-Galton frequeiicy curve of 

 variation and that a strong correlation exists between thickness of stem 

 (taken as a criterioji of vigorous development) and the mean fruit-length 

 of the plant, De Vries shows that the effect of manuring the plant in 

 increasing the mean fruit-length is not confined to one generation. He 

 gave his plants similar manurial treatment for three generations, and 

 his results are brought together in the following table : 

 Mean value for length of fruit: 



After 1st year without selection = 25*2 mm. 



After 2nd year combined with positive selection = 334 mm. 



After 2nd year combined with negative selection = 29-9 mm. 



After 3 generations without selection == 38-5 mm. 



It is thus seen that the effect of high nutrition continued through a 

 number of generations without selection proved itself superior to the 

 most stringent selection, and even when combined with negative selec- 

 tion it improved the mean. The effect of manuring the parent plant 

 was not confined to one generation. Under favourable nutritional con- 

 ditions the parent plants produced embryos which were well-nourished 

 during the "susceptible" period of seed-formation and the physiological 

 condition of the seed thus produced pre-determined to a considerable 

 degree the development of the plant produced after germination. It 

 will be seen that this result is essentially the same as that obtained by 

 Tracy. It shows, further, that the operation of high nutrition of the 

 seeds is capable of accumulation through two or more generations, and 

 the same is true of low or defective nutrition. 



8—2 



