Artificial and Natural Selection 815 



ternal causes. First come those that have 

 influenced the young germs of the plant 

 during its most sensitive period, when still 

 an embryo within the ripening seed. They 

 give a new limitation to the average con- 

 dition, which once and forever becomes fixed 

 for this special individual. In the second 

 place the young seedling is affected dur- 

 ing the development of its crown of leaves, 

 and of its roots, by numerous factors, 

 which cannot change this average, but may in- 

 duce deviations from it, increasing or decreas- 

 ing the amount of sugar, which will eventually 

 be laid down in the root. The best young beet 

 may be injured in many ways during periods of 

 its lifetime, and produce less sugar than could 

 reasonably be expected from it. It may be sur- 

 passed by beets of inferior constitution, but 

 growing under more favorable circumstances. 



Considered from this point of view the result 

 of the polarization-test is not a single value, but 

 consists of at least two different factors. It 

 may be equal to the algebraic sum of these, or 

 to their difference, according to whether the 

 external conditions on the field were locally and 

 individually favorable or unfavorable. A large 

 amount of sugar may be due to high individual 

 value, with slight subsequent deviation from it, 



