IV] HEREDITY 45 



parental mean. Selection, therefore, within the pure 

 line has no effect in altering the mean weight of the 

 seeds, for the differences in seed weight within the 

 line are not inherited. The probable cause of this 

 is that the differences between the seeds on a self- 

 fertilised plant are due to the action of external 

 circumstances ; the position of the beans in the 

 pod or the position of the pods on the plant cause 

 differences in the nutrition which allow some beans 

 to grow larger than others. These differences are 

 'acquired characters,' and we have here additional 

 evidence that such are not inherited. It is to variation 

 of this type that the term ' fluctuation ' is applied by 

 some authorities. 



It is clear then that if selection is made among 

 beans harvested from a mixed population, on the 

 whole the larger beans will belong to pure lines 

 having a higher mean, and thus selection for a few 

 generations will isolate pure lines having a high 

 value, and the mean of successive generations will 

 rise until the largest pure lines have been isolated. 

 Beyond that point further selection will have no 

 effect. This is precisely the result arrived at by 

 Prof. Pearson from a study of selection within a 

 mixed population ; the mean will rise rapidly on the 

 first selection, more slowly later, until in very few 

 generations it reaches a point at which selection 

 has no appreciable effect. Pearson calculates that if 



