22 



parentage, when considered in its entirety, inherits one-half its 

 characteristics from its parents, one-fourth from its grandparents, 

 one-eighth from its great-grandparents and so on. The law of 

 regression from mediocrity points out that the children of ex- 

 treme parents are not on the average so extreme as their parents, 

 though they deviate in the same direction from the mediocre con- 

 dition of the race. As an example of regression, take Galton's 

 results on sweet-peas : The diameter of parent seeds which pro- 

 duced plants having on the average seeds of the same diameter 

 was 3.94 mm. Assuming this to be the mediocre condition of 

 the strain he was using he found that whatever the parental de- 

 \'iation from this diameter the mean filial deviation was in the 

 same direction, but only one-third as great. Thus the offspring 

 from seeds 5.34 mm. in diameter produced seeds having an aver- 

 age diameter of 3.94 4- ^''^ "^ = 4.41 mm. (observed diam- 

 eter, 4.44 mm.). 



Johannsen obtained similar results in beans when he compared 

 the average weight of seeds in the offspring with the weight of 

 the parent seeds, if the latter were selected solely with reference 

 to the weight of the indix'idual seeds and without regard to 

 the pre-parental anccstrx' ; but when he separated the individual 

 " pure lines " he found that the mean weight of seeds in the off- 

 spring is the same on the average as that of the preceding genera- 

 tions in the .same "line," in other words, plants produced from 

 small seeds bear seeds of the same average weight as do plants 

 which are produced from large seeds having the same ancestr)'. 



By the " i)ure line" Johannsen means a scries of individuals 

 related only through the process of self-fertilization. On a 

 priori grounds it seems proper to apply the term to ever)' 

 series of individuals that do not combine the elements of two 

 or more ancestral lines through the ccjuivalent of a sexual 

 process. Thus, so far as hereditary qualities are concerned, 

 there should be no reason to expect in a self-fertilizing popula- 

 tion, conditions different from those in a population related through 

 budding or other method of vegetative reproduction, provided of 

 course, that the .self-fertilizing population has not been so re- 



