122 GENETICS 



parents tend to produce plus children ; but the progeny 

 of extreme parents, whether plus or minus, inherit the 

 parental peculiarities in a less marked degree than the 

 latter were manifested in the parents themselves. 



FIG. 23. Scheme to illustrate Gallon's law of regression. The 

 circles represent graded groups of parental height while the 

 arrowpoints indicate the average heights attained by the re- 

 spective offspring. The offspring of undersized parents are 

 taller, and of oversized parents are shorter than their respec- 

 tive parents. Based on data from Galton. 



2. THE IDEA OF THE PURE LINE 



It was Galton's law of regression that suggested 

 to the Danish botanist Johannsen a possible means of 

 controlling heredity. In his mind arose the question 

 whether it would not be possible by continually breeding 



