374 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



series of cases, the hereditary factors are fairly constant while the 

 environment differs. In this way the differential cause or causes of 

 any character may be located in heredity, in environment, or in both. 

 The observational and sta^tistical study of inheritance helped to 

 outline the problem but did little to solve it. Certain phenomena of 



Fig. 66. — Scheme to illustrate Galton's "Law of Filial Regression" as shown 

 in the stature of parents and children. The mean height of all parents is shown 

 by the dotted line between 68 and 69 inches. The circles through which the 

 diagonal line runs represent the heights of graded groups of parents, and the arrow- 

 heads indicate the average heights of their children. The offspring of undersized 

 parents are taller and of oversized parents are shorter than their respective parents. 

 {From Conklin, after Walter.) 



hereditary resemblances between ascendants and descendants were 

 made intelligible, but there were many peculiar and apparently irregu- 

 lar or lawless phenomena which could not be predicted before they 

 occurred nor explained afterward. For example when Darwin 

 crossed different breeds of domestic pigeons, no one of which had a 

 trace of blue in its plumage, he sometimes obtained offspring with 



