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 statistical study of inheritance helped to 
outline the problem but did little to solve it. Certain phenomena of 
Inches 
73 
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63 
Fic. 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, rio one of which had a 
trace of blue in its plumage, he sometimes obtained offspring with 
