The phenomenon of regression may be made somewhat clearer by the aid of a simple diagram--Fig. 10. Here are plotted first the heights, by inches, of a group of fathers, giving the series of dots joined by the diagonal _AB_. Next are plotted the average heights of the sons of each class of fathers: 62-inch fathers give 66-inch sons, 63-inch fathers 66.5-inch sons, 64-inch fathers 67-inch sons, and so for all the classes of fathers. These dots are then joined by the line _EF_. This is the _regression line_. Had it been the case that there was no regression in stature the different classes of fathers would have had sons averaging just the same as themselves and the line representing the heights of the sons would have coincided with the line _AB_. Or if regression had been complete the fathers of any class would have had sons averaging about 69 inches--just the same as the average of the whole group--and the line representing their heights would have had the position of _CD_ in the diagram. As a matter of fact, however, neither of these possibilities is actually realized and the regression line _EF_ is approximated in an actual series of data. A similar relation has been found for many characters other than stature. [Illustration: FIG. 10.--Diagram illustrating the phenomenon of regression. Explanation in text.] The fact of regression is of considerable importance for the theory of evolution as well as for the subject of Eugenics when describing the phenomena of heredity in this statistical manner in whole groups without paying attention to particular individuals. Regression is found in all characteristics observed in this way, psychic as well as purely physical. "The father [i. e., fathers] with a great excess of the character contributes [contribute] sons with an excess, but a less excess of it; the father [fathers] with a great defect of the character contributes [contribute] sons with a defect, but less defect of it."