THE INHERITANCE OF FAMILY TRAITS 27 



men. In respect of several of the special abilities the col- 

 laborators have volunteered a numerical grading as follows : 

 1, poor; 2, medium; 3, exceptionally good. These grades 

 are frequently referred to below. 



1. Eye Color 



This depends upon the condition of pigmentation of the 

 iris — the colored ring around the pupil. According to Mr. 

 Charles Roberts (1878, p. 134) ^ the iris has on its inner surface 

 "a layer of dark purple called the uvea . . . and in brown 

 eyes there is an additional layer of yellow (and, perhaps, 

 brown-red) pigment on its outer surface also, and in some 

 instances there is a deposit of pigment amongst the fibrous 

 structures. In the albino, where the pigment is entirely 

 absent from both surfaces of the iris, the bright red blood 

 is seen through the semi-transparent fibrous tissue of a pink 

 color; and in blue eyes, where the outer layer of pigment 

 is wanting, the various shades are due to the dark inner 

 layer of pigment — the uvea — showing through fibrous 

 structures of different densities or degrees of opacity. 



''The eyes of new born infants are dark blue, in conse- 

 quence of the greater delicacy and transparency of the 

 fibrous portion of the iris ; and as these tissues become thick- 

 ened by use and by advancing age the lighter shades of blue 

 and, finally, gray are produced, the gray, indeed, being 

 chiefly due to the color of the fibrous tissues themselves." 

 Yellow pigment is laid down upon the blue, forming yellow- 

 blue or green eyes. " In the hazel and brown eyes the uvea 

 and the fibrous tissues are hidden by increasing deposits 

 of yellow and brown pigment on the anterior surface of the 

 iris, and when this is very dense, black eyes are the result." 



While in most races of the globe brown pigment is heavily 



' For titles of works referred to in text, see Bibliography, at end of book. 



