THE CREATION OF SPECIES 



reappear in another just as does the blue eye pigment 

 or the white coat of the guinea pig. Thus, atrophy 

 of the optic nerve, which leads to total and incurable 

 blindness, may run through a family strain, appear- 

 ing in one generation and disappearing in the next. 

 So a person who has no eye defect, but who belongs 

 to a family having this defect, may transmit the 

 tendency; and the blindness of his children may with 

 full propriety be ascribed to heredity notwithstanding 

 the fact that both parents had normal eyes. The 

 same thing is true of various other eye defects, in- 

 cluding a very distressing anomaly known as colo- 

 boma, marked by a defect of the iris and an open 

 suture running right through the ball of the eye 

 from the pupil to the optic nerve. 



Professor Davenport gives charts showing pedi- 

 grees of families having this painful malformation of 

 the eye. One such chart shows that a man having 

 the defect married a normal woman, and of their 

 two children the boy had the defect and the girl was 

 normal. The boy married a normal woman and their 

 only son inherited the eye defect. The daughter, 

 whose eyes were normal, married a normal man; of 

 their offspring, seven in number, three sons had the 

 defect, and one son and the three daughters were 

 normal. Another chart shows the defect appearing 

 in a second and fifth generation, having skipped two 

 full generations. But in this case, the marriage of 

 cousins, introducing the defective strain from both 

 sides, and as it were, doubling its influence, probably 

 accounts for the reappearance of the malady in the 

 great-grandchildren of the afflicted person. 



203 



