118 HEREDITY IN RELATION TO EUGENICS 



parents produce no abnormal children. The condition that 

 makes for retinitis is something added to the normal con- 

 dition. 



The extent of the degeneration varies with the family. 



In a pedigree recorded by Leber (Fig. 85) the characteristic, 



I— 1^ throughout the family, was an increasing 



dimness of vision accompanied by night 



blindness; but later the degeneration was 



^_rn stayed. 



The eugenic instruction is clear. An 



I "l 1 affected man or woman should not marry 



O-r^ ^ I even into stock without taint of retini- 



I tis. Above all, in retinitis stock, cousins, 



I I especially if affected shotild by no means 



■ ■ marry. 



Fig. 85.— Pedigree m. Night Blindness (hemeralopia) . — 

 of retinitis pigmentosa -.,,.,. . . i i <• 



in a family in which This disease IS accompanied by no loss oi 

 the disease becomes perception of form, but at sunset the af- 



checked before bund- ^ ^ 



ness becomes com- fected persons must cease working. Ar- 

 plete. Leber, 1871. ^.g^.^j jjgj^^ j^^jp^ j-^^j^ ^^^ess very in- 

 tense. The lamps of the street are of no assistance in guid- 

 ing these people at night. Eventually, in most strains, the 

 affected persons become totally blind often with a retinitis. 

 This disease is probably due to a defect in the brain and not 

 as has been suggested merely to lack of the visual purple of 

 the retina (Bordley, 1908). 



Through the researches of Cunier (1838) and Nettleship 

 (1907) we have a pedigree of a night blind strain that is the 

 most extensive that has yet been compiled for any disease. 

 It includes 2,116 persons. A part of it is reproduced in 

 Fig. 86. Fig. 87 is a pedigree of an American (colored) 

 family furnished by Dr. Bordley. 



The disease is due to a positive factor. The normals lack 

 this factor. Usually, however, the factor must be duplex 



