112 HEREDITY IN RELATION TO EUGENICS 



the family strain cataract appears early, before the age of 

 the person who contemplates marriage, then such marriage 

 may be advised; but if in the given family the cataract occurs 

 late in life it is not possible to predict as to the immunity 

 of the parent, but in that case also, since the potential defect 

 will not greatly interfere with the effectiveness of the chil- 

 dren, fertile marriage may not be gainsaid. 



e. Displaced Lens (ectopia lentis). — This malposition of 

 the lens always causes distorted vision. Fortunately it is 

 not so common as cataract, for Loeb found only 42 families 



I 



I 



Fig. 78. — Pedigree of "coralliform" cataract. Affected persons repre- 

 sented by black symbols; &, male; 9, female; nimabers in circles indicate 

 number of individuals. From Nettleship, 1910. 



described, with 150 children, of whom 70 per cent were 

 affected. The details of the condition and the degree of 

 injury to sight vary from strain to strain (Fig. 79). 



In this case, also, it appears that the defect is due to some 

 positive factor and that when present in either parent it 

 will be present in about half the offspring; but if present in 

 neither parent it will be absent from all descendants. 



The eugenic teaching is clear; persons with displaced lens 

 should have no children; but normal persons of the same 

 strain will not reproduce it in their offspring. 



f. Degeneracy of the Cornea. — While several causes of 

 corneal opacity are known that seem not to be hereditary, 



