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THE AMERICAN NATURALIST [Vol. L 



of the person who contemplates marriage, then such marriage may be 



As regards congenital cataract, then, Davenport advises that 

 unaffected persons from affected stock can marry without fear 

 of producing affected children. Harman's tables show over 

 thirty matings of unaffected parents having at least one affected 

 child. 



No matter how unsatisfactory is the proof that cataract is a 

 simple recessive, it should be borne in mind that the data given 

 in Harman's tables do not stand the test when cataract is con- 

 sidered as a simple, dominant character. If the argument that 

 heterozygous individuals sometimes show the recessive character 

 is to be used to prove the dominance of cataract, it would be nec- 

 essary to use the assumption to explain thirty-one exceptional 

 families which have from one to eleven children of which 40 per 

 cent, of the total are affected. On the recessive hypothesis there 

 is only one exceptional family so far known to be explained. As 

 long as it is not a simple dominant character it makes no differ- 

 ence whether it is a simple or complex recessive or a dominant 

 governed by multiple factors, the eugenical recommendation 

 quoted above should not be made, and we still believe that Daven- 

 port can be justly criticized. 



Danforth objects to the disagreement between the observed 

 and the expected results in our table I, giving the progenies of 

 matings of normal by normal, and compares the goodness of fit 

 unfavorably with data given by Usher on retinitis pigmentosa. 

 In our results the disagreement lies in an excess of the actual 

 number of affected children over the expected number. If, as 

 Danforth says, ' ' a certain number of congenital cataracts are 

 produced by intrauterine poisoning without necessarily any ref- 

 erence to heredity ' ' the tendency would be to raise the actual 

 number of affected children above the expected. Also any cases 

 of origin de novo, to which he believes we did not give enough 

 consideration, would tend to have the same effect. Moreover, it 

 should be noticed that Usher has over twice as many individuals 

 to base his ratio upon, 320 as compared to 153 in our case. 



Danforth states two main conditions which he says our as- 

 sumption of cataract as a recessive character does not meet. The 

 first is the low probability of an individual carrying the abnor- 



3 Davenport, C. B., "Heredity in Kelation to Eugenics." Henry Holt 

 and Co., New York, 1911, pp. 111-112. 



