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THE AMERICAN NATURALIST [Vol. L 



or as due to faulty classification of the parents. 

 With reference to the origin de novo of characters it may be re- 

 called that one does not have to search the literature long to find 

 instances of the same mutation occurring repeatedly in different 

 stocks and at different times, or of certain stocks that seem to be 

 especially prone to mutation. 4 Congenital cataracts occur in 

 many races of man and in other mammals. So far as the writer 

 is aware we are not at present in a position to state, either on the 

 basis of observed data or from a priori consideration just how 

 frequently mutations may occur in the human germplasm. 



Again, since Jones and Mason elsewhere in the same paper 

 (p. 124) use the argument that "heterozygous individuals some- 

 times show the recessive character," we might, if necessary, use 

 the same argument to prove the dominance of cataract. On the 

 assumption that congenital cataract is dominant instead of re- 

 cessive it might be maintained that in those cases where both 

 parents of affected individuals seem to' be normal, one of them is, 

 after all, heterozygous, and affected children are therefore to be 

 expected. 



Finally it should be recalled that in their statistical study of 

 these 31 families Jones and Mason do not get the results that 

 their hypothesis demands. After having made the proper mathe- 

 matical corrections there still remains a discrepancy which they 

 do not adequately explain, the agreement between theoretical and 

 observed results being only .418 (p. 122). In order to test what 

 one should expect from the examination of such data when the 

 character is recessive, I have taken a paper by Usher 5 on retinitis 

 pigmentosa and summarized the charts in the same way that 

 Jones and Mason summarize those of Harman. Now retinitis 

 pigmentosa probably is a recessive character as is commonly be- 

 lieved. In the charts of Usher are recorded 44 families in which 



* To cite a single case, we may mentioi 

 ing fowls. In normally 4-toed races po 

 novo, but once having appeared is trans 

 Barfurth, Dietrich, " Experimented Unt< 

 Hyperdactylie bei Hiihnern. V. Mitteta 

 such ihrer Deutung nach den 1 

 mech. d. Organism., Bd. 



Cases," The Eo 1 



